“Horsetail” Inclusions in the Ural Demantoids: Growth Formations

Article “Horsetail” Inclusions in the Ural Demantoids: Growth Formations

Aleksander Yurevich Kissin *, Valery Vasilevich Murzin and Elizaveta Sergeevna Karaseva

A.N. Zavaritsky Institute of Geology and Geochemistry, Ural Branch of Russian Academy of Scences, Vonsovskogo str. 15, 620016 Ekaterinburg, Russia; [email protected] (V.V.M.); [email protected] (E.S.K.) * Correspondence: [email protected]

Abstract: The term “demantoid”, first proposed in 1856 by the famous Finnish mineralogist Nils von Nordensheld, refers to a highly dispersed yellow-green mineral from the Central Urals placers. In 1874, it was found to be a gem variety of andradite garnet. “Horsetail” inclusions are considered a sign of the Ural type demantoid. Although these inclusions are large (visible to the naked eye), their diagnostics remains debatable: some researchers attribute them to byssolite (amphibole-asbestos), others consider them chrysotile. We investigated the horsetail inclusions in the Ural demantoids through various methods: optical microscopy, scanning electron microscopy (SEM), Raman spec- trometry, X-ray powder diffraction, and thermal analysis. In most cases, “horsetail” inclusions in the Ural demantoid were represented by hollow channels and only the outcrops, on the demantoid surface, were occasionally filled with serpentine (established by SEM); in one case, magnetite was observed. Hollow canals were usually collected not in bundles, such as a “horsetail”, but in fans, sometimes curved into cones. The structure of the grains was spheroidal, sectorial, and sometimes had induction surfaces, which, to the periphery of the grain, were replaced by tubular channels assembled in a fan. The specifics of the growth of the “horsetail” inclusions of the demantoid grains Citation: Kissin, A.Y.; Murzin, V.V.; can be explained by the decompression conditions that arose when the ultrabasites (a crust-mantle Karaseva, E.S. “Horsetail” Inclusions mixture) were squeezed upwards during collision. in the Ural Demantoids: Growth Formations. Minerals 2021, 11, 825. Keywords: gemstones; demantoid; inclusions; the Urals https://doi.org/10.3390/min11080825

Academic Editors: Franca Caucia and Luigi Marinoni 1. Introduction Demantoid is a jewelry variety of andradite. Its chemical formula is Ca Fe Si O , Received: 9 July 2021 3 2 3 12 and it often contains admixtures of Cr, Ti, V, Al, and Mg. The refractive index of demantoid Accepted: 27 July 2021 Published: 29 July 2021 is 1.887, the dispersion is 0.057, the hardness is 6.5, and it is inert to UV-rays [1]. The demantoid was first discovered in gold-bearing alluvial placers of the Bobrovka

Publisher’s Note: MDPI stays neutral river, 100 km NNW from Yekaterinburg (Figure1). It was named in 1856 by the Finnish with regard to jurisdictional claims in mineralogist Nils von Nordensheld for its strong dispersion (0.057), comparable with that published maps and institutional affil- of diamonds (0.044). The mineral, however, was not identified. In 1874, demantoids were iations. discovered in gold-bearing placers near the Bobrovka river (a widespread name for small rivers in Russia) near the Poldnevaya village, 80 km south of Ekaterinburg (see Figure1), by V.G. and A.V. Kalugin. In 1879, after the study of the chemical composition of the demantoid by mining engineer A.A. Loesch, the mineral was identified as the andradite garnet. The primary sources of the demantoid were identified in the bedrock of placers in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the upper reaches of these rivers at the end of the 19th century. Political events in Russia at This article is an open access article the beginning of the 20th century stopped the development of the demantoid deposits in distributed under the terms and the Urals for many years. conditions of the Creative Commons In 1978–1979, one of the authors of this study, A.K., conducted a study of the Pold- Attribution (CC BY) license (https:// nevskoye demantoid deposit. This served as the beginning of the systematic prospecting for creativecommons.org/licenses/by/ the demantoid in the Middle Urals, which resulted in the discovery of the Korkodinskoye 4.0/). deposit. Later, the mineralization of the demantoid was established in the Verkhneivinskiy

Minerals 2021, 11, 825. https://doi.org/10.3390/min11080825 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 825 2 of 13

ultrabasic massif (the Central Urals, 50 km north-west of Ekaterinburg) [2], in the Polar Minerals 2021, 11, 825 Urals [3], on the Koryak highlands, and on the Kamchatka [4]. Demantoids are also known 2 of 13 to be present in Italy [5,6], Azerbaijan [7], Iran [8], Pakistan [9], the United States of America, China [10], Slovakia [11], Namibia [12], and Madagascar [13,14].

FigureFigure 1. Geographic 1. Geographic position position of the demantoid of the deposits demantoid in the Central deposits Urals. in the Central Urals.

At present, the gem demantoids related to (1) the massif of ultrabasic rocks (the Urals type) andIn (2)197 the8– skarns1979, (theone Namibian of the authors type) are recognized.of this study, The Ural A.K. demantoids, conducted are a study of the Pold- characterizednevskoye bydemantoid horsetail inclusions, deposit. which This are hair-likeserved inclusions as the beginning that diverge in of bunches the systematic prospecting for the demantoid in the Middle Urals, which resulted in the discovery of the Korkodinskoye deposit. Later, the mineralization of the demantoid was established in the Verkhneivinskiy ultrabasic massif (the Central Urals, 50 km north-west of Ekaterinburg) [2], in the Polar Urals [3], on the Koryak highlands, and on the Kamchatka [4]. Demantoids are also known to be present in Italy [5,6], Azerbaijan [7], Iran [8], Pakistan [9], the United States of America, China [10], Slovakia [11], Namibia [12], and Madagascar [13,14]. At present, the gem demantoids related to (1) the massif of ultrabasic rocks (the Urals type) and (2) the skarns (the Namibian type) are recognized. The Ural demantoids are characterized by horsetail inclusions, which are hair-like inclusions that diverge in bunches from the central part of the grain to its periphery (Figure 2). These inclusions became the key markers for identifying the demantoid and determining its geographical origin [15–17]. Horsetail inclusions were first discovered in the Ural demantoids, as these inclusions are: (1) always present, (2) numerous (the gem can completely lose its transparency), (3) often large and easily detected by the naked eye, and (4) sometimes colored in ochre and clearly visible. In spite of this, there is no consensus among researchers as to the origin or composition of these inclusions. Initially it was assumed that they were byssolite (actinolite-asbestos) and, under this name, they were mentioned in geological

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from the central part of the grain to its periphery (Figure2). These inclusions became the key markers for identifying the demantoid and determining its geographical origin [15–17]. Horsetail inclusions were ﬁrst discovered in the Ural demantoids, as these inclusions are: (1) always present, (2) numerous (the gem can completely lose its transparency), (3) often large Minerals 2021, 11, 825 and easily detected by the naked eye, and (4) sometimes colored in ochre and clearly visible.3 of 13

In spite of this, there is no consensus among researchers as to the origin or composition of these inclusions. Initially it was assumed that they were byssolite (actinolite-asbestos) and, underreports this and name, articles they [3,15,18 were mentioned]. In 1992, at in the geological International reports Gemological and articles Conference [3,15,18]. In (Paris) 1992,, at the International Gemological Conference (Paris), Gübelin (Switzerland) reported that Gübelin (Switzerland) reported that he managed to analyze these inclusions in a deman- he managed to analyze these inclusions in a demantoid from Val Malenco (Italy) by the toid from Val Malenco (Italy) by the X-ray diffraction method, and determined their chem- X-ray diffraction method, and determined their chemical composition to be chrysotile (a ical composition to be chrysotile (a serpentine group mineral) [19]. These results were serpentine group mineral) [19]. These results were conﬁrmed by microprobe analyses [20]. confirmed by microprobe analyses [20]. Chrysotile was also detected in the demantoid Chrysotile was also detected in the demantoid from Balochistan (Pakistan) [21]. from Balochistan (Pakistan) [21].

FigureFigure 2.2. “Horsetail”“Horsetail” inclusions in the demantoid demantoid from from the the Bobrovka Bobrovka riv riverer placer placer (Elizavetinskiy (Elizavetinskiy village, the Central Urals); image width 4 mm. Photo by E.S. Karaseva. village, the Central Urals); image width 4 mm. Photo by E.S. Karaseva.

AtAt thethe beginningbeginning ofof thethe 1990s,1990s, twotwo ofof thethe authorsauthors (A.K.(A.K. andand V.M.)V.M.) studiedstudied thethe “horse-“horsetail”tail” inclusionsinclusions of Ural demantoids and and came came to to the the conclusion conclusion that that they they are are tubular tubular in- inclusionsclusions [22 [22].].

2.2. GeologicalGeological SettingSetting AllAll finds finds of of thethe demantoiddemantoid in in thethe CentralCentral Ural Ural werewere confined confined to to the the ultrabasite ultrabasite massifs massifs ofof thethe MainMain UralsUrals andand thetheSerovsko-Maukskogo Serovsko-Maukskogo faults:faults: thethe NizhnetagilskiyNizhnetagilskiy massifmassif (Eliza-(Eliza- vetinskoyevetinskoye deposit), deposit), the the Verkhneivinskiy Verkhneivinskiy and and Korkodinskiy Korkodinskiy massifs massifs (the Poldnevskoye(the Poldnevskoye and Korkodinskoyeand Korkodinskoye deposits), deposits), and the and Ufaleiskiy the Ufaleiski massif.y massif. These The massifse massif structures structures are composed are com- ofposed dunites, of d clinopyroxenites,unites, clinopyroxenites and, more, and, rarely, more harzburgites rarely, harzburgites and, very and, rarely, very verlites rarely,and ver- websterites.lites and websterites. The rocks The are largelyrocks are serpentinized largely serpentinized and deformed. and deformed. VeinsVeins with with demantoiddemantoid mineralizationmineralization are rare rare:: a alonglong th thee strike, strike, they they are are traced traced for for 2– 2–33 m m and and are are completely completely wedged wedged out out,, while while along along the the dip, dip, they they are are traced traced to a to depth a depth of 2 of0– 20–3030 m (the m (the depth depth of the of thePoldnevskoye Poldnevskoye and and Korkodinskoye Korkodinskoye deposits). deposits). It is likely It is likely that these that thesedeposits deposits take the take form the formof ore of pillars ore pillars [23]. [23]. VeinVein serpentine serpentine (antigorite, (antigorite, clinochrysotile, clinochrysotile, and and lizardite), lizardite), carbonates carbonates (calcite, (calcite, dolomite, dolo- magnesite,mite, magnesite, and aragonite), and aragonite), magnetite, magnetite, brucite, brucite and, sometimes,, and, sometimes native, copper,native copper, copper andcop- nickelper and sulfides, nickel sulfides, cuprite, cuprite and other, and minerals other mineral are founds are found in association in association with demantoids.with deman- Demantoidstoids. Demantoid are rarelys arerepresented rarely represented by poorly by shapedpoorly rhombicshaped rhombic dodecahedron dodecahedron crystals and,crystals and, instead, are often composed of spherical aggregates made by of many individuals separated by veins of serpentine and calcite (Figure 3).

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Minerals 2021, 11, 825 instead, are often composed of spherical aggregates made by of many individuals separated4 of 13

by veins of serpentine and calcite (Figure3).

FigureFigure 3.3. SliceSlice ofof aa roundedrounded aggregate aggregate of of demantoid demantoid in in the the carbonate-serpentine carbonate-serpentine vein vein (Poldnevskoye (Pold- deposit).nevskoye Srp+Cal: deposit). serpentine Srp+Cal: serpentine and calcite, and Dm: calcite, andradite Dm: (brownandradite - Ti-containing, (brown - Ti-containing, light yellow-greenlight yellow-green-demantoid). Photo by E.S. Karaseva. demantoid). Photo by E.S. Karaseva.

3.3. MaterialsMaterials andand MethodsMethods TheThe facetedfaceted demantoidsdemantoids (Figure(Figure4 4a)a) were were borrowed borrowed from from the the collection collection of of A. A. Kissin. Kissin. SamplesSamples ofof untreated untreated (rough) (rough) demantoid demantoid (Figure (Figure4b) 4b) were were obtained obtained during during the explorationthe explora- worktion work carried carried out by out one by of one the of authors the authors (A.K.) (A.K.) in the in Poldnevskoye the Poldnevskoye deposit deposit in 1978–1979. in 1978– Additional1979. Additional materials material werescollected were collected by the by authors the authors during during visits visits to the to Poldnevskoye the Poldnevskoye and Korkodinskoyeand Korkodinskoye deposits deposits in 2018–2020. in 2018–2020. The investigations were carried out by optical methods using an Olympus microscope with DP2-TWAIN system at 10× to 400× magniﬁcation (Japan). The X-ray powder diffraction analysis of demantoid structure was performed at the Institute of Geology and Geochemistry of UrB RAS (Ekaterinburg, Russia). Thin sections of the demantoid, as well as the demantoid grain surface morphology, were studied using scanning electron microscopy (SEM). The analyses were carried out using a Jeol JSM-6390LV scanning electron microscope (Jeol, Tokyo, Japan) with an INCA Energy 450 X-Max 80 (Oxford Instruments, Abingdon, UK) energy-dispersive spectrometer at the Geoanalitik Collective Use Center of the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences (Ekaterinburg, Russia). The

operating conditions included an acceleration voltage of 20 kV, a beam current 85 µA, and an operating distance of 10 mm. Raman spectra in the region of 100–1000 cm−1 were obtained in back-scattering geometry using a Horiba LabRam HR800 Evolution (Horiba, Palaiseau, France) equipped with an Olympus BX-FM confocal microscope (Olympus, Tokyo, Japan), He-Ne laser (radiation wavelength 633 nm, with laser power of 2 mW) at the Geoanalitik Collective Use Center of the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences (Ekaterinburg, Russia). There was no photoluminescence observed with this excitation. An Olympus 100× objective (Olympus, Tokyo, Japan) with numerical aperture = 0.9 was used. The acquisition time was 10 s, with two accumulations per spectral segment. A diffraction grating of 1800 gr/mm and an electrically cooled charge-coupled device detector were used to record the spectra. The spectrometer calibration was guided along the Rayleigh line and the emission lines of a neon lamp.

Figure 4. Research material: (a) collection of faceted demantoids with “horsetail” inclusions sourced from Poldnevskoye (1-2), Elizavetinskoye (3; alluvial), and Korkodinskoye (4-11); and (b) untreated demantoid (rough material) from the Poldnevskoye deposit.

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Figure 3. Slice of a rounded aggregate of demantoid in the carbonate-serpentine vein (Pold- nevskoye deposit). Srp+Cal: serpentine and calcite, Dm: andradite (brown - Ti-containing, light yellow-green-demantoid). Photo by E.S. Karaseva.

3. Materials and Methods The faceted demantoids (Figure 4a) were borrowed from the collection of A. Kissin. Samples of untreated (rough) demantoid (Figure 4b) were obtained during the explora- Minerals 2021, 11, 825 tion work carried out by one of the authors (A.K.) in the Poldnevskoye deposit in 5197 of 138– 1979. Additional materials were collected by the authors during visits to the Poldnevskoye and Korkodinskoye deposits in 2018–2020.

FigureFigure 4. 4.Research Research material: material: ( a(a)) collection collection of of faceted faceted demantoids demantoids with with “horsetail” “horsetail inclusions” inclusions sourced fromsourced Poldnevskoye from Poldnevskoye (1–2), Elizavetinskoye (1-2), Elizavetinskoye (3; alluvial), (3; andalluvial), Korkodinskoye and Korkodinskoye (4–11); and (4 (-b11)) untreated; and (b) untreated demantoid (rough material) from the Poldnevskoye deposit. demantoid (rough material) from the Poldnevskoye deposit.

4. Results 4.1. Microscopic Characteristics The Ural demantoid rarely has well-formed crystals and, instead, usually manifests as rhombic dodecahedron with a rounded shape and unexpressive ribs (Figure5). It is very rare to ﬁnd relatively well-faceted crystals (Figure6). It is usually represented by grains with rounded irregular shape. Kidney-shaped formations consisting of many small grains with a sharply pronounced increase in size to the periphery of the aggregate are very typical (see Figure3). “Horsetail” inclusions are represented by hair-like formations of various sizes assembled in fan or sheaf-like bundles tapering toward the central part of the grain. Closely spaced inclusions form larger and denser formations. “Horsetail” inclusions may begin in the center of the grain from an inclusion of a dark ore mineral (chromospinel or magnetite, see Figure2), or without a central inclusion. Sometimes “horsetail” inclusions start from a facet of the phantom, which can reﬂect (Figure7a). “Horsetail” inclusions are often gathered not in a bundle (horsetail), but in a fan (Figure7b). This can be seen especially well on the facets of the stone. They can start at any depth of the stone, with shallow “horsetail” inclusions often being v-shaped (Figure7c). They are usually colorless and transparent, while in the demantoid from alluvial placers, horsetail inclusions are usually colored ochre-yellow (Figure7e,f). In the stones that underwent high-temperature annealing (to improve the coloring), the inclusions sometimes acquire a dark brown coloring and lose transparency (Figure7c,d). It is important to note that small disc-shaped cracks are not characteristic for annealed stones, although these cracks are a sign of heat treatment of the gemstones. The “horsetail” inclusions on the polished facets near the surface are dark colored and opaque (Figure7g), which is due to process dirt. Minerals 2021, 11, 825 5 of 13

The investigations were carried out by optical methods using an Olympus microscope with DP2-TWAIN system at 10× to 400× magnification (Japan). The X-ray powder diffraction analysis of demantoid structure was performed at the Institute of Geology and Geochemistry of UrB RAS (Ekaterinburg, Russia). Thin sections of the demantoid, as well as the demantoid grain surface morphology, were studied using scanning electron microscopy (SEM). The analyses were carried out using a Jeol JSM-6390LV scanning electron microscope (Jeol, Tokyo, Japan) with an INCA Energy 450 X-Max 80 (Oxford Instruments, Abingdon, UK) energy-dispersive spectrometer at the Geoanalitik Collective Use Center of the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences (Ekaterinburg, Russia). The operating conditions included an acceleration voltage of 20 kV, a beam current 85 µA, and an operating distance of 10 mm. Raman spectra in the region of 100–1000 cm−1 were obtained in back-scattering geometry using a Horiba LabRam HR800 Evolution (Horiba, Palaiseau, France) equipped with an Olympus BX-FM confocal microscope (Olympus, Tokyo, Japan), He-Ne laser (radiation wavelength 633 nm, with laser power of 2 mW) at the Geoanalitik Collective Use Center of the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences (Ekaterinburg, Russia). There was no photoluminescence observed with this excitation. An Olympus 100× objective (Olympus, Tokyo, Japan) with numerical aperture = 0.9 was used. The acquisition time was 10 s, with two accumulations per spectral segment. A diffraction grating of 1800 gr/mm and an electrically cooled charge-coupled device detector were used to record the spectra. The spectrometer calibration was guided along the Rayleigh line and the emission lines of a neon lamp.

Minerals 2021, 11, 825 6 of 13 Figure 5. Rhombic dodecahedron shape of the demantoid crystal from the Poldnevskoye deposit. Figure 5. Rhombic dodecahedron shape of the demantoid crystal from the Poldnevskoye deposit. SE image. SE image.

FigureFigure 6. The 6. demantoid The demantoid crystal from thecrystal Ufaleiskiy from manifestation. the Ufaleiskiy manifestation.

The study of “horsetail” inclusions in transparent, thin sections of the demantoid under a“ microscope,Horsetail with” inclusions and without an are analyzer, represented did not reveal by thehair presence-like formations of any of various sizes assem- mineral phases. When rotating the microscope table, only the reﬂection from the surface of thebled inclusions in fan were or observed. sheaf Sometimes,-like bundles the chips tapering made through toward the central the part central of the part of the grain. Closely demantoidspaced grain inclusions clearly showed form its sectoral-radiallarger and structure denser (Figure formations.8a,b): the individuals “Horsetail” inclusions may begin in of the demantoid diverged from the center along radians expanding rapidly. In some places,the center induction of surfaces the grain with coarse from transverse an inclusion striations were of clearlya dark visible. ore Theremineral were (chromospinel or magnetite, alsosee longitudinal Figure thin2), grooves,or without the number a central and size ofinclusion. which increased Sometimes rapidly toward “ thehorsetail” inclusions start from peripherya facet of o thef the grain phantom, (see Figure8b). which These arecan the reflect “horsetail” (Figure inclusions. 7a). “Horsetail” inclusions are often gathered not in a bundle (horsetail), but in a fan (Figure 7b). This can be seen especially well on the facets of the stone. They can start at any depth of the stone, with shallow “horsetail” inclusions often being v-shaped (Figure 7c). They are usually colorless and transparent, while in the demantoid from alluvial placers, horsetail inclusions are usually colored ochre-yellow (Figure 7e,f). In the stones that underwent high-temperature annealing (to improve the coloring), the inclusions sometimes acquire a dark brown coloring and lose transparency (Figure 7c,d). It is important to note that small disc-shaped cracks are not characteristic for annealed stones, although these cracks are a sign of heat treatment of the gemstones. The “horsetail” inclusions on the polished facets near the surface are dark colored and opaque (Figure 7g), which is due to process dirt.

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Figure 6. The demantoid crystal from the Ufaleiskiy manifestation.

“Horsetail” inclusions are represented by hair-like formations of various sizes assembled in fan or sheaf-like bundles tapering toward the central part of the grain. Closely spaced inclusions form larger and denser formations. “Horsetail” inclusions may begin in the center of the grain from an inclusion of a dark ore mineral (chromospinel or magnetite, see Figure 2), or without a central inclusion. Sometimes “horsetail” inclusions start from a facet of the phantom, which can reflect (Figure 7a). “Horsetail” inclusions are often gathered not in a bundle (horsetail), but in a fan (Figure 7b). This can be seen especially well on the facets of the stone. They can start at any depth of the stone, with shallow “horsetail” inclusions often being v-shaped (Figure 7c). They are usually colorless and transparent, while in the demantoid from alluvial placers, horsetail inclusions are usually colored ochre-yellow (Figure 7e,f). In the stones that underwent high-temperature annealing (to improve the coloring), the inclusions sometimes acquire a dark brown coloring and lose transparency (Figure 7c,d). It is important to note that small disc-shaped cracks are not Minerals 2021, 11, 825 characteristic for annealed stones, although these cracks are a sign of heat treatment of the 7 of 13 gemstones. The “horsetail” inclusions on the polished facets near the surface are dark colored and opaque (Figure 7g), which is due to process dirt.

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Figure 7. Features of “horsetail” inclusions revealed by optical methods: (a) “horsetail” inclusions start from the reflecting Figure 7. a surfaceFeatures of the ofcrystal “horsetail” phantom inclusions (stone 2 inrevealed Figure 4a); by (b optical) curtains methods: of threadlike ( ) “horsetail” inclusions; ( inclusionsc) v-shaped startdark- frombrown the inclu- reﬂecting surfacesion of the in annealed crystal phantom stone; (d) (stone inclusion 2 in in Figure annealed4a); (stone;b) curtains (e,f) colored of threadlike inclusions inclusions; in the demantoids (c) v-shaped from dark-brownalluvial placers; inclusion in annealedand (g) stone; dark, opaque (d) inclusion ends of in“horsetail annealed” inclusions, stone; (e exiting,f) colored to the inclusionspolished surface in the (stone demantoids 1 in Figure from 4a). alluvial placers; and (g) dark, opaque ends of “horsetail” inclusions, exiting to the polished surface (stone 1 in Figure4a). The study of “horsetail” inclusions in transparent, thin sections of the demantoid under a microscope, with and without an analyzer, did not reveal the presence of any mineral phases. When rotating the microscope table, only the reflection from the surface of the inclusions were observed. Sometimes, the chips made through the central part of the demantoid grain clearly showed its sectoral-radial structure (Figure 8a,b): the individuals of the demantoid diverged from the center along radians expanding rapidly. In some places, induction surfaces with coarse transverse striations were clearly visible. There were also longitudinal thin grooves, the number and size of which increased rapidly toward the periphery of the grain (see Figure 8b). These are the “horsetail” inclusions.

Minerals 2021, 11, 825 8 of 13 Minerals 2021, 11, 825 8 of 13

FigureFigure 8. Features 8. Features of the of the structure structure of ofthe the Ural Ural demantoid demantoid grains: grains: (a) (photoa) photo of chipping of chipping through through the the central central part part of the of the demantoid grain, showing induction surfaces; (b) same, sketch from photo (a); (c) chipping through the central part of the demantoid grain, showing induction surfaces; (b) same, sketch from photo (a); (c) chipping through the central part of demantoid grain (induction surfaces); (d) induction surfaces on the polychrome demantoid grain recovered from the ag- the demantoid grain (induction surfaces); (d) induction surfaces on the polychrome demantoid grain recovered from the gregate; and (e) cavity in the central part of the demantoid grain. aggregate; and (e) cavity in the central part of the demantoid grain. Figure 8c shows a similar chipping on the induction surfaces of another demantoid Figure8c shows a similar chipping on the induction surfaces of another demantoid grain. grain. Here, there is no coarse cross hatching, but fine hatching, creating inclusions of the Here, there is no coarse cross hatching, but fine hatching, creating inclusions of the “horsetail” “horsetail” type, is shown quite distinctly. Note the rounded shape of the grain. Figure 8d type, is shown quite distinctly. Note the rounded shape of the grain. Figure8d shows shows an individual polychrome demantoid with distinct induction surfaces on the dark an individual polychrome demantoid with distinct induction surfaces on the dark green green(interior) (interior) area, area, and and abundant abundant “horsetail” “horsetail inclusions” inclusions in its in light its yellow–greenlight yellow–green (exterior) (exte- area. rior)These area inclusions. These inclusions begin only begin when only the when induction the induction surfaces surfaces with coarse with coarse transverse transverse hatching hatchindisappear.g disappear. Sometimes, there is a cavern in the central part of the demantoid grain, from which “horsetail” inclusions start (Figure 8e). A drop of water placed in the cavern is instantly

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Sometimes, there is a cavern in the central part of the demantoid grain, from which absorbed“horsetail” by inclusions the stone startdue to (Figure the presence8e). A drop of an of effective water placed capillary in the system cavern caused is instantly by the “absorbedhorsetail” by inclusions. the stone due to the presence of an effective capillary system caused by the “horsetail” inclusions. 4.2. X-ray Powder Diffraction and Thermal Analysis 4.2. X-ray Powder Diffraction and Thermal Analysis X-ray powder diffraction of the demantoid samples with numerous “horsetail” in- clusionsX-ray (loss powder of transparency) diffraction detect of theed demantoid only andradite. samples These with results numerous were “horsetail” confirmed in- by thermalclusions research (loss of transparency)methods. detected only andradite. These results were conﬁrmed by thermal research methods.

4.3.4.3. Electron Electron Scanning Scanning Microscope Microscope TheThe study study of of hair hair-like-like inclusions onon freshfresh chipschipsof of the the Ural Ural demantoid demantoid grains grains under under anan electron electron scanning scanning microscope showedshowed veryvery interestinginteresting results. results. EvenEven the the seemingly seemingly monocrystalmonocrystal andradite andradite (see (see Figure Figure 55)) hadhad anan aggregateaggregate structure.structure. WhenWhen crushingcrushing suchsuch a crystal,a crystal, many many fragments fragments were were cone cone-shaped-shaped (Figure (Figure 9a)9 a)with with thin thin longitudinal longitudinal grooves grooves and knolls.and knolls. Note Notealso the also transverse the transverse irregularities irregularities (steps) (steps) enveloping enveloping the cone, the cone, which which are char- are acteristiccharacteristic of the of induction the induction surfaces surfaces of crystals. of crystals. The The longitudinal longitudinal section section of of“horsetail “horsetail”” inclusionsinclusions is shown is shown in inFigure Figure 9b:9b: many many subparallel subparallel groovesgrooves ofof different different lengths lengths and and depths depths werewere observed; observed; there there were split andand relativelyrelatively shortshort groovesgrooves which which disappeared disappeared in in both both directions.directions.

FigureFigure 9. 9.BSEBSE-images-images of of demantoids: demantoids: (a (a)) cone cone-shaped-shaped fragment; fragment; ( b) surface ofof thethe chipchip inin the the “horsetail” “horsetail” inclusions inclusions plane; plane; and (c) chip through the center of an opaque andradite crystal with negative-shaped ribs (d). and (c) chip through the center of an opaque andradite crystal with negative-shaped ribs (d).

TheThe appearance ofof a chipa chip of brownof brown andradite andradite crystal crystal (Figure (Figure9c) from 9c) the Poldnevskoyefrom the Pold- nevskoyedeposit is deposit very like is a very “trapiche” like a crystal“trapiche of corundum” crystal of or corundum beryl: ﬂat faces or beryl: with deepflat faces grooves with deep grooves in place of the ribs of the crystal (Figure 9d). Here, too, “horsetail” inclusions were visible, but in some areas they were parallel and also v-shaped.

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in place of the ribs of the crystal (Figure9d). Here, too, “horsetail” inclusions were visible, but in some areas they were parallel and also v-shaped. The mineral composition of the “horsetail” inclusions was not detected, or they were The mineral composition of the “horsetail” inclusions was not detected, or they represented by singlewere inclusions, represented which by single were inclusions, not identified which due were to not their identified small size due to(Figure their small size 10a,d). The vast majority(Figure 10of a,d).the “ Thehorsetail vast majority” inclusions of the were “horsetail” hollow inclusionschannels were(Figure hollow 10). channels The configuration (Figureof their 10 sections). The configuration was very bizarre, of their but sections closest was to very slit bizarre,-like (Figure but closest 10b). to slit-like The walls of the channels(Figure 10wereb). The corrugated, walls of the resembling channels were the corrugated,faces of negative resembling crystals the faces (Fig- of negative ure 10c,d). crystals (Figure 10c,d).

FigureFigure 10. BSE-images 10. BSE-images of “horsetail” of “horsetail inclusions” inclusions in fresh chipping in fresh ofchipping the demantoid: of the demantoid: (a) point-like ( hollowa) point channels-like (one containshollow a needle-like channels inclusion (one contains of an unknown a needle mineral);-like inclusion (b) slit-like of an hollow unknown channels; mineral); (c) morphology (b) slit-like of cross hollow sections of pointchannels; and slit-like (c) channels; morphology and (d of) mineral cross sections inclusion of in point a hollow and channel slit-like (fragment channels; of Figureand (d 10) minerala). inclusion in a hollow channel (fragment of Figure 10a).

4.4. Raman Spectroscopy Raman spectroscopy of a mineral filling in the large “horsetail” inclusions (Figure 11a) did not detect in the depth ranges of 0–100 µm. The obtained Raman spectra corresponded to the reference data of the demantoid (Figure 11b). In the Ca3Fe2(SiO4)3 demantoid spectra (space group Ia3d), 15 out of the 25 Raman-active vibrational modes were

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4.4. Raman Spectroscopy Raman spectroscopy of a mineral ﬁlling in the large “horsetail” inclusions (Figure 11a) did not detect in the depth ranges of 0–100 µm. The obtained Raman spectra corresponded to the reference data of the demantoid (Figure 11b). In the Ca3Fe2(SiO4)3 demantoid spectra (space group Ia3d), 15 out of the 25 Raman-active vibrational modes were collected, which Minerals 2021, 11, 825 were predicted by group-theoretical analysis [24]. No photoluminescence occurred at the12 of 15

selected excitation wavelength.

Figure 11. Raman spectrum of the Demantoid from the Poldnevskoye deposit: (a) optical image of the “horsetail” inclusion in the demantoid (thin sections); and (b) Raman spectra of inclusions in the depth range of 0–100 µm.

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−1 In the region 100–400 cm , modes were associated with the translation of SiO4—T(SiO4), 3+ 3+ the rotation of SiO4—R(SiO4), and the translation of YO6(Y = Fe , Cr )—T(M). The bending −1 vibrations of SiO4 tetrahedra – ν2, in the region 493–516 cm , with bending vibrations of −1 SiO4 tetrahedra – ν4 and ν2, while in the region 816–874 cm , were associated with stretching vibrations of the SiO4 tetrahedra – ν3 [11,24].

5. Discussion This study on the Ural demantoids, via various methods, showed that “horsetail” inclusions are usually represented by hollow tubular formations. This can explain the problems associated with identifying these inclusions by optical microscopy, X-ray powder diffraction, thermal methods, and Raman spectroscopy. The presence of hollow channels should be already inferred from the analysis of the cause of coloring of “horsetail” inclusions in the demantoids from alluvial placers (see Figure7e,f), those subjected to high-temperature annealing (see Figure7c,d), or, sometimes, those exiting on the polished surface of a cut stone (see Figure7g). In the first case, the staining is due to the penetration of iron hydroxides into the stone, while in the second and third cases it is due to combustion products and technological dirt, respectively. This would be impossible if the “horsetail” inclusions were filled with solid mineral matter. This is also indicated by the absence of disc-shaped cracks around the inclusions in the annealed demantoids, which are inevitable in the presence of solid mineral inclusions. It is noteworthy that similar conclusions were reached by Fritz et al. [25], who studied andradite garnet from Namibia. The formation of hollow channels in the demantoid could not be the result of dissolution of the mineral inclusion under natural conditions, because the carbonate-serpentine vein masses that host the demantoid (see Figure3) showed no signs of dissolution, even calcite. Moreover, serpentine filled the “horsetail” inclusions on the surface of the demantoid grain, as its envelopes (see Figure3). The mouths of hollow channels can also be filled with other minerals. For example, we observed the junction of demantoid and a large magnetite crystal: the contact of the mouths of the “horsetail” inclusions were filled with magnetite. “Horsetail” inclusions often have a fan-shape, in the plane of which induction surfaces are sometimes observed (see Figure8a–d), and the demantoid grains themselves have an aggregate sectoral structure. It is the sectoral growth of the demantoid crystals that explains the appearance of “horsetail” inclusions. Even in well-formed crystals (see Figure6), the sectoral structure is observed (Figure8a,b). The reason for the sectoral growth of the demantoid crystals is caused, in our opinion, by mineral formation under the conditions of decompression during the rise of the crust- mantle mixture, experiencing autometamorphism and autometasomatism [26,27]. At the Poldnevskoye deposit, rounded grains of calcite and, sometimes, magnetite, in association with demantoid in the carbonate-serpentine vein mass, had a similar structure. The mechanism of such crystal growth under decompression conditions, however, requires further study.

6. Conclusions 1. “Horsetail” inclusions in the Ural demantoids are represented by tubular hollow formations, sometimes containing serpentine; 2. The formation of “horsetail” inclusions is caused by the spherical growth of the demantoid crystals under conditions of decompression, autometamorphism, and autometasomatism of the up-moving ultrabasite massif; 3. The mechanism of split growth of the demantoid crystals and the formation of “horsetail” inclusions is not well understood and requires additional research.

Author Contributions: Conceptualization, A.Y.K.; methodology, V.V.M. and E.S.K.; software, E.S.K.; visualization, V.V.M. and E.S.K.; writing—original draft, A.Y.K.; writing—review and editing, A.Y.K. and V.V.M. All authors have read and agreed to the published version of the manuscript. Minerals 2021, 11, 825 13 of 13

Funding: The work was performed within the framework of topic no. AAAA-A18-118052590028-9 of the IGG UB RAS state assignment. Data Availability Statement: Not applicable. Acknowledgments: The editor of Minerals and guest editors are thanked for organizing the Special Issue on “Gems and Gem Minerals” and extending an invitation to submit a contribution for consid- eration. We thank the Reviewers for the important comments and constructive suggestions, which helped us to improve the quality of the manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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