Garnets from Val D'ala Rodingites, Piedmont, Italy

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Article Garnets from Val d’Ala Rodingites, Piedmont, Italy: An Investigation of Their Gemological, Spectroscopic and Crystal Chemical Properties

Valeria Diella 1,* , Rosangela Bocchio 2, Nicoletta Marinoni 2, Franca Caucia 3, Maria Iole Spalla 2 , Ilaria Adamo 4, Antonio Langone 5 and Lucia Mancini 6 1 Institute of Environmental Geology and Geoengineering (IGAG), Section of Milan, National Research Council, 20133 Milan, Italy 2 Department of Earth Sciences “Ardito Desio”, University of Milan, 20133 Milan, Italy; [email protected] (R.B.); [email protected] (N.M.); [email protected] (M.I.S.) 3 Department of Earth Sciences, University of Pavia, 27100 Pavia, Italy; [email protected] 4 Italian Gemological Institute (IGI), 20123 Milan, Italy; [email protected] 5 Institute of Geosciences and Earth Resources (IGG), Section of Pavia, National Research Council, 27100 Pavia, Italy; [email protected] 6 Elettra-Sincrotrone Trieste S.C.p.A., 34149 Basovizza (Trieste), Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-02-5031-5621

Received: 6 November 2019; Accepted: 23 November 2019; Published: 26 November 2019

Abstract: In Val d’Ala (Piedmont, Western Alps, Italy), the more interesting rocks for the mineralogical research are represented by rodingites (rich in mineralized veins and fractures) associated with serpentinites in the eclogitized oceanic crust of Piemonte Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, garnets are the most appreciated as mineral specimens and, in less degree despite their vivid and rich colors, for their potential as gem-quality materials. This study provides a complete gemological characterization of five faceted samples and offers new information by means of Synchrotron X-ray computed micro-tomography imaging gem features. Electron-probe microanalysis (EMPA) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) established that the chemical composition of garnets from different localities, resulted both close to pure andradite, enriched in light rare earth elements (LREE) with a positive Eu anomaly, and grossular-andradite solid solution (grandite), enriched in heavy rare earth elements (HREE). X-ray powder diffraction analyses indicate the possible coexistence of almost pure grossular and andradite. A spectroscopic approach, commonly used with gem-like material, by Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, completes the characterization of the samples. The new data on the textural and geochemical features of the grandite and andradite garnets suggest local growth processes under various chemical and oxidation conditions of metasomatic and metamorphic fluids interacting with the host-rocks. Garnets represent long-lasting mineral records of the complex geological history of the Val d’Ala rodingitic dikes during their oceanic- and subduction-related metamorphic evolution.

Keywords: garnet; rodingites; Val d’Ala; Piedmont; Western Alps; Italy

1. Introduction Val d’Ala (Piedmont, Italy) is internationally renowned for mining activity of minerals used for practical and decorative purposes. The more interesting minerals, garnet, “mussite” (a ﬁbrous variety name of diopside), vesuvianite, and epidote have been well known since the 18th century. The orange–brown grossular-garnets, known with the variety name “hessonite”, were used in the past as precious gemstones in the traditional costumes of the valley and as a sign of engagement. The grossular

Minerals 2019, 9, 728; doi:10.3390/min9120728 www.mdpi.com/journal/minerals MineralsMinerals 20192019, 9, x9 ,FOR 728 PEER REVIEW 2 of 192 of 18 grossular‐garnets, known with the variety name “hessonite”, were used in the past as precious gemstones incrystals, the traditional up to 2 costumes cm in size, of the are valley generally and foundas a sign in of rodingites engagement. and showThe grossular a color fromcrystals, dark up brown to 2 cm to in size,red are orange generally with found large in variation rodingites in theand habit show anda color complex from dark micro brown crystal to red faces. orange Most with of large the deposits variation inproduced the habit euhedraland complex crystals, micro that crystal have faces. been Most sampled of the for deposits mineral produced collections, euhedral scientiﬁc crystals, investigations, that have beenand sampled jewelry (Figure for mineral1). A secondcollections, type scientific of garnet, investigations, variety name “topazolite”,and jewelry belonging(Figure 1). toA thesecond andradite type of garnet,end member, variety name is also “topazolite”, present in this belonging valley but to the andradite small size end of the member, crystals is prevents also present their in use this in valley jewelry. but theThe small crystals size showof the only crystals rhombic-dodecahedral prevents their use in habit jewelry. and coverThe crystals the walls show of thin only and rhombic irregular‐dodecahedral fractures habitoccurring and cover in the the mussite walls of with thin and associated irregular magnetite. fractures Rareoccurring andradite in the crystalsmussite occurwith associated as “melanite” magnetite. and Rare“demantoid” andradite varieties.crystals occur as “melanite” and “demantoid” varieties.

FigureFigure 1. 1.(a)( aOrange) Orange hessonite hessonite crystals crystals filling ﬁlling fractures fractures in rodingite in rodingite from from Val Vald’Ala d’Ala (Museum (Museum of the of Earth the SciencesEarth Sciences Department, Department, University University of Milan); of (b Milan);) close‐up (b) showing close-up the showing mineral the assemblage mineral assemblage filling the fracture. ﬁlling Photosthe fracture. by R. Bocchio. Photos by R. Bocchio.

InIn the the first first decades decades of of last last century, detaileddetailed studiesstudies havehave beenbeen performed performed mainly mainly focusing focusing on on the the morphologicalmorphological and and chemical chemical properties properties of of the the garnet, and onon thethe moremore notable notable occurrences occurrences of of rodingite rodingite layerslayers and and lenses lenses along along the the valley, valley, representing thethe mainmain sourcessources of of high-quality high‐quality crystals crystals [ 1[1–3].–3]. Recently, Recently, MalettoMaletto and and Piccoli Piccoli [4] [4 ]published published a a monograph of greatgreat interestinterest for for museum museum and and private private collections collections dedicateddedicated to to all all Val Val d’Ala d’Ala minerals. minerals. TheThe significant significant scientific scientific and and economic economic value value of garnet of garnet from from Val Val d’Ala d’Ala is documented is documented in literature, in literature, even if evenin recent if in times recent the times mineralogical, the mineralogical, petrological, petrological, and structural and structural investigations investigations on this mineral on this are mineral lacking. Inare this lacking. light, we In have this light, undertaken we have this undertaken study, aimed this to study, provide aimed a complete to provide mineralogical a complete and mineralogical gemological characterizationand gemological that characterization may enhance their that potential may enhance use as their gem potential material and use astheir gem role material in metamorphism. and their role The chemicalin metamorphism. and physical The properties chemical andelucidated physical in properties this study elucidated may further in this the study understanding may further of the the metamorphicunderstanding kinematics of the metamorphic experienced kinematics by the Val experienced d’Ala ultramafic by the Valand d’Alarodingite ultramafic host rocks, and rodingite and their evolution in the subduction of Mesozoic ocean lithosphere during the Alpine orogeny. In particular, the host rocks, and their evolution in the subduction of Mesozoic ocean lithosphere during the Alpine different geodynamic environments in which rodingites can form must be considered. In fact, rodingites orogeny. In particular, the different geodynamic environments in which rodingites can form must be are Si‐undersaturated and Ca‐rich rocks that can form as a result of the metasomatic interaction between considered. In fact, rodingites are Si-undersaturated and Ca-rich rocks that can form as a result of the serpentinites and mafic dykes at the ocean floor or after fluid‐assisted recrystallization during subduction metasomatic interaction between serpentinites and mafic dykes at the ocean floor or after fluid-assisted and collision [5–14]. Garnet is indeed a litmus paper as it is always present in the diagnostic assemblages in recrystallization during subduction and collision [5–14]. Garnet is indeed a litmus paper as it is always each of these different petrogenetic environments. present in the diagnostic assemblages in each of these different petrogenetic environments. The studied samples belong to the Bazzi historical collection preserved in the Museum of the The studied samples belong to the Bazzi historical collection preserved in the Museum of Department of Earth Sciences of the University of Milan, Italy. The faceted samples were characterized by standardthe Department gemological of Earthmethods, Sciences Synchrotron of the UniversityX‐ray computed of Milan, micro Italy.‐tomography The faceted (SR‐μCT) samples and Raman were spectroscopy.characterized Mineral by standard fragments gemological coming from methods, the same Synchrotron localities and X-ray garnet computed country micro-tomography‐rocks were analyzed (SR-µCT) and Raman spectroscopy. Mineral fragments coming from the same localities and garnet country-rocks were analyzed by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier 2

Minerals 2019, 9, 728 3 of 18 transform spectroscopy (DRIFTS), and electron microprobe (EMPA–WDS) combined with laser ablation-inductively coupled plasma-mass (LA–ICP–MS) techniques.

2. Geological Setting Val d’ Ala (Figure2) is an east valley underlain by metaophiolites of the Piemonte Zone of the Western Alps between the Gran Paradiso (to the North) and Dora–Maira Penninic continental nappes. These Mesozoic oceanic rock sequences mainly consist of ultramafites, metagabbros, metabasalts, and calcschists, which underwent Alpine metamorphism under greenschist- and blueschists-facies conditions. These multiple facies reflect the polyphase evolution recorded from Mesozoic ocean lithosphere deformation in the Tertiary Alpine subduction and collisional tectonics ([15–17] and references therein). During Alpine subduction, Val d’Ala metaophiolites re-equilibrated at P 1.3 GPa ≥ and T = 425–550 ◦C, as estimated from eclogite relics variably preserved during exhumation-related greenschist-facies [17]. Mineral assemblages of metabasites and ultramafites ([15–17] and references therein) indicate that these ophiolites recorded an ocean floor-related metamorphic imprint, under conditions (zeolite-, prehnite-pumpellyite- and greenschist-facies) similar to those suggested for other metaophiolites from Western Alps [10]. Their qualitative PT (Pressure-Temperature) evolution as deduced from the literature is synthesized in Figure2d where the low- and high-pressure stability fields of different garnet-bearing assemblages inferred from polymetamorphic rodingites of Betic Cordillera, in Southern Spain [18] are reported (light-pink fields). The ultramafic rocks consist of antigorite serpentinite bodies that represent the most widespread rock-type of Val d’Ala, and contain from decametric to metric lenses and layers of rodingites. These rocks are intersected by abundant veins that have attracted the interest of many researchers since the beginning of 1800 ([6,19] and references therein). Rodingites occur in many localities all along Val d’Ala, from the high part of the valley, West of the village of Balme, to the lower part of the valley, East of the village of Ala di Stura near Mount Curbassera. In all occurrences, the rodingites are very heterogeneous with structural and mineralogical features that record a complex tectonic origin. The rodingite-forming minerals are mainly Ca-rich garnet, epidote, diopside, chlorite, and vesuvianite, with minor titanite and opaque minerals [16]. A complete list of all the minerals as well as the detailed description of the main occurrences of rodingites are already reported by [3]. The mineralogical richest and most known locations for finest grossular are Testa Ciarva and Roch Neir (I and II; labelled No.1, 2, and 3 in Figure2c) where the serpentinite cli ffs contain rodingite layers up to 2 m that are commonly boudinaged. South of Roch Neir, in Mount Tovo serpentinites (labelled 4 in Figure2c), mineralized veins host garnets with a rounded and globular shape. Finally at Curbassera (labelled 5 in Figure2c), grossular concentrations occur both in lenses of brown rodingite and in blocks of debris in a landslide fallen in the 18th century. Our samples came from the above-mentioned famous localities. Minerals 2019, 9, 728 4 of 18 Minerals 2019, 9, x FOR PEER REVIEW 4 of 19

FigureFigure 2. Geologic outline outline of of Val Val d’Ala d’Ala garnet garnet-bearing‐bearing rocks: rocks: (a) ( atectonic) tectonic sketch sketch map map of the of the European European Alps; Alps;(b) tectonic (b) tectonic scheme scheme of the Western of the Western Alps with Alps Val with d’Ala Val location; d’Ala location; (c) simplified (c) simplified geological geological map of Val map d’Ala ofmodified Val d’Ala after modified [16,20]. afterLegend: [16, 20Metaophiolites]. Legend: Metaophiolites of the Piemonte of theZone: Piemonte 1 = calcschist; Zone: 1 2= =calcschist; metabasalt; 2 =3 = metabasalt;metagabbro; 3 4= =metagabbro; serpentinite; 4 5= =serpentinite; metagranitoid 5 = withmetagranitoid K‐feldspar with porphyroclasts K-feldspar porphyroclasts(Gran Paradiso (Grannappe– ParadisoPenninic nappe–Penniniccontinental crust); continental 6 = fine‐grained crust); 6gneiss= fine-grained (Sesia‐Lanzo gneiss Zone–Austroalpine (Sesia-Lanzo Zone–Austroalpine continental crust); continentalsampling localities: crust); sampling red asterisks: localities: 1 = Testa red Ciarva; asterisks: 2, 3 1 == RochTesta Neir Ciarva; I, II; 2,4 = 3 Mount= Roch Tovo; Neir 5 I, =Curbassera. II; 4 = Mount (d) Tovo;Qualitative 5 = Curbassera. P‐T evolution (d) Qualitativeof Val d’Ala P-T metaophiolites evolution of Valas inferred d’Ala metaophiolites from [10,17]. See as inferred discussion from in [ 10the,17 text.]. SeeOrange discussion square = in estimated the text. metamorphic Orange square conditions= estimated of eclogite metamorphic facies in Val conditions d’Ala [17]. of eclogiteMetamorphic facies facies in Valare d’Alaafter [21]: [17 ].A Metamorphic= amphibolite, Bs facies = blueschist, are after E [21 = ]:eclogite, A = amphibolite, EA = epidote Bs‐amphibolite,= blueschist, G = Egranulite,= eclogite, Gs = EAgreenschist,= epidote-amphibolite, PP = prehnite‐pumpellyite, G = granulite, Z = zeolite. Gs = greenschist, Light‐pink fields PP = = prehnite-pumpellyite,low‐ and high‐pressure Zconditions= zeolite. for Light-pinkgarnet growth fields in =polymetamorphiclow- and high-pressure rodingites conditions from Betic for Cordillera–Southern garnet growth in polymetamorphic Spain [18]. rodingites from Betic Cordillera–Southern Spain [18]. 3. Materials and Methods 3. Materials and Methods Twelve samples from different localities were selected for this study and are described in Table 1. The samplesTwelve range samples from light from yellow different to dark localities red in were color. selected The reddish for this garnets, study andfrom are 1 cm described to 3 cm in Tablesize with1. Thevarious samples crystalline range fromfaces light (cube, yellow trapezoedron, to dark red rhombic in color.‐dodecahedron, The reddish garnets, and hexoctahedron), from 1 cm to 3may cm in be sizedeformed with various whereas crystalline the light facesyellow (cube, samples trapezoedron, are smaller, rhombic-dodecahedron, up to 4 mm in size, clear and and hexoctahedron), only rhombic‐ maydodecahedron be deformed in shape. whereas the light yellow samples are smaller, up to 4 mm in size, clear and only rhombic-dodecahedron in shape.

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Table 1. Description of the studied garnets.

Sample Locality Color Analytical Techniques 1 Roch Neir I dark red electron microprobe (WDS) powder XRD LA–ICP–Mass 2 Roch Neir I light yellow electron microprobe (WDS) powder XRD DRIFT spectroscopy spectrometry from light yellow to jacinth LA–ICP–Mass 3 Testa Ciarva electron microprobe (WDS) powder XRD DRIFT spectroscopy and dark red spectrometry LA–ICP–Mass 4 Roch Neir II dark red electron microprobe (WDS) DRIFT spectroscopy spectrometry 5 Roch Neir II dark red electron microprobe (WDS) from light yellow to jacinth LA–ICP–Mass 6 Curbassera electron microprobe (WDS) and dark red spectrometry from light yellow–green to LA–ICP–Mass 7 Mount Tovo electron microprobe (WDS) powder XRD DRIFT spectroscopy dark red spectrometry 8 faceted stone Roch Neir II orange red specific gravity refractive index Micro-Raman spectroscopy 9 faceted stone Roch Neir II orange red specific gravity refractive index Micro-Raman spectroscopy Microfocus X-ray computed µ-tomography 10 faceted stone Roch Neir II orange red specific gravity refractive index Micro-Raman spectroscopy Microfocus X-ray computed µ-tomography 11 faceted stone Testa Ciarva reddish orange specific gravity refractive index Micro-Raman spectroscopy Microfocus X-ray computed µ-tomography 12 faceted stone Testa Ciarva reddish orange specific gravity refractive index Micro-Raman spectroscopy Minerals 2019, 9, 728 6 of 18

Five faceted samples (three from Roch Neir II, samples 8–10, and two from Testa Ciarva, samples 11, 12) were examined by standard gemological methods at the laboratories of the University of Pavia, Italy, in order to determine their refractive index, hydrostatic specific gravity and microscopic features. The refractive indices were measured with a Krüss refractometer (1.45–1.80 range) using ordinary light source with a sodium filter (589 nm) and a methylene iodide as a contact liquid (n = 1.80). A Mettler hydrostatic balance was used to determine the specific gravity in bi-distilled water. The ultraviolet fluorescence was investigated with a short (254 nm) and long (366 nm) wavelength ultraviolet Wood lamp. The color was determined with a GIA Color Table; often the crystals are slight polychrome. The Microfocus X-ray computed micro-tomography (CT) analyses of two cut samples (10 and 11) were performed at laboratory X-ray µCT scanning using the TomoLab station at the Elettra synchrotron facility in Basovizza (Trieste, Italy). The µCT system is equipped with a sealed microfocus X-ray source (L9181, Hamamatsu Photonic, Iwata, Japan) and delivers a polychromatic beam in a Voltage range of 40 kV to 130 kV with a maximum current of 300 µA and a minimum focal spot size of 5 µm. The used detector consisted of a full frame CCD imager (4008 2672 pixels) coupled to a Gadox scintillator × screen by a fiber-optic taper. The water-cooled CCD camera has a 12-bit dynamic range and an effective pixel size of 12.5 µm2 12.5 µm2. The experimental parameters used for the X-ray µCT scans are × reported in Table2. The slice reconstruction was carried out using the commercial software COBRA (Exxim Computing, Pleasanton, CA, USA) based on the Feldkamp algorithm [22], which also allows us to correct beam hardening artefacts [23]. 3D visualization through volume rendering of the analyzed samples were obtained by the commercial software VGStudio 2.2 (Volume Graphics).

Table 2. Experimental parameters used for the laboratory X-ray microtomography measurements (d1 and d2 represent the source-to-sample and the source-to-detector distances, respectively).

d d Voltage Current Voxel Size Al Filter Thickness Exposure Sample 1 2 (mm) (mm) (kV) (µA) (µm) (mm) Time (sec) 10 80 400 130 61 5.0 1.5 6.1 11 80 500 90 89 4.0 0.75 8.25

Seven samples from different localities were polished and analyzed by electron microprobe (EMPA) and inductively coupled plasma–mass spectroscopy (LA–ICP–MS): five massive garnets samples varying from 1–2 mm (2 and 7) to 3–4 cm in size (1, 4 and 5) embedded in epoxy resin, and two thin sections (3 and 6) of rock containing garnets in their matrix. Backscattered electron (BSE) images and quantitative chemical analyses of major and minor elements were obtained with the JEOL JXA-8200 electron microprobe in wavelength dispersion mode (EMPA–DS) at the laboratory of the Department of Earth Sciences of the University of Milan, Italy, under the following conditions: 15 kV accelerating voltage, 5 nA beam current, and a count time of 60 s on peak and 30 s on the background, with a 1 µm diameter beam. Natural minerals were used as standards and the rough data corrected for matrix effects using a conventional ΦρZ routine in the JEOL software package (Japan Electron Optics Laboratory). Rare earth and selected trace elements were determined by laser ablation–inductively coupled plasma–mass spectroscopy (LA–ICP–MS) at IGG-CNR Laboratory of Pavia, Italy. Wherever possible measurement spots were chosen at the same or close to the EMPA points. The instrument consisted of a 193 nm Excimer laser (Geolas) coupled to an Agilent 8900 triple quadrupole ICPMS. The spot size was 55 µm, using NIST SRM 610 glass as an external standard and 29Si as an internal standard, as analyzed by microprobe. Precision and accuracy estimated on the USGS basaltic glass standard BCR2 and NIST612 were better than 10%. Four fragments (~500 µm) from samples 1–3, and 7 were selected for powder X-ray diffraction measurements. The analyses were performed at the Department of Earth Sciences of the University of Milan, by a Panalytical X’Pert-PROMPD X’Celerator X-ray powder diffractometer, using CuKα radiation (λ = 1.518 Å), at a beam voltage of 40 kV and a current of 40 mA. X-ray powder diffraction Minerals 2019, 9, 728 7 of 18

Minerals 2019, 9, x FOR PEER REVIEW 7 of 19 patterns were collected over the 9–120◦ range of the scattering angle 2θ, with steps of 0.01◦ 2θ and a step.count The timelattice of parameters 50 s per step. of The the latticegarnet parameters samples were of the determined garnet samples by using were Si determined (NBS SRM by 640b) using as Si an internal(NBS standard SRM 640b) and as the an internalGeneral standardStructure and Analysis the General System Structure (GSAS) software Analysis was System used (GSAS) to process software XRPD datawas [24,25]. used to process XRPD data [24,25]. BesidesBesides standard standard gemological gemological testing, testing, four four faceted faceted samples samples were were analyzed analyzed by by Raman Raman spectroscopy at the atlaboratories the laboratories of the ofUniversity the University of Pavia. of Pavia. Micro Micro-Raman‐Raman scattering scattering measurements measurements were wereconducted conducted using a Horibausing Jobin a Horiba‐Yvon Xplora Jobin-Yvon Plus single Xplora monochromator Plus single monochromator spectrometer (with spectrometer a grating (with of 2400 a gratinggroove/mm) of equipped2400 groove with /anmm) Olympus equipped BX41 with microscope. an Olympus Raman BX41 microscope.spectra were Ramanexcited spectraby the were532 nm excited line. by The –1 spectrometerthe 532 nm was line. calibrated The spectrometer to the silicon was Raman calibrated peak at to 520.5 the silicon cm–1. The Raman spectral peak resolution at 520.5 cm was.  The2 cm–1 –1 andspectral the instrumental resolution wasaccuracy ~2 cm in determiningand the instrumental the peak accuracy positions in determiningwas 0.56 cm the–1. peakRaman positions spectra was were –1 –1 –1 collected~0.56 cmin the. Ramanspectral spectra range 50–1250 were collected cm–1 and in 3000–3800 the spectral cm range–1 for 50–1250 5 s averaging cm and over 3000–3800 40 accumulations. cm for 5Diffuse s averaging reflectance over 40 infrared accumulations. Fourier transform spectroscopy (DRIFTS) was performed on four samples at the ItalianDiffuse Gemological reflectance Institute infrared (IGI), Fourier using transform a Perkin spectroscopy Elmer Frontier (DRIFTS) FTIR, equipped was performed with the on Praying four Mantissamples diffuse at thereflection Italian accessory. Gemological The Institute spectra (IGI),of the usingsamples a Perkin (2–4, and Elmer 7) were Frontier collected FTIR, between equipped 8000 with cm– 1 to the450 Praying cm–1, using Mantis LiTaO diff3use as reflectiondetector. A accessory. resolution The of spectra 4 cm–1 ofwas the adopted samples and (2–4, a andtotal 7) of were 128 collectedscans were –1 –1 –1 recordedbetween and 8000 averaged cm tofor 450 each cm sample., using LiTaO3 as detector. A resolution of 4 cm was adopted and a total of 128 scans were recorded and averaged for each sample. 4. Results and Discussion 4. Results and Discussion 4.1. Gemological Properties and 3D Visualisation 4.1. Gemological Properties and 3D Visualisation The vivid, intense and rich colors, ranging from reddish orange to orange red, make the faceted The vivid, intense and rich colors, ranging from reddish orange to orange red, make the samples of gemological interest (Figure 3). The luster is vitreous; the diaphaneity is transparent/translucent. faceted samples of gemological interest (Figure3). The luster is vitreous; the diaphaneity is The gems have been cut from samples with a combination of the rhombic dodecahedron {100} and transparent/translucent. The gems have been cut from samples with a combination of the rhombic trapezohedron {211} crystal form. The inhomogeneous color of some samples may be due to the presence dodecahedron {100} and trapezohedron {211} crystal form. The inhomogeneous color of some of crystalline inclusions such as clinochlore, diopside, calcite veinlets, or fractures. Their physical and samples may be due to the presence of crystalline inclusions such as clinochlore, diopside, calcite optical properties are reported in Table 3. The measured gravity ranges from 3.63 g/cm3 to 3.66 g/cm3, in veinlets, or fractures. Their physical and optical properties are reported in Table3. The measured agreement with the literature data for solid solution grossular/andradite [26]. All the analyzed gems are gravity ranges from 3.63 g/cm3 to 3.66 g/cm3, in agreement with the literature data for solid solution inert to long and short UV. grossular/andradite [26]. All the analyzed gems are inert to long and short UV.

FigureFigure 3. 3. FacetedFaceted garnets garnets described described in in Table Table 33..

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Table 3. Physical and optical data.

MineralsGem2019, 9,Weight 728 (ct) Gravity (g/cm3) Refraction Index Color (GIA) Cut/Shape 8 of 18 8 3.04 3.66 >1.81 orange red Brilliant oval 9 5.6 3.63 >1.81 orange red Fantasy triangular Table 3. Physical and optical data. 10 4.22 3.66 >1.81 orange red Emerald rectangular 11 0.45Weight 3.65Gravity Refraction>1.81 reddish orange Brilliant round Gem Color (GIA) Cut/Shape 12 0.33 (ct) 3.66(g/cm 3) Index>1.81 reddish orange Brilliant round 8 3.04 3.66 >1.81 orange red Brilliant oval More 9details were 5.6 provided by 3.63 the 3D visualisation>1.81 of faceted orange samples red 10 and Fantasy 11 and triangular their renderings are shown10 in Figure 4. 4.22 In Figure 4a, 3.66 the tomographic>1.81 results highlight orange the red occurrence Emerald of small rectangular and irregular crystals (labeled11 garnet 0.45 11‐2) widespread 3.65 on the> 1.81surface of reddishthe gem orange (garnet 11‐1). Brilliant The lighter round gray‐scale values associated12 to garnet 0.33 11‐2 might 3.66 reflect its> 1.81slightly different reddish chemical orange composition Brilliant roundwith respect to garnet 11‐1 and attributed to the presence of a Fe‐richer phase. This hypothesis is consistent with the XRPD refinementMore detailsresults of were sample provided 3 from bythe thesame 3D locality, visualisation indicating of the faceted coexistence samples both 10 of and grossular 11 and and their of renderingsandradite (see are below; shown 4.2 in paragraph). Figure4. In Figure4a, the tomographic results highlight the occurrence of Furthermore, the 3D rendering with the corresponding iso‐surfaces of the fractures and pores of small and irregular crystals (labeled garnet 11-2) widespread on the surface of the gem (garnet 11-1). sample 10 are shown in Figure 4b. A widespread microcracking is highlighted inside the garnet and is The lighter gray-scale values associated to garnet 11-2 might reflect its slightly different chemical characterized by cracks mostly iso‐oriented. Pores appear micrometric in size with an irregular shape and composition with respect to garnet 11-1 and attributed to the presence of a Fe-richer phase. This randomly distributed inside the whole volume. The reconstructed slices show that the cracks are mostly hypothesisempty and, is only consistent in few cases, with the are XRPD filled with refinement mineral results phases, of which sample have 3 from grayscale the same values locality, very indicatingsimilar to thegarnet coexistence 11‐2 and could both ofbe grossularattributed andto the of crystallization andradite (see of below; andradite. Section 4.2).

FigureFigure 4. 4. GarnetGarnet 3D 3D visualization: visualization: (a) volume (a) volume renderings renderings of sample of sample 11; (b) volume 11; (b) renderings volume renderings and segmented and segmentedpores and cracks pores of and sample cracks 10. of sample 10.

4.2. ChemicalFurthermore, Composition the 3D and rendering Crystallographic with the Properties corresponding iso-surfaces of the fractures and pores of sampleWe 10 analyzed are shown five in massive Figure4 b.samples A widespread and two microcrackingthin sections where is highlighted the mineral inside paragenesis the garnet consists and is characterizedmainly of garnet, by cracksdiopside, mostly clinochlore, iso-oriented. amphibole, Pores and appear rutile/titanite. micrometric Fabric in size of the with host an‐rodingites irregular varies shape andfrom randomly massive to distributed foliated, generally inside the banded whole (summarized volume. The description reconstructed of thin slices sections show thatare synthesized the cracks are in mostlyTable 4). empty and, only in few cases, are ﬁlled with mineral phases, which have grayscale values very similar to garnet 11-2 and could be attributed to the crystallization of andradite.

4.2. Chemical Composition and Crystallographic Properties We analyzed five massive samples and two thin sections where the mineral paragenesis consists mainly of garnet, diopside, clinochlore, amphibole, and rutile/titanite. Fabric of the host-rodingites varies from massive to foliated, generally banded (summarized description of thin sections are synthesized in Table4). The relative quantities of grossular and andradite compared to all remaining components are plotted in Figure5 and averaged chemical analyses for samples 2, 7, 1, 4, 5 and selected analyses8 for thin sections, samples 3 and 6, are reported in Table5. The rare earth elements (REE) and trace elements concentrations are presented in Table6. Minerals 2019, 9, 728 9 of 18 Minerals 2019, 9, x FOR PEER REVIEW 9 of 19 Minerals 2019, 9, x FOR PEER REVIEW Minerals 2019, 9, x FOR PEER REVIEW 9 of 19 9 of 19 Minerals 2019, 9, x FOR PEER REVIEW 9 of 19 Table 4. Textural and mineralogical characters of studied thin sections from samples 3 and 6. The short side of thin sections microphotographs is 6 mm and 10 mm Table 4. TexturalTable 4. Textural and mineralogical and mineralogical characters Table 4. of Textural studied of studied andthin mineralogicalsections thin from sections characterssamples from3 andof studied 6. samples The thinshort sections side 3 and of thinfrom 6. sections samples The short microphotographs 3 and side6. The ofshort thin isside 6 mm sectionsof thin and sections 10 mm microphotographs microphotographs is 6 is mm 6 and mm 10 andmm 10 mm for Table 4. Textural and mineralogicalfor samples characters3 and 6, respectively. of studied thin sections from samples 3 and 6. The short side of thin sections microphotographs is 6 mm and 10 mm for samples 3 and 6, respectively. for samples 3 and 6, respectively. samples 3for and samples 6, respectively. 3 and 6, respectively. Mineral Assemblages Texture Mineral Assemblages MineralTexture Assemblages Texture SamMineralple Assemblages3 Texture Sample 3 MineralSample Assemblages 3 Texture Sample 3 Massive: garnetMassive: occurs in garnetMassive:Massive: occurs garnet garnet in occurs aggregates occurs in in Massive: garnet occurs in aggregates or in veins interstitialor inaggregates veins aggregates interstitial or in or veins in between veins interstitial interstitial aggregates or in veins interstitial between diopside polygonal andbetween between diopside diopside polygonal polygonal and and between diopside polygonaldiopside and polygonal and seriate seriate aggregates. Large grainsizedseriate aggregates. Large grainsized seriate aggregates. Large grainsizedaggregates.seriate aggregates. Large grainsized Large grainsized Garnet, clinopyroxene, Mg‐ diopsideGarnet, is cloudy clinopyroxene, and wrapped Mg‐ by diopside is cloudy and wrapped by Garnet,Garnet, clinopyroxene, clinopyroxene, MgGarnet,‐ diopside Mg-richclinopyroxene, is cloudy and Mgdiopside wrapped‐ diopside by is cloudy is cloudy and and wrapped wrapped by by rich chlorite, tremolite, clearrich fine chlorite,‐grained tremolite, polygonal clear fine‐grained polygonal rich chlorite, tremolite, rich chlorite,clear fine tremolite,‐grained polygonal clear fine‐grained polygonal calcite,chlorite, magnetite, tremolite,rutile. calcite,diopsidecalcite, grains. magnetite, Garnetsclear rutile. fine-grainedshow diopside polygonal grains. Garnets diopside show calcite, magnetite, rutile. calcite,diopside magnetite, grains. rutile. Garnets showdiopside grains. Garnets show magnetite, rutile.complex Grs–Adr zoninggrains. (different Garnets complex Grs–Adr show complex zoning (different complex Grs–Adr zoning (differentcomplex Grs–Adr zoning (different gray patchy zones in the Grs–AdrBSE image gray zoning patchy (di zonesfferent in the gray BSE image gray patchy zones in the BSE image on the left) that are intersectedgray by patchyon the zonesleft) that in arethe intersected BSE image by on the left) that are intersectedpatchy zones by in the BSE image on the late Adr‐rich veins. on the left) thatlate areAdr ‐intersectedrich veins. by late Adr‐rich veins.left) that are intersected by late late Adr‐rich veins. Adr-rich veins.

Sample 6 Sample 6 Foliated: garnetite Foliated:vein at high garnetite Foliated: vein garnetite at high vein angle at high Sample 6 Foliated: garnetite vein at high Sample 6 angle with the Alpine foliation that angle with the Alpine foliation that angle with the Alpine foliationwith the thatFoliated: Alpine garnetite foliation vein that at high is is marked by chlorite‐diopside is marked by chlorite‐diopside is marked by chlorite‐diopsidemarkedangle with by chlorite-diopside the Alpine foliation that lenticular aggregates and garnet lenticular aggregates and garnet lenticular aggregates andlenticular garnet aggregates and garnet Garnet, clinopyroxene, Mg‐ trailsGarnet, and elongated clinopyroxene, aggregates. Mg‐is markedtrails andby chloriteelongated‐diopside aggregates. Garnet,Garnet, clinopyroxene, clinopyroxene, Mg‐ trails Mg-rich and elongated aggregates.trails and elongated aggregates. rich chlorite, tremolite, Compositionalrich chlorite, pattern tremolite, of garnet lenticular is Compositional aggregates pattern and ofgarnet garnet is rich chlorite, tremolite, Compositional pattern of garnet is magnetite,chlorite, rutile, tremolite, titanite,Garnet, magnetite, complex: clinopyroxene,magnetite, rutile, in garnetite rutile, MgCompositional titanite,veins‐ Adr trails‐ complex:and patternelongated in garnetite of aggregates. garnet veins is Adr ‐ magnetite, rutile, titanite, complex: in garnetite veins Adr‐ and titanite,calcite. andrich calcite.rich chlorite, grains tremolite,occur,and calcite. whereascomplex: in Compositionalfine in‐ rich garnetite grains pattern occur, veins whereas of Adr-rich garnet in fineis ‐ and calcite. rich grains occur, whereas in fine‐ grained aggregates grainscomplex occur, and grained whereas aggregates in ﬁne-grained complex and magnetite,grained rutile, aggregates titanite, complex andcomplex: in garnetite veins Adr‐ oscillatory Grs–Adr zoning occurs, oscillatory Grs–Adr zoning occurs, oscillatoryand calcite. Grs–Adr zoningaggregates occurs,rich grains complex occur, and whereas oscillatory in fine‐ as shown in the BSE image on the as shown in the BSE image on the as shown in the BSEGrs–Adr image on grainedthe zoning aggregates occurs, ascomplex shown and in left. left. left. theoscillatory BSE image Grs–Adr on the zoning left. occurs, as shown in the BSE image on the

left. 9 9 9

Minerals 2019, 9, 728 10 of 18

Table 5. Electron microprobe analyses of the Val d’Ala garnets (wt %).

2 7 1 4 5 3 6 Sample Average Average Average Average Average st. dev. st. dev. st. dev. st. dev. st. dev. c1-6 c1-1 c3b-42 c3b-43 5 pts 50 pts 8 pts 13 pts 43 pts

SiO2 35.98 0.24 35.53 0.24 38.25 0.24 37.84 0.37 37.75 0.43 38.67 39.67 37.32 38.35 TiO2 0.02 0.02 0.35 0.14 1.14 0.13 1.58 0.38 1.54 0.41 0.54 0.18 1.53 0.74 Al2O3 0.60 0.23 0.15 0.05 15.07 0.52 14.03 0.82 14.20 0.97 17.75 20.06 15.00 19.99 Cr2O3 0.01 0.01 0.04 0.04 0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.05 0.14 FeOtot 27.39 0.40 27.75 0.27 9.52 0.36 10.53 0.79 10.30 0.81 7.43 3.59 11.79 11.40 MnO 0.10 0.03 0.06 0.04 0.26 0.05 0.27 0.04 0.28 0.05 0.22 0.11 0.78 0.51 MgO 0.11 0.02 0.16 0.04 0.12 0.04 0.17 0.04 0.18 0.06 0.43 0.27 0.17 1.25 CaO 33.21 0.10 33.33 0.11 35.17 0.25 34.94 0.34 34.76 0.31 34.71 36.20 32.48 27.54 Total 97.42 97.38 99.55 99.38 99.03 99.78 100.08 99.12 99.91 Number of ions on the basis of 24 oxygens Si 6.032 5.976 5.945 5.917 5.923 5.935 6.002 5.867 5.905 Ti 0.003 0.044 0.133 0.185 0.182 0.063 0.021 0.181 0.085 Al 0.118 0.031 2.761 2.586 2.626 3.211 3.577 2.779 3.627 Cr 0.001 0.005 0.002 0.001 0.002 0.003 - 0.007 0.017 Fe3+ * 3.812 3.899 1.087 1.212 1.176 0.791 0.381 1.122 0.379 Fe2+ * 0.029 0.004 0.150 0.166 0.180 0.163 0.073 0.428 1.089 Mn 0.015 0.008 0.035 0.035 0.038 0.029 0.014 0.103 0.067 Mg 0.027 0.039 0.027 0.040 0.041 0.098 0.061 0.040 0.286 Ca 5.964 6.006 5.856 5.854 5.844 5.708 5.868 5.471 4.544 Prp 0.45 0.64 0.44 0.66 0.68 1.64 1.01 0.66 4.77 Alm 0.48 0.07 2.48 2.72 2.95 2.71 1.21 7.08 18.20 Sps 0.24 0.13 0.57 0.58 0.62 0.49 0.23 1.71 1.12 Adr 95.78 97.73 26.71 29.83 28.84 18.81 9.35 25.68 7.16 Uv 0.03 0.13 0.05 0.03 0.06 0.06 0.00 0.15 0.32 Grs 2.96 0.18 66.49 61.62 62.41 74.80 87.68 60.58 66.81 Shorl 0.07 1.12 3.39 4.75 4.65 1.57 0.53 4.57 2.12 * calculated on the basis of stoichiometry [27]. Minerals 2019, 9, x FOR PEER REVIEW 10 of 19

The relative quantities of grossular and andradite compared to all remaining components are plotted in Figure 5 and averaged chemical analyses for samples 2, 7, 1, 4, 5 and selected analyses for thin sections, samples 3 and 6, are reported in Table 5. The rare earth elements (REE) and trace elements concentrations are presented in Table 6. The deficiency of silica (stoichiometric Si values lower than 6 in Table 5) may indicate the possible occurrence of OH in the tetrahedral site supporting the presence of water as resulted from Mineralsthe Raman2019, spectra9, 728 (see below). 11 of 18

Figure 5.5.Compositional Compositional ternary ternary diagram diagram of grossular, of grossular, andradite andradite and the and sum the of remainingsum of remaining end-members end‐ ofmembers studied of samples studied (a samples) and enlargements (a) and enlargements (b,c). (b,c). Table 6. LA–ICP–MS analyses of the Val d’Ala garnets (ppm).

2 7 1 3 6 Sample Average 6 pts Average 13 pts Average 5 pts Average 5 pts Average 9 pts Sc 0.39 8.47 24.81 26.18 41.36 Ti 141.4 2296 8686 2255 8295 V 276.5 573.9 299.3 177.1 379.4 Cr 5.04 271.5 55.26 13.93 400.1 Co 0.01 0.06 1.58 3.78 2.14 Ni 0.14 1.16 0.70 0.51 0.88 Zn 0.06 0.45 1.96 2.30 4.60 Sr 0.14 0.07 0.36 0.19 1.54 Y 0.88 1.00 25.06 6.06 43.98 Zr 0.72 1.68 10.79 7.71 14.64 La 0.86 0.19 0.01 0.01 0.12 Ce 2.20 1.52 0.37 0.22 0.75 Pr 0.22 0.19 0.31 0.19 0.42 Nd 0.99 1.12 5.40 3.25 7.64 Sm 0.19 0.28 3.95 1.34 5.39 Eu 0.06 0.41 1.68 0.78 2.61 Gd 0.16 0.24 5.27 1.54 7.72 Tb 0.02 0.03 0.77 0.21 1.21 Dy 0.16 0.18 5.59 1.43 9.12 Ho 0.03 0.03 0.90 0.23 1.59 Er 0.09 0.07 2.77 0.62 4.90 10 Tm 0.01 0.01 0.32 0.06 0.58 Yb 0.10 0.09 2.68 0.46 4.72 Lu 0.01 0.01 0.31 0.05 0.55 P REE 5.10 4.36 30.33 10.39 47.31 Minerals 2019, 9, 728 12 of 18

The deficiency of silica (stoichiometric Si values lower than 6 in Table5) may indicate the possible occurrence of OH in the tetrahedral site supporting the presence of water as resulted from the Raman spectra (see below). The results show two different compositions of garnets. The first group, including samples 2 and 7, is very close to the end member andradite; the second one (massive samples 1, 4, 5 and thin sections 3 and 6) belongs to a solid solution grossular–andradite (grandite), never reaching the grossular pure end member. The aspect and the analyses of sample 2 (Adr96) from Roch Neir II confirm the previous classification of garnet as topazolite, the variety of andradite greenish–yellow in color. Sample 7 from Mount Tovo with globular and rounded crystal shapes, reported in literature as grossular [3], is here determined to be 95–99% andradite. Sample 7 exhibits a hue of color ranging from green to orange–brown with higher content of the chromophore minor elements, such as Ti, Cr, and V. The higher content of these elements was determined in the brown zones of the crystals. In the second group, including samples from deep red-brown to light orange in color and classified as hessonite variety, grossular content ranges from 41% to 88%, andradite content from 4% to 48% and almandine up to 20%. The higher grossular component was detected in samples from Testa Ciarva (sample 3) and Curbassera (sample 6) where garnets consist of large red brown crystals covered by smaller garnet crystals paler in color in respect to the samples from Roch Neir I (1), II (4,5). The TiO2 content is up to 2.4 wt %, Cr2O3 ranges from nil to 0.1 wt %, and MnO from 0.3 wt % to 0.8 wt %. The variations, occurring in all samples, are not related to a regular or harmonic zoning from core to the rim. The lower content of chromophore elements (Ti, V, and Cr) of sample 3 (Testa Ciarva) may explain the color of faceted samples 11 and 12 coming from the same locality, lighter than those from Roch Minerals 2019, 9, x FOR PEER REVIEW 13 of 19 Neir II (9–11). ExamplesExamples of of the the compositional compositional variations variations in Adrin Adr and Grsand componentsGrs components are depicted are depicted in BSE imagesin BSE (samplesimages (samples 3 and 6): 3 theand garnet 6): the appears garnet appears heterogenous heterogenous with lighter with parts lighter enriched parts enriched in andradite in andradite content (Figurecontent6 a,b;(Figure Table 6a,b;5, C1-6 Table and 5, C3b-42 C1‐6 and analyses) C3b‐42 in analyses) respect to in the respect darker to ones the (Figure darker6 a,b;ones Table (Figure5, C1-1 6a,b; andTable C3b-43 5, C1‐ analyses).1 and C3b‐43 analyses).

FigureFigure 6.6. BSEBSE imagesimages ofof garnetsgarnets (dark(dark andand lightlight gray)gray) inin samplesample 33 ((aa)) andand 66 ( (bb).). TheThe numbersnumbers inin thethe photosphotos referrefer to to microprobe microprobe point point analyses analyses in in Table Table5 .5.

Chondrite-normalizedChondrite‐normalized REE REE patterns patterns [ 28[28]] of of the the examined examined samples samples are are plotted, plotted, as as mean mean values, values, in Figurein Figure7a. 7a. The The patterns patterns of granditeof grandite garnets garnets (1,3,6) (1,3,6) display display an an increase increase of of heavy heavy rare rare earth earth elements elements (HREE)(HREE) andand wellwell matchmatch thatthat ofof grossulargrossular garnet. garnet. InIn contrast,contrast, andraditeandradite samplessamples (2,7)(2,7) showshow thethe enrichmentenrichment inin LREELREE withwith respectrespect toto HREE.HREE. AsAs alreadyalready discusseddiscussed byby [[29]29] thisthis behaviorbehavior isis controlledcontrolled 3+ byby crystalcrystal chemistrychemistry andand indicateindicate thatthat inin aa calcic calcic garnet garnet the the substitution substitution of of Fe Fe3+ for Al atat thethe [[YY]] sitesite enlargesenlarges the the eightfold eightfold [X ][X site] site and and favors favors the incorporation the incorporation of LREE. of Moreover,LREE. Moreover, the positive the Eupositive anomaly, Eu anomaly, more evident in sample 7 and in the andradite‐rich areas of sample 3, implies the presence of Eu in the reduced form (Eu2+) according with the reaction Fe2+ + Eu3+  Fe3+ + Eu2+ [30] and its fractionation from the HREE. In all samples, total REE content correlates positively with Y indicating, at least at local level, an equilibrium state [31]. Yttrium is plotted in the chondrite‐normalized diagram together with the concentrations of Sr, Zr and of trace elements forming the “first transition series” (i.e., Sc, Ti, V, Cr, Co, Ni, and Zn), arranged according to the atomic number increasing (Figure 7b). The patterns are typically U‐shaped and in all samples Cr, Co, Ni, Zn, and Y are below the C1 values whereas Sc, Ti, V, and Zr show a slight enrichment mainly in grandites. Other measured trace elements (Cs, Ba, Pb, Nb, Hf, Ta, Th, and U) resulted below the detection limits.

Minerals 2019, 9, 728 13 of 18 more evident in sample 7 and in the andradite-rich areas of sample 3, implies the presence of Eu in the reduced form (Eu2+) according with the reaction Fe2+ + Eu3+ Fe3+ + Eu2+ [30] and its fractionation → fromMinerals the 2019 HREE., 9, x FOR PEER REVIEW 14 of 19

Figure 7. REE patterns (a) and multi-element diagram (b) for the analyzed samples. Figure 7. REE patterns (a) and multi‐element diagram (b) for the analyzed samples. In all samples, total REE content correlates positively with Y indicating, at least at local level, an equilibriumThe calculated state [a31 unit]. Yttrium‐cell parameter is plotted from in theXRPD chondrite-normalized analyses, crystallite diagram size and together microstrain with of the the concentrationsfour analyzed samples of Sr, Zr are and reported of trace in elements Table 7. forming The a‐value the “ﬁrst of samples transition 2 and series” 7 are in (i.e., agreement Sc, Ti, V, with Cr, Co,the Ni, end and‐member Zn), arranged andradite, according whereas to in the sample atomic 1 numberthe cell increasingedge results (Figure between7b). the The grossular patterns areand typicallyandradite U-shaped values reported and in all in samples literature Cr, Co,(andradite Ni, Zn, a and = 12.0630(1) Y are below Å, the value C1 valuesfrom [32]; whereas grossular Sc, Ti, a V, = and11.8505(4) Zr show Å, a value slight enrichmentfrom [33]). XRPD mainly pattern in grandites. of sample Other 3 measuredsuggests the trace coexistence elements (Cs,of two Ba, separate Pb, Nb, Hf,garnets, Ta, Th, an and almost U) resulted pure grossular below the (3‐ detection1) associated limits. with an andradite (3‐2). Note the occurrence of a secondThe generation calculated aofunit-cell little light parameter yellow from andradite XRPD‐ analyses,garnets formed crystallite on sizethe andolder microstrain and bigger of thered fourgrossular analyzed as suggested samples are by reported [3]. in Table7. The a-value of samples 2 and 7 are in agreement with the end-memberThe microstructural andradite, whereas features in sampleof garnet 1 the3‐2 cellwere edge not results reported between because the of grossular the low andstatistics andradite of its valuesdiffraction reported peaks in in literature the XRPD (andradite pattern thata = 12.0630(1)do not allow Å, us value to obtain from [reliable32]; grossular microstructurala = 11.8505(4) results. Å, valueThe from mean [33]). XRPDcrystal pattern size is of larger sample for 3 suggests grossular the (3 coexistence‐1) and andradite of two separate (2,7) than garnets, for an the almost most intermediate grossular‐andradite solid solution (1), reflecting the larger coherence of their diffracting domains. Conversely, the microstrain is smaller for grossular (3‐1) and andradite (2,7) whereas the intermediate solid solution shows smaller crystallite sizes and larger strain values.

Minerals 2019, 9, x FOR PEER REVIEW 15 of 19

Table 7. Crystallographic data.

Sample a (Å) Crystal Size (Å) Microstrain 2 and 7 12.0633(3) 1610 0.004 Minerals 2019, 9, 728 14 of 18 1 11.9394(4) 1440 0.055 3‐1 11.8826(3) 1510 0.031 pure grossular (3-1) associated3‐2 with12.0650(3) an andradite ‐ ‐ (3-2). Note the occurrence of a second generation of little light yellow andradite-garnets formed on the older and bigger red grossular as suggested by [3]. The presence of garnets with different morphology and crystal‐chemistry is related to changing of chemical and physical conditions duringTable 7. theCrystallographic calcic metasomatism, data. which represents the main cause of the occurrence of rodingites in this area and/or during the subsequent Alpine deformation and Sample a (Å) Crystal Size (Å) Microstrain metamorphism. The grandite garnets probably were formed in presence of low fO2 fluids (low Adr/Alm ratio) during2 and the 7 serpentinization 12.0633(3) of the host rocks. 1610 On the contrary, 0.004 the smaller and well‐shaped 1 11.9394(4) 1440 0.055 andradites may be the product of an increase of fluid activity (fO2~HM buffer) producing a 3-1 11.8826(3) 1510 0.031 progressive Al mobility3-2 and removal, 12.0650(3) accompanied by -a significant increase - in the iron oxidation (high Adr/Alm ratio) [18,34,35].

4.3. RamanThe microstructural and DRIFT Spectroscopy features of garnet 3-2 were not reported because of the low statistics of its diffraction peaks in the XRPD pattern that do not allow us to obtain reliable microstructural results. –1 –1 TheWe meananalyzed crystal all sizethe faceted is larger garnets for grossular using (3-1) the Raman and andradite spectroscopy (2,7) than from for the100 mostcm intermediateto 1100 cm –1 –1 –1 grossular-andraditeand from 3500 cm to solid 3700 solutioncm ; the (1),range reflecting 100–1100 the cm larger is usually coherence divided of theirfor garnets diffracting in two domains. regions: –1 –1 Conversely,the first below the 400 microstrain cm (internal is smaller vibrations) for grossular and the (3-1) second and above andradite 400 cm (2,7)(external whereas vibration the intermediate region). solidAll the solution spectra shows resulted smaller very crystallitesimilar, so sizes only andthat largerrelative strain to sample values. 9 is shown in Figure 8. –1 TheIn Figure presence 8a, the of garnets vibrations with up di tofferent 320 cm morphologyare attributed and crystal-chemistry to SiO4 tetrahedra is and related divalent to changing cations –1 –1 –1 –1 –1 oftranslations chemical (bands and physical at 177 cm conditions, 241 cm during, 275 cm the) calcicand between metasomatism, 320 cm and which 400 representscm to the librations the main –1 –1 causeof the ofSiO the4 units occurrence (360, Si–O–O of rodingites bending inmotions this area within and /theor duringSiO4 groups). the subsequent Between 400 Alpine cm deformationand 650 cm one observes the O–Si–O bending modes (bands at 494 cm–1, 534 cm–1, 561 cm–1, 619 cm–1) whereas at and metamorphism. The grandite garnets probably were formed in presence of low f O2 fluids –1 –1 –1 –1 (lowfrequencies Adr/Alm higher ratio) than during 800 thecm serpentinizationthe Si–O stretching of the modes host rocks. (bands On at the820 contrary, cm , 878 the cm smaller, 942 cm and, 1001 cm–1) [36–38]. Table 8 presents the main peaks of the grossular and andradite end‐members and well-shaped andradites may be the product of an increase of fluid activity (f O2~HM buffer) producing athose progressive present Alin our mobility samples and (indicated removal, accompaniedas “This work”) by resulted a significant consistent, increase by in comparison, the iron oxidation with a (highsolid solution Adr/Alm of ratio) grossular [18,34 and,35]. andradite.

4.3. RamanTable 8. and Raman DRIFT frequency Spectroscopy (cm–1) of selected peaks of andradite and grossular end members and our results. We analyzed all the faceted garnets using the Raman spectroscopy from 100 cm–1 to 1100 cm–1 and

from 3500Peak cm #1–1 Thisto 3700 Work cm Peak–1; the #2–#3 range This 100–1100 Work Peak cm –1#4 isThis usually Work dividedPeak #5 forThis garnets Work inPeak two #6 regions: This Work Andradite 236 352–370 516 816 874 the ﬁrst below 400 cm240–1 (internal vibrations)370 and the second above534 400 cm–1 (external820 vibration region).878 Grossular 247 373–376 550 827 880 All the spectra resulted very similar, so only that relative to sample 9 is shown in Figure8.

a 3600 370 b 3606

820 878 3572 intensity intensity 534

3636 3663 240 323 772 1001 494 942 177 275 619

200 400 600 800 1000 3500 3550 3600 3650 3700

‐1 ‐1 Wavenumber (cm ) Wavenumber (cm ) Figure 8. Raman spectra of sample 9 in the two considered ranges of frequency: (a) 100–1100 cm–1 and 15 –1 (b) 3500–3700 cm .

–1 In Figure8a, the vibrations up to 320 cm are attributed to SiO4 tetrahedra and divalent cations translations (bands at 177 cm–1, 241 cm–1, 275 cm–1) and between 320 cm–1 and 400 cm–1 to the Minerals 2019, 9, 728 15 of 18

–1 librations of the SiO4 units (360, Si–O–O bending motions within the SiO4 groups). Between 400 cm and 650 cm–1 one observes the O–Si–O bending modes (bands at 494 cm–1, 534 cm–1, 561 cm–1, 619 cm–1) whereas at frequencies higher than 800 cm–1 the Si–O stretching modes (bands at 820 cm–1, 878 cm–1, 942 cm–1, 1001 cm–1)[36–38]. Table8 presents the main peaks of the grossular and andradite end-members and those present in our samples (indicated as “This work”) resulted consistent, by comparison, with a solid solution of grossular and andradite.

Table 8. Raman frequency (cm–1) of selected peaks of andradite and grossular end members and our results.

Minerals 2019, 9Peak, x FOR PEERThis REVIEWPeak This Peak This Peak This Peak 16This of 19 #1 Work #2–#3 Work #4 Work #5 Work #6 Work AndraditeFigure 8. 236Raman spectra of sample352–370 9 in the two considered516 ranges of frequency:816 (a) 100–1100874 cm–1 and 240 370 534 820 878 Grossular(b) 3500–3700247 cm–1. 373–376 550 827 880

Moreover, the analysis of the Raman spectrum in the range from 3500 cm–1 to 3700 cm–1 allowed Moreover, the analysis of the Raman spectrum in the range from 3500 cm–1 to 3700 cm–1 allowed us to determine the presence of OH, that, in grossular garnets and especially in rodingites, may be us to determine the presence of OH, that, in grossular garnets and especially in rodingites, may be present (expressed as weight % in H2O) in a range from 12.8 wt % to less than 0.005 wt % but typically present (expressed as weight % in H O) in a range from 12.8 wt % to less than 0.005 wt % but typically less than 0.3 wt % [39]. In Figure 8b,2 we can observe the internal OH‐stretching modes that derive less than 0.3 wt % [39]. In Figure8b, we can observe the internal OH-stretching modes that derive–1 from the O4H4 groups. The OH‐stretching region consists of two OH bands around 3663 cm–1 and from the–1 O4H4 groups.–1 The OH-stretching region consists of two OH bands around 3663 cm and 3600 cm–1 . The 3663 cm –1 band is connected to vibrations of O4H4 groups that are surrounded by other 3600 cm . The 3663 cm band is connected to vibrations of O4H4 groups that are surrounded by O4H4 groups and the 3600 cm–1 band to O4H4 clusters adjacent to their SiO4 neighbors [39]. The more other O H groups and the 3600 cm–1 band to O H clusters adjacent to their SiO neighbors [39]. The complex4 bands4 at 3606 cm–1 and 3636 cm–1 are4 OH4 ‐stretching modes [40]. The 4band located at 3572 more complex bands at 3606 cm–1 and 3636 cm–1 are OH-stretching modes [40]. The band located at cm–1 may represent hydrogrossular substitution [41,42]. 3572 cm–1 may represent hydrogrossular substitution [41,42]. The infrared spectra between 1100 cm–1 and 450 cm–1 of samples 2 and 7 and of samples 3 and 4 The infrared spectra between 1100 cm–1 and 450 cm–1 of samples 2 and 7 and of samples 3 and 4 are very similar to each other. Spectra of samples 2 and 3 are shown in Figure 9. The sample 2 exhibits are very similar to each other. Spectra of samples 2 and 3 are shown in Figure9. The sample 2 exhibits peaks at 888 cm–1, 830 cm–1 and 815 cm–1, 589 cm–1, 511 cm–1, and 478 cm–1, which are related to the peaks at 888 cm–1, 830 cm–1 and 815 cm–1, 589 cm–1, 511 cm–1, and 478 cm–1, which are related to the vibrational mode of the SiO4 tetrahedra [43–45]. The position of these peaks are in agreement with vibrational mode of the SiO tetrahedra [43–45]. The position of these peaks are in agreement with measurements relative to andradite4 (94%) of [43] and are consistent with our chemical results on this measurements relative to andradite (94%) of [43] and are consistent with our chemical results on this sample (andradite 96%). Instead, the spectra of sample 3, resulted 61% grossular, shows a shift of sample (andradite 96%). Instead, the spectra of sample 3, resulted 61% grossular, shows a shift of these these SiO4 vibrational modes towards higher frequency, i.e., 908 cm–1, 853 cm–1 and 837 cm–1, 615 cm–1, SiO vibrational modes towards higher frequency, i.e., 908 cm–1, 853 cm–1 and 837 cm–1, 615 cm–1, 5384 cm–1, and 468 cm–1, consistent with data reported in [45] for grossular and in [43] for 74% 538 cm–1, and 468 cm–1, consistent with data reported in [45] for grossular and in [43] for 74% grossular. grossular. The change of the energies of these mid‐IR bands is due to difference in the chemical The change of the energies of these mid-IR bands is due to diﬀerence in the chemical composition, as composition, as well known for garnets. well known for garnets.

a b 853 837 888 830 908 815

468

511 538

478 intensity intensity 615

589

1100 1000 900 800 700 600 500 1100 1000 900 800 700 600 500

Wavenumber (cm‐1) Wavenumber (cm‐1)

Figure 9. Infrared absorption spectra of samples 2 (a) and 3 (b). Figure 9. Infrared absorption spectra of samples 2 (a) and 3 (b).

5. Conclusions Ca‐rich garnets are the only documented garnets occurring in the rodingites of Val d’Ala. The orange–red garnets from Testa Ciarva and the light yellow colored andradite (topazolite variety) from Roch Neir I are prized among gemstone collectors. The variation in color make them very attractive and valuable for gemological purposes. However, the translucent diaphaneity and the presence of defects and fractures (well evidenced by the CT) which characterize their singular charm, motivated the scarce consideration on the market for these garnets. The complete characterization we carried out leads to the following conclusions: Chemical composition ranges from grossular‐andradite solid solution to pure andradite. These compositional variations resulted irregular and indicate, together with the different size of the crystals, a growth in partial chemical equilibrium conditions, as expected in polyphasic metamorphic 16

Minerals 2019, 9, 728 16 of 18

5. Conclusions Ca-rich garnets are the only documented garnets occurring in the rodingites of Val d’Ala. The orange–red garnets from Testa Ciarva and the light yellow colored andradite (topazolite variety) from Roch Neir I are prized among gemstone collectors. The variation in color make them very attractive and valuable for gemological purposes. However, the translucent diaphaneity and the presence of defects and fractures (well evidenced by the µCT) which characterize their singular charm, motivated the scarce consideration on the market for these garnets. The complete characterization we carried out leads to the following conclusions: Chemical composition ranges from grossular-andradite solid solution to pure andradite. These compositional variations resulted irregular and indicate, together with the different size of the crystals, a growth in partial chemical equilibrium conditions, as expected in polyphasic metamorphic rocks. The crystallization of both Grs rich and pure Adr garnet suggests a change from internally buffered metamorphic conditions to infiltration processes of oxidation fluids, probably released by the hosting serpentinite, both during oceanization and subduction processes, in agreement with the metamorphic evolution inferred from the literature (Figure2d). Indeed, Val d’Ala metaophiolites recorded low-pressure re-equilibration during Jurassic-Cretaceous oceanization, followed by eclogite- and greenschist-facies imprints during Alpine subduction and successive exhumation. Crystallographic data highlight the coexistence of almost pure andradite and grandite very close to grossular end-member enlarging the variation range resulted from microprobe analyses. The DRIFT spectra result adequate to the chemical compositions. Raman spectra of faceted samples resulted in andradite-grossular solid solution and evidenced the presence of OH but at a very low level. Compositional variation, microstructure, and growth zoning of garnet may be used as a key to understanding the timing and duration of the metamorphic events or the partial chemical equilibrium during the crystallization. In a complex geological context such as that of Val d’Ala, the present data, suggest a growth of garnets during one or more metasomatic/metamorphic events, involving increasing of Ca and variation of Al and Fe mobility due to different influx of hydrothermal fluids and/or different chemical and physical interaction with the host rocks. A forthcoming planned structural and metamorphic analysis supporting a strategic sampling, focused on defining the relative chronology of overlapping tectono-metamorphic imprints, will drive towards a comprehensive interpretation of the geological and petrological processes occurred in Val d’Ala metaophiolites.

Author Contributions: V.D. performed EMPA analyses; N.M. the XRD analysis; L.M. the Synchrotron X-ray computed micro-tomography; M.I.S. is responsible for thin sections analysis and geological outline; A.L. collected LA–ICP–MS data; F.C. characterized the gemological materials and performed the Raman analyses; I.A. performed the DRIFT analyses; V.D., R.B., M.I.S., N.M., and F.C. analyzed the data and wrote the paper. All authors contributed to the final version and gave their approval for submission. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Repossi, E. La Val d’Ala e i suoi minerali. Riv. Sci. Nat. 1919, 10, 89–132. 2. Grill, E. Quarzo, granato, clorite di Val d’Ala. Atti Soc. Ital. Sci. Nat. 1922, 61, 215–240. 3. Grill, E.; Repossi, E. Itinerari Mineralogici. Riv. Sci. Nat. 1942, 33, 96–111. 4. Maletto, G.; Piccoli, G.C. Minerali in Val d’Ala; Ass. Amici del Museo F. Eusebio: Cuneo, Italy, 2014; p. 224, ISBN-10 8890380985, ISBN-13 9788890380983. 5. Coleman, R.G. Low-Temperature Reaction Zones and Alpine Ultramaﬁc Rocks of California, Oregon, and Washington; U.S. Geological Survey Bulletin: Washington, DC, USA, 1967; Volume 1247, pp. 1–49. 6. Dal Piaz, G.V. Le «granatiti» (rodingiti l.s.) nelle serpentine delle Alpi occidentali italiane. Mem. Soc. Geol. Ital. 1967, 6, 267–313. Minerals 2019, 9, 728 17 of 18

7. Austrheim, H.; Prestvik, T. Rodingitization and hydration of the oceanic lithosphere as developed in the Leka ophiolite, north–central Norway. Lithos 2008, 104, 177–198. [CrossRef] 8. Frost, B.R.; Beard, J.S.; McCaig, A.; Condliﬀe, E. The Formation of Micro-Rodingites from IODP Hole U1309D: Key to Understanding the Process of Serpentinization. J. Petrol. 2008, 49, 1579–1588. [CrossRef] 9. Piccardo, G.B.; Messiga, B.; Cimmino, F. Antigoritic serpentinites and rodingites of the Voltri Massif: Some petrological evidences for their evolutive history. Oﬁoliti 1980, 5, 111–114. 10. Li, X.-P.; Rahn, M.; Bucher, K. Metamorphic Processes in Rodingites of the Zermatt-Saas Ophiolites. Int. Geol. Rev. 2004, 46, 28–51. [CrossRef] 11. Li, X.-P.; Rahn, M.; Bucher, K. Eclogite facies metarodingites—Phase relations in the system

SiO2-Al2O3-Fe2O3-FeO-MgO-CaO-CO2-H2O: An example from the Zermatt-Saas ophiolite. J. Metamorph. Geol. 2008, 26, 347–364. [CrossRef] 12. Ferrando, S.; Frezzotti, M.L.; Orione, P.; Conte, R.C.; Compagnoni, R. Late-Alpine rodingitization in the Bellecombe meta-ophiolites (Aosta Valley, Italian Western Alps): Evidence from mineral assemblages and serpentinization-derived H2-bearing brine. Int. Geol. Rev. 2010, 52, 1220–1243. [CrossRef] 13. Zanoni, D.; Rebay, G.; Bernardoni, J.; Spalla, M.I. Using multiscale structural analysis to infer high-/ultrahigh-pressure assemblages in subducted rodingites of the Zermatt-Saas Zone at Valtournanche, Italy. J. Virtual Explor. 2012, 41, 2–30. [CrossRef] 14. Zanoni, D.; Rebay, G.; Spalla, M.I. Ocean floor and subduction record in the Zermatt-Saas rodingites, Valtournanche, Western Alps. J. Metamorph. Geol. 2016, 34, 941–961. [CrossRef] 15. Leardi, L.; Rossetti, P.; Compagnoni, R. Geochemical study of a metamorphic ophiolite sequence from the Val d’Ala di Lanzo (internal Piedmontese Zone, Graian Alps, Italy). Mem. Soc. Geol. Ital. 1984, 29, 93–105. 16. Leardi, L.; Rossetti, P. Caratteri geologici e petrografici delle metaofioliti della Val d’Ala (Valli di Lanzo, Alpi Graie). Boll. Dell’assoc. Min. Subalp. 1985, XXII, 422–441. 17. Sandrone, R.; Leardi, L.; Rossetti, P.; Compagnoni, R. P-T conditions for the eclogitic re-equilibration of the metaophiolite from Val d’Ala (internal Piemontese zone, Western Alps). J. Metamorph. Geol. 1986, 4, 161–178. [CrossRef] 18. Laborda-López, C.; Sánchez-Vizcaíno, V.L.; Marchesi, C.; Gómez-Pugnaire, M.T.; Garrido, C.J.; Jabaloy-Sánchez, A.; Padrón-Navarta, J.A.; Hidas, K. High-P metamorphism of rodingites during serpentinite dehydration (Cerro del Almirez, Southern Spain): Implications for the redox state in subduction zones. J. Metamorph. Geol. 2018, 36, 1141–1173. [CrossRef] 19. Dal Piaz, G.V. Filoni rodingitici e zone di reazione a bassa temperatura al contatto tettonico tra serpentine e rocce incassanti nelle Alpi occidentali italiane. Rend. Soc. Ital. Mineral. Petrol. 1969, 25, 262–315. 20. Mattirolo, E.; Novarese, V.; Franchi, S.; Stella, A. Carta Geologica D’italia Alla Scala 1:100.000; Foglio 55; Servizio Geologico d’Italia: Susa, Italy, 1910. 21. Spear, F.S. Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths; Mineralogical Society of America Special Publication: Washington, DC, USA, 1993; p. 799. 22. Feldkamp, L.A.; Davis, L.C.; Kress, J.W. Practical cone-beam algorithm. J. Opt. Soc. Am. A 1984, 1, 612–619. [CrossRef] 23. Kitchen, M.J.; Pavlov, K.M.; Siu, K.K.W.; Menk, R.H.; Tromba, G.; Lewis, R.A.; Pavlov, K. Analyser-based phase contrast image reconstruction using geometrical optics. Phys. Med. Biol. 2007, 52, 4171–4187. [CrossRef] 24. Rietveld, H.M. A profile refinement method for nuclear and magnetic structures. J. Appl. Crystallogr. 1969, 2, 65–71. [CrossRef] 25. Toby, B.H.; Von Dreele, R.B. GSAS-II: The genesis of a modern open-source all purpose crystallography software package. J. Appl. Crystallogr. 2013, 46, 544–549. [CrossRef] 26. Johnson, M.L.; Boehm, E.; Krupp, H.; Zang, J.W.; Kammerling, R.C. Gem-Quality Grossular-Andradite: A New Garnet from Mali. Gems Gemol. 1995, 31, 152–166. [CrossRef] 27. Droop, G.T.R. A general equation for estimating Fe3+concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria. Mineral. Mag. 1987, 51, 431–435. [CrossRef] 28. Anders, E.; Grevesse, N. Abundances of the elements: Meteoritic and solar. Geochim. Cosmochim. Acta 1989, 53, 197–214. [CrossRef] 29. Bocchio, R.; Adamo, I.; Diella, V. The profile of trace elements, including the REE, in gem-quality green andradite from classic localities. Can. Miner. 2010, 48, 1205–1216. [CrossRef] Minerals 2019, 9, 728 18 of 18

30. Sverjensky, D.A. Europium redox equilibria in aqueous solution. Earth Planet. Sci. Lett. 1984, 67, 70–78. [CrossRef] 31. Park, C.; Song, Y.; Kang, I.-M.; Shim, J.; Chung, D.; Park, C.-S. Metasomatic changes during periodic fluid flux recorded in grandite garnet from the Weondong W-skarn deposit, South Korea. Chem. Geol. 2017, 451, 135–153. [CrossRef] 32. Armbruster, T.; Geiger, C.A. Andradite crystal chemistry, dynamic X-site disorder and structural strain in silicate garnets. Eur. J. Miner. 1993, 5, 59–72. [CrossRef] 33. Rodehorst, U.; Geiger, C.A.; Armbruster, T. The crystal structures of grossular and spessartine between 100 and 600 K and the crystal chemistry of grossular-spessartine solid solutions. Am. Miner. 2002, 87, 542–549. [CrossRef] 34. Basso, R.; Cimmino, F.; Messiga, B. Crystal chemical and petrological study of hydrogarnets from a Fe-gabbro metarodingites (Gruppo di Voltri, Western Liguria, Italy). Neues Jahrb. Mineral. Abh. 1984, 150, 247–258. 35. Dubińska,E.; Bylina, P.; Kozłowski, A. Garnets from Lower Silesia rodingites: Constraints from their chemistry. Mineral. Soc. Pol. Spec. Pap. 2004, 24, 135–139. 36. Kolesov, B.A.; Geiger, C.A. Raman scattering in silicate garnets: An investigation of their resonance intensities. J. Raman Spectrosc. 1997, 28, 659–662. [CrossRef] 37. Kolesov, B.A.; Geiger, C.A. Raman spectra of silicate garnets. Phys. Chem. Miner. 1998, 25, 142–151. [CrossRef] 38. Bersani, D.; Ando, S.; Vignola, P.E.; Moltifiori, G.; Marino, I.-G.; Lottici, P.P.; Diella, V. Micro-Raman spectroscopy as a routine tool for garnet analysis. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2009, 73, 484–491. [CrossRef] 39. Rossmann, G.R.; Aines, R.D. The hydrous component in garnets: Grossular–hydrogrossular. Am. Mineral. 1991, 76, 1153–1164. 40. Thomas, S.-M.; Davidson, P.; Reichart, P.; Koch-Müller, M.; Dollinger, G. Application of Raman spectroscopy to quantify trace water concentrations in glasses and garnets. Am. Mineral. 2008, 93, 1550–1557. [CrossRef] 41. Lager, G.A.; Armbruster, T.; Rotella, F.J.; Rossmann, G.R. OH substitution in garnets: X-ray and neutron diffraction, infrared and geometric modeling studies. Am. Mineral. 1989, 74, 840–851. 42. Andrut, M.; Wildner, M.; Beran, A. The crystal chemistry of birefringent natural uvarovites. Part IV. OH defect incorporation mechanisms in non-cubic garnets derived from polarized IR spectroscopy. Eur. J. Miner. 2002, 14, 1019–1026. [CrossRef] 43. Moore, R.K.; White, W.B.; Long, T.V. Vibrational spectra of the common silicates: I. The garnets. Am. Mineral. 1971, 56, 54–71. 44. Hofmeister, A.; Chopelas, A. Vibrational spectroscopy of end-member silicate garnets. Phys. Chem. Miner. 1991, 17, 503–526. [CrossRef] 45. Bosenick, A.; Geiger, C.A.; Schaller, T.; Sebald, A. A 29 Si MAS NMR and IR spectroscopic investigation of synthetic pyrope-grossular garnet solid solutions. Am. Mineral. 1995, 80, 691–704. [CrossRef]

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