minerals Article Crystal Chemistry of Birefringent Uvarovite Solid Solutions Sytle M. Antao * and Jeffrey J. Salvador Department of Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada * Correspondence: [email protected] Received: 28 May 2019; Accepted: 27 June 2019; Published: 28 June 2019 Abstract: The crystal chemistry of five optically anisotropic uvarovite samples from different localities (California, Finland, Russia, and Switzerland) were studied with electron-probe microanalysis (EPMA) and the Rietveld method. Monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data were used, and Rietveld refinement was carried out with the cubic [8] [6] [4] [4] space group, Ia3d. The general formula for garnet is X3 Y2 Z3 O12. Uvarovite has the ideal formula, Ca3Cr2Si3O12, which may be written as Ca3{Cr,Al,Fe}S2[Si3O12] because of solid solutions. HRPXRD traces show multiple cubic garnet phases in each sample that has a heterogeneous chemical composition. The optical and back-scattered electron (BSE) images and elemental maps contain lamellar and concentric zoning as well as patchy intergrowths. With increasing a unit-cell parameter for uvarovite solid solutions, the Z–O distance remains constant, and the average <X–O> distance increases slightly in response to the Cr3+ Al3+ cation substitution in the Y site. The Y–O distance , increases most because Cr3+ (radius = 0.615 Å) is larger than Al3+ (radius = 0.545 Å) cations. The Fe3+ (radius = 0.645 Å) cation is also involved in this substitution. Structural mismatch between the cubic garnet phases in the samples gives rise to strain-induced optical anisotropy. Keywords: uvarovite; grossular; andradite; garnet; optical anisotropy; intergrowths of multiple cubic phases; Rietveld refinements; Synchrotron high-resolution powder X-ray diffraction (HRPXRD); structure; chemistry 1. Introduction Several studies have documented birefringent garnets with lamellar or oscillatory features, which were referred to as “chemical zoning” instead of separate phases, e.g., [1–4]. Such birefringent garnets appear to contain a few cubic garnet phases that grow as oscillatory zoning or by re-sorption and re-precipitation that gives rise to patchy features, e.g., [5–7]. Birefringence in garnet was reported over a century ago, e.g., [8,9], but the origin remains questionable. Some almandine, grossular, spessartine, andradite, uvarovite, and hydrogarnet samples are anisotropic under cross-polarized light, which may indicate that they are not optically cubic, e.g., [10–13]. Several reasons were given for the birefringence, but the main one appears to be cation orders in the X and Y sites that cause a symmetry reduction, e.g., [1,10,14,15]. Other suggested reasons for the birefringence in garnet were discussed and are not repeated here [16]. Six birefringent uvarovite samples were investigated with various experimental techniques, and the results were presented [17–19]. The water content in these samples ranges from 0.07 to 0.34 wt. %, and they exhibit no compositional zoning. Their selected-area electron diffraction (SAED) patterns are consistent with cubic symmetry. Microtwinning was not observed in their high-resolution transmission electron microscopy (HRTEM) images. Hydroxyl groups are not the primary cause for the anisotropic behavior in uvarovite, but cation ordering on the Y octahedral positions causes birefringence, as indicated by single-crystal X-ray diffraction studies [19]. The structure of their uvarovite samples was Minerals 2019, 9, 395; doi:10.3390/min9070395 www.mdpi.com/journal/minerals Minerals 2019, 9, x FOR PEER REVIEW 2 of 21 crystal X-ray diffraction studies [19]. The structure of their uvarovite samples was refined in various noncubic space groups, including triclinic, monoclinic, and orthorhombic, and it was concluded that partial long-range Cr3+/Al ordering on the Y site was the most prominent noncubic feature. The structure of a Minerals 2019, 9, 395 2 of 21 synthetic uvarovite was refined in the cubic space group [18]. Crystal structures that were refined in unnecessarily low symmetry space groups were heavily criticized [20,21]. refinedDiffraction in various peaks noncubic from garnets space groups,showing including split reflections triclinic, monoclinic,were observed and orthorhombic,, e.g., [11,22–27 and]. Recently, it was multiconcluded-phase intergrowths that partial of long-range two or three Cr 3cubic+/Al orderinggarnet phases on the were Y site observed was the with most HRPXRD; prominent all noncubicbirefringent garnetsfeature. show Theed split structure reflections of a synthetic [5,7,16,28,2 uvarovite9]. was refined in the cubic space group [18]. Crystal structuresThe general that formula were refined for common in unnecessarily silicate low garnet symmetry is [8]X3 space[6]Y2[4]Z groups3[4]O12, wereZ = 8, heavily and space criticized group [20 ,퐼푎21].3̅푑, where theDi ffeightraction-fold peaks coordinated from garnets dodecahedral showing splitX site reflections contains wereMg, observed,Fe2+, Mn2+, e.g., or Ca [11 cations,22–27].; Recently,the six-fold coordinatedmulti-phase octahedral intergrowths Y site contains of two orAl, three Cr3+, Fe cubic3+, Mn garnet3+, Ti4+ phases, or Zr4+ werecations observed; and the withfour- HRPXRD;fold coordinated all tetrahedralbirefringent Z site garnets contains showed Si or split Fe3+ reflections cations or [5 (O,7,164H,428) ,groups29]. , e.g., [7,30–33]. The structure consists of [8] [6] [4] [4] alternatingThe ZO general4 tetrahedra formula and for YO common6 octahedra silicate with garnet X atoms is formingX3 Y2 XOZ83 dodOecahedra12,Z = 8, ( andFigure space 1). The group eight 2+ 2+ O atomsIa3d, wherein the XO the8 eight-folddodecahedron coordinated occur at dodecahedralthe corners of a X distorted site contains cube Mg,. Each Fe O atom, Mn is, four or Ca-coordinated cations; tetrahedrallythe six-fold by coordinated two X, one Y, octahedral and one Z Y cation site contains. The O Al,atom Cr occupies3+, Fe3+, Mna general3+, Ti4 +position, or Zr4,+ whereascations; the and cation the s 3+ are four-foldlocated at coordinatedspecial positions tetrahedral with fixed Z site atom contains coordinates. Si or Fe If substitutioncations or with (O4H a4 )different groups, size e.g., cation [7,30– occurs33]. on theThe Y structure site in uvarovite consists solid of alternating solutions, ZOfor4 example,tetrahedra then and the YO Y–6Ooctahedra distance changes with X atoms significantly, forming whereas XO8 the dodecahedraZ–O and average (Figure 1<X). The–O> eightdistances O atoms change in the by XO minor8 dodecahedron amounts occur[7]. Nomenclature at the corners of of a distortedthe garnet supergroupcube. Each was O recently atom is given four-coordinated [34]. tetrahedrally by two X, one Y, and one Z cation. The O atomThis occupiesstudy examines a general the crystal position, chemistry whereas of thefive cations optically are anisotropic located at uvarovite special positions samples from with different fixed localitiesatom. coordinates.Each sample Ifcontains substitution multiple with cubic a di ffgarneterent sizephas cationes, but occursone crystal on the fragment Y site in is uvarovite an isotropic solid cubic phasesolutions,. This work for example,is part of a then large the study Y–O on distance garnet- changesgroup minerals significantly, [35–39 whereas]. Briefly, thethis Z–O study and indicates average that intergrowth<X–O> distances of multiple change cubic by phases minor in amounts a crystal [7 of]. Nomenclatureuvarovite cause ofs thestrain garnet-induced supergroup optical wasanisotropy. recently given [34]. Figure 1. Part of the garnet structure viewed down the c axis, showing the linkages of the Figure 1. Part of the garnet structure viewed down the c axis, showing the linkages of the various polyhedra various polyhedra (X = yellow dodecahedra, Y = pink octahedra, Z = green tetrahedra, and grey (X =spheres yellow dodecahedra,= O atoms). Y = pink octahedra, Z = green tetrahedra, and grey spheres = O atoms). This study examines the crystal chemistry of five optically anisotropic uvarovite samples from different localities. Each sample contains multiple cubic garnet phases, but one crystal fragment is an isotropic cubic phase. This work is part of a large study on garnet-group minerals [35–39]. Briefly, this study indicates that intergrowth of multiple cubic phases in a crystal of uvarovite causes strain-induced optical anisotropy. Minerals 2019, 9, 395 3 of 21 2. Experimental Methods 2.1. Sample Description The localities and some characteristics of the five uvarovite samples used in this study are given (Table1). The crystals were a few millimeters in size and varied in color from light to dark green. The specimen of sample 5 contained crystals that were either dark (5a and 5b were intergrowths of two cubic phases) or light green (5c was a cubic, isotropic phase). All the other samples were birefringent, so they were not optically cubic (Figure2). Table 1. Localities of the uvarovite samplesy used in this study. Sample # Locality Sarany, near Perm, Russia; Royal Ontario Museum (ROM #M51847) 1. Russia-A sample. Minerals 2019, 9, x FOR PEER REVIEW 3 of 21 2. Switzerland Zermatt area, Switzerland (ROM #M33537). 3. Russia-B Sarany, Urals, Russia. 2. Experimental Methods 4. Finland Outokumpo, Finland. Jacksonville, Toulumne Co., California, USA. These crystals are dark 5a, 5b. California 2.1. Sample Description green. The localities and some characteristicJacksonville,s of the Toulumne five uvarovite Co., California, samples used USA. in The this crystals study are are given light (Table 1). The crystals5c. California were a few millimetersgreen, isotropic, in size and and var chemicallyied in color homogeneous. from light to Alldark crystals green. fromThe specimen sample of 5 are from the same hand specimen. sample 5 contained crystals that were either dark (5a and 5b were intergrowths of two cubic phases) or light greenyAll (5c the was samples a cubic, (a few isotropic millimeters phase). in size) All are the light other to darksamples green were in color. birefringent, They are birefringent so they were and not chemically optically heterogeneous, except for sample 5c. cubic (Figure 2).