Heinz-Jürgen Bernhardt

Boralsilite, Al16B6Si2O37, and “boron-mullite:” Compositional variations and associated phases in experiment and nature

Abstract Boralsilite, the only natural anhydrous ternary B2O3-Al2O3-SiO2 (BAS) phase, has been synthesized from BASH gels with Al/Si ratios of 8:1 and 4:1 but variable B2O3 and H2O contents at 700-800 °C, 1-4 kbar, close to the conditions estimated for natural boralsilite (600-700 °C, 3-4 kbar). Rietveld refinement gives monoclinic symmetry, C2/m, a = 14.797(1), b = 5.5800(3), c = 15.095(2) Å, β = 91.750(4)°, and V = 1245.8(2) Å3. Boron replaces 14% of the Si at the Si site, and Si or Al replaces ca. 12% of the B at the tetrahedral B2 site. A relatively well-ordered boralsilite was also synthesized at 450 °C, 10 kbar with dumortierite and the OH analogue of jeremejevite. An orthorhombic phase (“boron-mullite”) synthesized at 750 °C, 2 kbar has mullite-like cell parameters a = 7.505(1), b = 7.640(2), c = 2.8330(4) Å, and V = 162.44(6) Å3. “Boron-mullite” also accompanied disordered boralsilite at 750-800 °C, 1-2 kbar. A possible natural analogue of “boron-mullite” is replacing the Fe-dominant analogue of werdingite in B-rich metapelites at Mount Stafford, central Australia; its composition extends from close to stoichiometric Al2SiO5 to Al2.06B0.26Si0.76O5, i.e., almost halfway to Al5BO9. Boralsilite is a minor constituent of pegmatites cutting granulite-facies rocks in the Larsemann Hills, Prydz Bay, East Antarctica, and at Almgotheii, Rogaland, Norway. Electron-microprobe analyses (including B) gave two distinct types: (1) a limited solid solution in which Si varies inversely with B over a narrow range, and (2) a more extensive solid solution containing up to 30% (Mg,Fe)2Al14B4Si4O37 (werdingite). Boralsilite in the Larsemann Hills is commonly associated with graphic tourmaline-quartz intergrowths, which could be the products of rapid growth due to oversaturation, leaving a residual melt thoroughly depleted in Fe and Mg, but not in Al and B. The combination of a B-rich source and relatively low water content, together with limited fractionation, resulted in an unusual buildup of B, but not of Li, Be, and other elements normally concentrated in pegmatites. The resulting conditions are favorable in the late stages of pegmatite crystallization for precipitation of boralsilite, werdingite, and grandidierite instead of elbaite and B minerals characteristic of the later stages in more fractionated pegmatites.

Texture formation and element partitioning in trapiche ruby

Abstract Based on textural and compositional investigations on ruby single crystals showing textures with six arms and six growth sectors (trapiche ruby), it has been analysed how the unique texture was formed, and the element partitioning was governed by the growth mechanism. The arms were formed earlier by dendritic growth on rough interfaces, and the growth sectors by lateral growth on smooth interfaces. The arm portions consist of plural mineral phases but show a low and almost uniform Cr content in corundum throughout the extension of the arm and its branches, whereas the growth sectors are single ruby phase, but show Cr zoning parallel to the growth surfaces. Element partitioning in the earlier dendritic growth is governed by thermodynamic parameters, whereas that of the latter layer-by-layer growth by kinetics.

Authigenic burbankite in the Cioclovina Cave sediments (Romania)

The complex rare-earth-bearing anhydrous carbonate burbankite, A 3 B 3(CO3)5, occurs as microcrystalline yellow greyish aggregates in the lower part of a lacustrine-like sediment sequence in the Cioclovina Cave, Romania. From this occurrence, foggite, churchite-(Y) and colourless or milky-white needle-like brushite and gypsum were also documented. The empirical formula (calculated from the electron-microprobe results on the basis of five carbonate groups pfu ) is (Na2.46Ca0.98Sr1.71Ba0.32Y0.05Ce0.17 La0.08Nd0.08Pr0.02Th0.09)∑ = 5.96(CO3)5. Single-crystal X-ray investigations gave a = 10.514(3) and c = 6.477(2) A, space group P 63 mc , Z = 2. The structural refinement converged at R 1 = 0.030 for 827 F 0 > 4σ( F 0 ). The crystal structure refinement was performed to verify ordering at the two sites, A and B . The A site is [6 + 2] coordinated with an average A— O bond length of 2.491 A; the B site is [10] coordinated, the average B— O bond length is 2.678 A. As expected the Na and Ca atoms are concentrated in the smaller A O8 polyhedron whereas the larger cations occupy the B site. The three crystallographically different carbonate groups are planar within the accuracy of structure refinement, C—O bonds vary from 1.268(4) to 1.294(3) A. The δ13C and δ18O values for Cioclovina burbankite compare to other low-temperature cave carbonates, and thus clearly distinguish it from the more common burbankite occurring in igneous alkaline rocks. Precipitation of burbankite in cave settings is attributed to the reaction between percolating REE s, Sr- and Na-rich solutions and carbon dioxide, in an alkali-balanced environment, under dry and poor or no drainage conditions.

Garnets from Madagascar with a Color Change of Blue-Green to Purple

© 1999 Gemological Institute of America stones are usually small (0.1–0.8 ct), many stones over 1 ct have been reported. The largest faceted topquality blue-green color-change garnet seen to date is 9.5 ct (T. Hainschwang, pers. comm., 1999). In general, garnets that show a distinct color change from green to bluish green in day (or fluorescent) light and red to reddish purple in incandescent light are subdivided into two groups according to their chemical composition. The first group consists of chromium-rich pyropes with chromium contents above 3 wt.% Cr2O3. Chemical properties of samples from this first group have been reviewed by Schmetzer et al. (1980), but faceted gem material has been seen only rarely in the trade. Occasionally, garnet inclusions in diamond (see Fryer, 1982) reveal this chemical composition. The second group of color-change garnets is more commonly seen as faceted gem material. This group is formed by members of the pyrope-spessartine solid-solution series that also contain minor molecular percentages of almandine and grossular. The color change in these garnets always is associated with small amounts of V2O3 and/or Cr2O3 (Schmetzer and Ottemann, 1979; Schmetzer et al., 1980; Stockton, GARNETS FROM MADAGASCAR WITH A COLOR CHANGE OF BLUE-GREEN TO PURPLE

Pink to Pinkish Orange Malaya Garnets from Bekily, Madagascar

296 MALAYA GARNETS FROM MADAGASCAR GEMS & GEMOLOGY WINTER 2001 spite” have been applied to intermediate pyropespessartine garnets in this color range, but only “malaya” has found international acceptance in the trade and the literature (see, e.g., references above and Curtis, 1980; Stockton and Manson, 1985; Keller, 1992; and Hänni, 1999). Limited amounts of rough are still mined in northern Tanzania and marketed as malaya garnet (Karl Egon Wild, pers. comm., 2001). In this article, the term malaya is used for garnets that are pink to pinkish orange, as well as orange to red, that are composed primarily of pyrope-spessartine. Note, however, that there is no established definition of this term based on a precise compositional range. [Editor’s note: Although a trade name, because of its long acceptance in the trade, for the balance of the article malaya will not be enclosed in quotations marks or capitalized.] By Karl Schmetzer, Thomas Hainschwang, Lore Kiefert, and Heinz-Jürgen Bernhardt

Purple to Purplish Red Chromium-Bearing Taaffeites

Gems & Gemology, Vol. 36, No. 1, pp. 50–59.

Serendibite from Sri Lanka

from secondary deposits in the Ratnapura area in the second half of the 1990s increases the number of gems containing essential boron from this island (tourmaline, dumortierite, boron-rich kornerupine [or prismatine, according to recent mineralogical studies], and sinhalite). To date, serendibite has been reported from 11 localities: Sri Lanka (type locality near Kandy), United States (three occurrences), Russia (two occurrences), Ukraine, Tanzania, Canada, and Madagascar (two localities). At all of these localities, serendibite occurs in metasomatic high-temperature calc-silicate rocks (skarns), mostly of granulite facies (Grew et al., 1990, 1991a,b; Grew, 1996). The occurrence of gemquality serendibite crystals in secondary deposits near Ratnapura, Sri Lanka, is consistent with the derivation of these gem gravels from granulite-facies metamorphic rocks. According to modern mineralogical examination (see, e.g., Kunzman, 1999), serendibite is a triclinic Ca-Mg-Al-B-silicate which can also contain distinct amounts of Fe2+ and Fe3+. Since 1997, gemologist and gem dealer D. Palitha Gunasekera of Ratnapura has reported encountering three gem-quality samples of serendibite, weighing 0.35, 0.55, and 0.56 ct as faceted stones( pers. comm., 1997–2001). A merchant in Kolonne showed him the first (smallest) sample as a 1.25 ct pebble. This rough sample was said to originate from Ginigalgoda, near Kolonne in the Ratnapura district. Mr. Gunasekera submitted the 0.35 ct stone faceted from this piece of rough (figure 1) to the GIA Gem Trade Laboratory for identification in autumn of 1996 (Reinitz and Johnson, 1997). It was subsequently purchased by one of the present authors (EG) and identified independently as serendibite by KS in 1997, also using X-ray powder diffraction. A review By Karl Schmetzer, George Bosshart, Heinz-Jurgen Bernhardt, Edward J. Gubelin, and Christopher P. Smith