Carlosturanite, a New Asbestiform Rock-Forming Silicate from Val Yaraita, Italy

AmericanMineralogist, Volwp 70,pages 767-772, 1985

Carlosturanite,a new asbestiformrock-forming silicate from Val Yaraita, Italy

Rornnro Colap,lcNoNr Dipartimento di Scienze ilella Terra Uniuersitd della Calabria, Castiglione scalo,87030 Cosenza,Italy

Grov,tNNt FnnnlRts Dipartimento di Scienze della Terra Uniuersitd di Torino, uia S. Massimo 22, 10123 Torino, Italy

,C,NDMARCSLIO MELLINI C.N.R., C.S. Geolo gia Strutturale Dinamic a dell Appennino uia S. Maria 53,56100 Pisa,Italy

Abctract Carlosturaniteis a new rock-forming silicate occurring in a network of veins crosscutting the antigorite serpentiniteof Sampeyrein the Monviso ophiolite, Italy. It is light-brown, asbestiform,and the [010] fibers are paralleled by fibrous diopside and chrysotile. The mineral is monoclinicCm, with a : 36.70,b :9.41, c :7.2914, f : 101.1".The strongest lines in the X-ray powder diffraction pattern are: 18.02(25X200),7.1(100X001,201), 3.595(45X10.00,W2), 3.397(55t202), 256440X8O2),2.280(35X14.01,16.01). Similar refractive indicesare measuredalong (1.605)and across(1.600) the fiber axis. Carlosturanitedehydrates upon heating, forming chrysotile and hematite (,100.C)and finally forsterite (770"C).The infrared pattern shows absorption bands due to hydroxyl anions and silicate tetrahedra. Chemicaldata lead to the empirical chemicalformula

(Mgrr.rrFe2r+rrTif.lnMnfr.!rCr3.12h41.54(Si22.e2Alo.81h2t.rcHrz.srorzu (D""r" : 2.606, Dob" : 2.63glcmt) or, ideally and according to a structural model, MztffrzOze(OH)41(OH)3o'}{2O(Z:2). Carlosturaniteor carlosturanite-likephases may be expectedto develop in serpentinitecompositions under low grade matamorphic conditions.

Introduction Names. The holotype material is kept at the Museo Re- Specimensof the new mineral were fust collected on the gionaledelle Scienze(Torino). working face of a chrysotile-asbestosmining prospect,lo- catedsome 5 km westof Sampeyre(Val Varait4 Piedmont, Occurrence Italy). The mine is situated in a metamorphic serpentinite Carlosturaniteis quite common in the Sampeyreserpen- belongingto a southernportion of the Monviso ophiolite, tinite, where it occursover an area of a few squarekilome- which is part of the internal Piemonte-Liguria Mesozoic ters. The zone underwenta polyphasemetamorphic evolu- ophiolitic belt, in the pennidicdomain o[ the WesternAlps. tion characterizedby an Early Alpine event under greens- The new mineral strongly resemblesfibrous serpentine, chist facies conditions (Hunziker, 1974). Structural and for which it can be mistakenin the field; it has commonly mineralogical relics indicate that the antigoritic serpen- been called metaxite or metaxitic serpentine (Zucchetti, tinite derivesfrom an upper-mantlespinel lherzolite partly 1968)or xylotile, i.e.,the samenames used for balangeroite, re-equilibratedunder the plagioclaseperidotite facies,like another macroscopicallysimilar fibrous silicate from Ba- most serpentinitesof the ophiolitic belt of the WcsternAlps langero (Lanzo Valley, Piedmont) described by Com- (Compagnoniet al. 1985). pagnoniet al. (1983). Carlosturanitedevelops in a closenetwork ofveins (from The name carlosturanite is after Carlo Sturani (1933- 0.1 mm to severalcentimeters thick) randomly crosscutting 1976),Professor of Geology at the University of Torino, the serpentinite(Fig. 1). Usually it occurs together with whose untimely death occurred in a fatal accident during chrysotile, diopside, and opaque ore minerals (magnetite field work. The name and the specieshave been approved and native NiFe alloys); locally, clinohumite, perovskite, by the I.M.A. Commissionon New Minerals and Mineral and a green uvarovite garnet are also found. Frequently, 0003-{04x/8s/0708-o767$02.00 767 768 COMPAGNONI ET AL.: CARLOSTURANITE

ed determination of the hardness.Parallel intergrowth between [010] carlosturanite, t0011 diopside (Fig. 2), and [100] chrysotilefibers occursin extremelyvariable degtees from macroscopicto TEM scale;at the latter scalebrucite occurs in the presenceof abundant chrysotile (Fig. 3). Single fibers of carlosturanite are seldom larger than 0.2 pm. Very good {001} cleavageand an approximate {010} fracture ate observed.A densityof 2.63(2)g/cm3 was determined by the heavyliquid method. Optical measurementsare very dillicult and imprecise.In thin section carlosturanite is transparent and pleochroic with orange brown and pale orange brown parallel and perpendicular to [010], respectively.The extinction is parallel and the elongationpositive. Because ofthe texture, only an average refractive index, 1.600(5),has becn ob- servedperpendicular to [101]; 1.605(5)has beenmeasured along [010], Birefringenceis low (first order orange) to very low (grey), though greyish-blue anomalous inter- fsrenc€colors are locally shown by sectionscut parallel to the fiber axis. Interferencefigures, performed on fiber bundles cut perpendicularto the fiber axis, are definitely posi- tive, but may appeareither pseudo-uniaxialor biaxial with small to moderateoptic axial angle.Microscopically, carlosturanite is very similar to balangeroite,from which it may be distinguished by lower refractive indices (n : 1.68 in balangeroite)and by weakerpleochroism (a : pale yellow- ish red-brown and,y: reddishbrown in balangeroite). Only the powder diffraction pattern and the repetition period along the fiber axis could be obtained by X-ray diffraction. The b-rotation photographs(CuKa radiation) show strong zero and third layer lines, all the other layer lines being definitely weak (Fig. 4); such a pattern can be Fig. 1. Deformedvein (upper) and bundles(lower) of carlostu- compared with that of antigorite around b (Ar$a, L946) ranitecut paralleland perpendicular to the fiberaxis, respectively c (Compagnoni et al', 1983). (planepolarized light). and of balangeoitearound Superpositionof 5.2A layer lines of chrysotile occurs with some samples.Only continuous lines are observedon the Weissenbergphotographs, indicating complete rotational the carlosturanite-bearingveins appear to have been de- disorder around the elongation direction. Selectedarea formed andlor reactivatedseveral times, resulting in a com- electron diffraction patterns indicated C2fm, Cm, ot C2 plex structural and mineralogicaltexture. On the one hand symmetry(Mellini et al., 1985)and supplementedthe a, c, these veins appear to be crosscut by the later mono- and p starting valuesused to index the powder diffraction mineralic veins of antigorite, chrysotile,brucite, and mag- pattern (Table 1); a least-squaresunit cell refinementbased nesite;on the other hand they seemto reactivateor cross- on the powder data gave a:36.7V3), b:9.41(2), c: cut earlier veins of antigorite, of chrysotile and of olivine 7.2916)4,and B :101.1(1f. Becauseof the fibrousnature + clinohumite + diopside+ opaques.Microscopic obser- of the material,preferred orientation may affectthe powder vations indicate that carlosturanite,as well as the other diffraction patterns. In particular, most reflections with serpentine minerals, is frequently replaced by brucite. k # 0 have been observed only with Guinier-Lenn6 TransmissionElectron Microscope(TEM) images,showing camera; thesereflections are starred in Table 1. Undoubt- brucite crystalsapparently pseudomorphic after singlestu- edly, the unit cell dimensionsand the distribution of the ranite fibers and embayedby abundant chrysotile,suggest strongestreflections recall the serpentineminerals (Whitta- the following breakdown reaction: carlosturanite+ bru- ker and Zussman,1956), but the overall powder pattern is cite * chrysotile. definitely characteristic for carlosturanite. In particular, supplementarylower angle reflections (18.02 and 9'01A) Physical and crystallographic properties occur in the new mineral. Carlosturanite is light-brown, flexible, and develops Thermal and infrared study [010] fibers severalcentimeters long, commonly gathered in folded bundles.The streak is whitish. and the luster is The weight loss at 1000oC,as determinedby Thermogra- vitreous-pearly.The fibrous nature of the mineral prevent- vimetric Analysis (TGA) (Fie. 5), is 16.85%.Comparison COM P AGNON I ET AL.: C ARLOSTURAN I'r E

Fig. 2. TEM imageof parallelintergrowth of diopside(di) and carlosturanite(cst). The opaqueinclusion (m) is chromianmagnetite.

with Differential Thermogravimetric Analysis (DTG) re- just above room temperature,suggests that H2O may be veals that the weight loss begins at rtO'C and continues presentas well. quite smoothly with minor flexes at 75"C (1%), 105'C (1.8%) and 195"C (2.5%). After further loss to 5.6Voat 380'C, a smoother slope begins,and then a step (600'C) Compositional data brings the weight loss to l3Vo at 750'C; the last part of the Table 2 reports the average results of fifteen electron curve has a flex at 900'C- microprobe analyses obtained by wavelength dispersive A continuousX-ray powder pattern recordedfrom 20 to analysis on a fully automated ARL-SEMQ instrument, 1100'C (20'C/hour) showsa practically constantpattern of using olivine (Si, Mg, Fe), jadeite (Al), ilmenite (Ti), Mn- carlosturaniteup to about 400'C whereit vanishes,leaving hortonolite (Mn), and Cr-garnet (Cr) as standards;H2O is at least two other phases:a serpentinephase, which disap- from TGA. No other element with Z > 11 was shown by pearsat about 500"C,and hematite,which is still presentat TEM/EDS analyses.Some variation, particularly for the 1100"C. The latter is the only crystalline phase evident minor constituents,was detectedwith the microprobe; this from 5fi) to 700'C, when forsteriteappears. Minor shifts of variation is probably due to submicroscopicintergrowths lines towards higher anglescan be detectedaround 200'C. or to the heterogeneitiesdiscussed in the companionpaper Keeping in mind the different rates of heating, the two by Mellini et al. (1985).Determination of Fe3+ was not different high-temperatureexperiments can be regardedas attempted.On the basis of 126 oxygen atoms per unit cell consistentwith oneanother. (see below) the following empirical formula was derived: The infrared spectrum(Fig. 6) displaysabsorption bands (Mgrr..,rF el+rrTif.inMnfr .irCr3.1 2L4 1. s4(S i 22j2Alo.E rh:x..7 3 due to the presenceof hydroxyl anions and silicate tetra- Hrr.rrOrzu.From this formula D""to: 2.606g/cm3 is ob- hedra; as a whole, it is similar to that of chrysotile,with tained with F.W. : 3873.45.The Gladstone-Dalerelation- major differencesin the low frequency side of the SiO. ship givesK:0.229 comparedwith 0.230calculated from 1. stretchingvibrations between950 and 850 cm- The wide the empirical formula with Mandarino's (1976) refracti- rangeobserved for the temperatureofdehydration, starting vities. 770 COMPAGNONI ET AL.: CARLOSTURAN I'l E

TEM imageshowing association of fibrous carlosturanite(cst), brucite (b) and chrysotile(ch) as seenalong the fiber axis.

Fig. 4. b-rotation photograph of carlosturanite(Ni-filtered CuKa radiation).The 5.2A layer lines of intergrown chrysotileare shown by arrows. COMPAGNONI ET AL.: CARLOSTURANITE

Table l. X-ray powder data for carlosturanitg obtained by powder diffractomcter and Guinier-Lenn€ camera (CuKa radiation). Intensities (/o) on relative scale, observed (dj and calculated (d) interplanar spacings with their assiped tr&l indices are reported. Starred do values refer to reflections that arc observed only in Guinier-Lenn6 patterns.

ro do(R) dc(?) hkt

25 14.02 18,01 200 5 9.O1 9,OO 400 100 7.77 7.!6i7.14 OO1;2O1 10 6.28 6.25;6,21 2O1;401 5.76i5.72t5,50 111;510;111 5. 14; 5.11 401;601 5 4,22 4,22i4.19 601;8O1 10 4.71* o20 Fig. 5. TGA (top)and DTG (bottom)curves in air; 5.25mg of 20 3.637 202 carlosturaniteat a rate of 20"C/min. 45 3.595 3.601;3.578 10.OO;002 t0 3.513 3.518;3.498 aO1; 10.01 55 3.397 3.387 202 (a further 1.9%) without collapse of the structure (cf. the 1s 3.096 3.106 802 5 2.944 3 . OO1; 2 . 994 ; 2 . 979 12.OO; 10. 01 ; 12. 01 shrinkageof the unit cell at about 2fi)'9; (2) breaking of 5 2,849 2.844i2,8A3 331;602 the inter-strip octahedrawith loss of (OH)- and releaseof 5 2.818 2.824 10.02 the correspondingcations; (3) formation of hematite and, 5 2.674* 2.678 730 15 2.586 2.584 14. 01 possibly, of other undetectedoxides; (4) condensationof 40 2.562 2.577 802 the strips to form a serp€ntinephase; (5) collapse of this 20 2.539 t2.02 phase with formation of forsterite. Steps (2), (3) and (4) are 10 2,425 2 ,424 i2 . A24i2,426 dsr ; loe; 2oa 10 2.308 2 .308 203 simultaneous.The exact balanceof the weight loss in the 15 2.293 2.294 803 steps(2) and (3) cannot be determined theoretically, in view 35 2.280 2,285i2.276 14.01 ; 16.01 of the unknown cation content of the broken octahedra 5 2. t-O1* 2. 1O9;2. 101 12.02; 15.11 (seebelow). In any case,the weight loss at the end of step 10 2.065* 2 .067 i2.065 L7,1-Oi823 10 1.9373 1 .9452; 1 .9280 14.03;932 (3) cannot exceedthe corresponding TGA value measured 15 1-.9223 L.9272tL,9217 l-4.02i333 at the temp€rature of the formation of hematite. 5 1.9030 1.9096 15.30 The appearanceof hematite with serpentine means that 5 1.4170 I .8147; 1 .8123 2O4i604 15 1.709a* I.7LL7 i1-.708L 18.21;10.23 iron is released at this stage, presumably together with t 5 1.6030 )-.6024 14.03 other cations: in fact, the 2.2 Fe atoms of the empirical 20 1.5679* 1 .5683 060 formula arc not suffrcient to fill th€ six inter-strip octahedra 5 1,3995 1.4005:1.3968 il. os;2a-.os 5 1.3671 1,3671;1.3675 17.osiI7,za per unit cell that must be broken to form serp€ntine (cf. 5 1.2435 1,2454;1.2813 16.04;463 5 L.2790 1.2768i1.2790 di.oail6.az Table 2. Electronmicroprobe analysis of carlostiranite.HrO da tcrminedbv TGA.

Crystal chemisfy

Carlosturanite a, b, and c lattice parameters closely cor- Mco 36.7 - 4r.3 39.28 respond to 7a, b, and c of the fundamental serpentine or- Feo 3,2 - 5.8 4.O3 thorhombic oell. On the basisof this relationship,the other TiO2 r.0 - 4.1 2.24 described properties, and further high-resolution TEM re- rho 0.5 - 1.2 0.72 sults, a structural model is proposed by Mellini et al. - (1985). It is based on the serpentinestructur€ with the C.2O3 0.2 O.3 o.24 - tetrahedral sheet split into strips consisting of [010] triple sio2 31.9 37.2 35.53 silicate chains which result from introduction of ordered AlzO3 1.O - r.3 1.07 vacancies.According to this model, the empirical formula - 16.85 "2o of carlosturanite can be written (Mg, Fe, Ti, Mn, Cr, Total 99.96 tl)zr(Sr, Al)12O28(OH)nl(OH)ro.H2O with two formula per units unit cell; bracketsenclose the silicate polyanion l. Min/nmx Z utejAhtA 6otL 15 In light of this formula, the thermal behavior of carlostura- e2ec.ttton nhnopu be anoLAAu, nite can be interpretedaccording to the following steps:(1) 2. Avetnge Z ute,Lght. loss of H2O ( - lYo) and of the (OH)- in the silicate strip COMPAGNONI ET AL.: CARLOSTURANITE

^80 z I Hoo t,z tr,$ t z EN

Fig. 6. Infrared spectrumofcarlosturanite (KBr disk).

Fig. 3 in Mellini et d., 1985).Since the cations other tban attention to the fibrous mineral and supplied us with the first Mg can have ionization numbers higher than 2 and their specimen.Professors R. Rinaldi and A. Alberti (Istituto di Miner- performed sum is smaller than 6, a plausible hypothesisis that they alogia e Petrografia dell'Universitd di Modena) kindly and TGAIDTG analyses,respectively' are located in the inter-strip octahedralsites, where some electron microprobe vacanciesalso should be presentto compensatethe highly References chargedcations. Aruja, E. (1945)An X-ray study of the crystal-structureof antigor- Conclusions itc. Mineralogical Magazine, 27, 65_74. Compagnoni,R., Ferraris,G. and Fiora, L. (1983)Balangeroite, a Carlosturanite, a major constituent of the serpentinite new fibrous silicate related to gageite from Balangero, Italy. from Sampeyre,is found to be closelyrelated to the serpen- American Mineralogist, 68, 214-219. tine minerals in its physical and chemical properties and Compagnoni,R., Piccardo,G. 8., and Sandrone,R. (1985)Ophi' probable formation conditions and phaseequilibria. It can olites from the Western Alps and the Apennines. In N. Bogda- be defined as a Si-poor, highly-hydrated, serpentinelike nov, Ed., Ophiolites of Contincnts and Comparable Oceanic phase.Most probably, carlosturanite,or carlosturanite-like Floor Rocks. Elsevier,in press. D. Dana. John phases,are not rare and may be found in many similarly Dana, E. S. (1914)The Systemof Mineralogy of J' matamorphosedserpentinites. In fact, a searchof old litera- Wiley, New York. Hintze, C. (1897) Handbuch der Mincralogie. Verlag Von Veit' ture data on serpentine(Hintze, 1897;Darna,1914) shows I'eipzrg. HrO-rich analysesthat recall severalcases of Si-poor and Hunziker, J. C. (1974) Rb-Sr and K-Ar age determinations and "species the composition of carlosturanite.In some cases the alpine tectonic history of the Western Alps. Memorie names"such as hydrophite and enophite(Dana, 1914)were dell'Istituto di Geologia e Mineralogia dell'Universitti di proposed as well. However, identification of such oc- Padova,3l,l-54. cuffencesas carlosturaniteis not possible,because defini- Mandarino, J. A. (1976) Gladstone-Dale relationship-Part I: tive diffraction data are not available. Derivation of new constants. Canadian Mineralogist, 14, 498- Textural evidencesuggests that carlosturanite,chrysotile, 502. and fibrous diopside grew together,forming a stable min- Mellini, M., Ferraris G., and Compagnoni,R. (1985)Carlostura- polysomatic seriesincluding ser- eral assemblage.The stability field of diopside + chrysotile nite: HRTEM evidenceof a pentine.American Mineralogist, 70, 773-781. is consideredto lie approximately between250 and 3fi)'C Trommsdorff, V. (1983) Petrologic aspectsof serpentinitemeta- : Pu"o: 2 kbar (Trommsdorfl 1983);therefore, at Pxn.16 morphism. Rendiconti della Societri Italiana di Mineralogia e may be similar very low grade metamorphic conditions Petrologia,38, 54F559. suggested also for cailosturanite. Within the diop- Whittaker, E. J. W., and Zussman,J. (1956)The characterization side + chrysotilestability field, the growth ofcarlosturanite of serpentine minerals by X-ray ditrraction. Mineralogical possibly is controlled by chemical parameters. Among Magazine,31,197-126. them, a significant role may be played by Ti. In fact, this Zucchetti, S. (1968) Mineralizzazioni nichelifere a ferro-nickel e elementis a pervasiveconstituent of carlosturanite,while solfuri nel giacimento asbestifero di Sampeyre (Cuneo) ed in systematicallyabsent from the associatedminerals (Mellini altre serpentinitialpine. Bollettino della AssociazioncMineraria et al., 1985). Subalpina,5,10G120. Acknowledgments The authorsare gratefulto ProfessorE. Occella(Dipartimento Manuscript receioeil, MaY 4, 1984; di Georisorsee Territorio, Politecnicodi Torino),who calledour acceptedfor publicatiory January 28, 1985.