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

Edward S. Grew^1,*, Heribert A. Graetsch², Birgit Pöter², Martin G. Yates¹, Ian Buick³, Heinz-Jürgen Bernhardt⁴, Werner Schreyer^2,, Günter Werding², Christopher J. Carson³ and Geoffrey L. Clarke⁶

¹ Department of Earth Sciences, University of Maine, 5790 Bryand Research Center, Orono, Maine 04469-5790, U.S.A.
² Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany
³ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
⁴ Zentrale Elektronen-Mikrosonde, Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D-44801 Bochum, Germany
⁶ School of Geosciences, University of Sydney, NSW 2006, Australia

View larger version (30K): [in this window] [in a new window]   FIGURE 1. Plot in mol% of phases in the B2O3-Al2O3-SiO2 system. Data on synthetic (Si < 2 per 37 O) and natural boralsilite (only compositions with Si > 2 per 37 O, sample 121501E) are taken from Figure 6. Gielisse and Foster (1961) confirmed the solid solution (dashed line) between Al18B4O33 (A9B2) and Al6Si2O12 (3:2 mullite), which had been suggested by the syntheses of "boron-mullite" from a 3:1 gel (Letort 1952), with melt (Dietzel and Scholze 1955), and from mullite (Gelsdorf et al. 1958). Other "boron-mullite" synthesized from gels (B4; W4) is taken from Figure 7. Solid solution in the Mount Stafford "boron-mullite" is based on the least-squares fits in Figure 9. Stoichiometries of binary Al borates are based on reported compositions (e.g., Scholze 1956; Gielisse and Foster 1962; Sokolova et al. 1978; Ihara et al. 1980; Garsche et al. 1991; Mazza et al. 1992); the number of discrete phases could be fewer than the six shown owing to solid solutions.

View larger version (28K): [in this window] [in a new window]   FIGURE 2. Traces of WDS scan of wavelengths where BK peak is located. (a) Scan of boralsilite (sample 010602a1; grain 3). The measured peak area lies between the two vertical bars, beyond which is background. (b) Scans of minerals containing no B showing increase of background with increasing SiO2 content.

View larger version (21K): [in this window] [in a new window]   FIGURE 3. Pressure-temperature diagram summarizing syntheses of boralsilite and "boron-mullite" in Table 1. Filled squares = synthesis of ordered boralsilite, which includes B25 (Table 1). Open squares = synthesis of disordered boralsilite (Table 1). Gray diamonds = experimental products reported as boralsilite originally, but not reexamined to assess degree of order (only shown for 10 kbar runs). Filled circles = experimental products reported as "boron-mullite" in Table 1 and by Werding and Schreyer (1992, their Al8B2Si2O19) and Pöter et al. (1998); XRD pattern similar to the pattern for W6 (Fig. 4d). Gray triangles = experimental products reported as "boron-mullite" by Werding and Schreyer (1984, 1996) and Wodara and Schreyer (2001); no XRD pattern given (only shown if no overlap with later studies). X = breakdown of boralsilite for which degree of order was not re-examined. Superimposed symbols are separated for clarity. Box labeled "natural" indicates the P-T conditions estimated for natural boralsilite. Sources of other data: Al2SiO5 relations (Pattison 1992), dumortierite breakdown to quartz (Qtz) + "boron-mullite" (B-Mul) + fluid (fl) (Werding Schreyer 1996), dravite breakdown (Robbins and Yoder 1962; Krosse 1995; Werding and Schreyer 1996), upper stability limit of schorldravite tourmaline (Tur) suggested for the Ryoke metamorphic belt, Japan (Kawakami 2004).

View larger version (22K): [in this window] [in a new window]   FIGURE 4. XRD patterns of experimental products. (a) Ordered boralsilite with a few reflections indexed. (b) Disordered boralsilite and "boron-mullite." m = reflections of "boron-mullite" not overlapping with boralsilite reflections. (c) Disordered boralsilite with minor H3BO3 impurity. (d) "Boron-mullite" with a few reflections indexed, minor quartz and H3BO3 impurities. The 101 quartz reflection is a shoulder on the 210 "boron-mullite" reflection.

View larger version (84K): [in this window] [in a new window]   FIGURE 5. Images obtained by scanning electron microscopy of run products. (a) Prism of ordered boralsilite among grains of the amorphous phase. (b) Very fine-grained prisms on the surface of a prism of ordered boralsilite. (c) An aggregate of disordered boralsilite. (d) A close-up of an aggregate of disordered boralsilite. (e) Aggregate of "boron-mullite." (f) Same aggregate as e but magnified.

View larger version (18K): [in this window] [in a new window]   FIGURE 6. (a) Compositions of synthetic boralsilite from runs B1 (ordered, filled symbols) and W2 (ordered, unfilled symbols). Gray-filled symbols represent the composition of W2 refined by the Rietveld method (assuming that only Al replaces B at the B2 site). (b) Compositions of boralsilite in samples 121502E, 020602A, 121102B, 121102C, and holotype (8812905-1 and -2) from the Larsemann Hills (Type I). Open squares indicate ideal composition Al16B6Si2O37. The lines indicate the ideal substitutions.

View larger version (22K): [in this window] [in a new window]   FIGURE 7. Compositions of "boron-mullite" synthesized from gels at 1050 °C and 1 bar (B4, W4, this study) and at 930 °C and 1 bar (3:1 gel, Letort 1952) plus compositions of disordered boralsilite (B3) in mol% oxide. Trend line is based on least-squares fit to the nine points for "boron-mullite" B4 and W4.

View larger version (59K): [in this window] [in a new window]   FIGURE 8. Back-scattered electron images of borosilicates in metapelites from Mount Stafford, central Australia. (a) "Boron-mullite" replacing werdingite in a cordierite matrix. Light grains are hercynite (subequant) and ilmenite (tabular). Sample 95-175D. (b) Banded "boron-mullite" has replaced werdingite adjacent to andalusite. Werdingite shows indistinct zoning. Sample MST1002. (c) Banded "boron-mullite" has replaced werdingite. Sample MSTgran. (d) Banded "boron-mullite" has replaced werdingite. Lightest bands (S) approach sillimanite in composition. Sample MST1002. Abbreviations: A = andalusite, BM = "boron-mullite", C = cordierite, H = hercynite, S = sillimanite-like, W = werdingite.

View larger version (21K): [in this window] [in a new window]   FIGURE 9. Compositions of "boron-mullite" in B-rich metapelitic rocks from Mount Stafford, central Australia. Plotted data are the results of individual analyses on three or four grains in each of the three sections; only analyses totaling 98–102 wt% and containing less than 0.5 wt% MgO and 1.2 wt% FeO, i.e., negligible werdingite impurity, are plotted. Ratios refer to Al2O3: SiO2 (mol) in mullite. Sil = sillimanite. Lines are least-squares fits to the data.

View larger version (56K): [in this window] [in a new window]   FIGURE 10. Geologic maps of B-rich metamorphic rocks in the Stornes Peninsula (modified from Carson and Grew 2007) showing localities for boralsilite collected during the 2003–2004 season (open circles; GPS coordinates are given in Appendix Table 3). The holotype sample was collected near locality 121102 (Douglas Thost, personal communication, 1991). Boralsilite was identified using optical microscopy or the electron microprobe except 122005. D4 indicates the one locality where borasilite occurs in second-generation pegmatite veins; the veins at 122005 could also be D4. Prs = prismatine; Tur = tourmaline, Qtz-Fsp = quartzofeldspathic. The order in which the metasedimentary units are listed does not correspond to relative depositional age, which remains indeterminate.

View larger version (145K): [in this window] [in a new window]   FIGURE 11. Photographs of mineral associations in pegmatites from the Larsemann Hills, East Antarctica. (a) Graphic tourmaline-quartz intergrowth (black and white) in coarse-grained red microcline, D2-D3 pegmatite at locality 113001, about 100 m SW of locality 112906. Lens cap is 5 cm in diameter. (b) Photomicrograph of twinned prisms of boralsilite overgrown by secondary tourmaline, which is at extinction. Cross-polarized light, sample 120902G. (c) Radiating fibrous bundle of boralsilite (stained yellow above pointer) adjacent to graphic tourmaline-quartz intergrowth in a D2-D3 pegmatite at locality 121102. (d) Photomicrograph of acicular, partially altered boralsilite adjacent to a graphic tourmaline-quartz intergrowth. Boralsilite is largely engulfed in olive and blue secondary tourmaline. Diameter of section is 2.5 cm. Plane polarized light, sample 120405D. (e) Photomicrograph of aggregates of boralsilite in quartz near grandidierite. Secondary andalusite encloses boralsilite (A+B). Pyrite is partially replaced by oxide. Plane-polarized light, sample 112906B. Abbreviations: A = andalusite, B = boralsilite, G = grandidierite, P = pyrite, Q = quartz, T = tourmaline.

View larger version (163K): [in this window] [in a new window]   FIGURE 12. Images of mineral associations in the pegmatite from locality 121102. (a) Photomicrograph of graphic tourmaline-quartz intergrowth, prismatine and boralsilite with quartz (not marked) in section 121102C2, plane light. Matrix is plagioclase. (b) Enlargement of the center of (a), but taken with section in a different orientation, which resulted in a difference in the colors of prismatine and tourmaline. (c) Photomicrograph of sillimanite with fringes of werdingite and boralsilite in section 121102B, plane-polarized light. Dark-blue tourmaline is replacing grandidierite. Matrix is plagioclase. (d) Photomicrograph of sillimanite with fringes containing werdingite, boralsilite, and grandidierite in section 121102B, plane light. Matrix is plagioclase (f); area in back-scattered electron image. (e) Back-scattered electron image of a fringe of boralsilite around sillimanite in section 121102B. Matrix is plagioclase with minor quartz (not marked). (f) Back-scattered electron image of a portion of fringe in (d) showing intergrowth of boralsilite, werdingite and grandidierite, and a tiny grain of apatite. Abbreviations: A = apatite, B = boralsilite, G = grandidierite, Pl = plagioclase, Prs = prismatine, Q = quartz, T = tourmaline, W = werdingite.

View larger version (48K): [in this window] [in a new window]   FIGURE 13. Back-scattered electron images of boralsilite in pegmatites from the Larsemann Hills, East Antarctica. (a) Boralsilite (type I) shows faint banding parallel to prism length. Sample 010602A. (b) Acicular boralsilite (type II) enclosed in quartz surrounded by feldspars. Sample 010205J4. (c) Aggregate of boralsilite prisms (type I) showing indistinctly lighter cores. Sample 121502E. (d) Aggregate of boralsilite prisms (type 2) showing patchy light areas. Sample 112906B. Abbreviations: B = boralsilite, K = K-feldspar, Pl = plagioclase, Q = quartz, T = tourmaline.

View larger version (18K): [in this window] [in a new window]   FIGURE 14. (a) Compositions of boralsilite in samples 112906B and 010205J4 from the Larsemann Hills (Type II). (b) Compositions of boralsilite in samples HE138B2 and HE138B3 from Almgjotheii, Rogaland Complex, SW Norway. Squares indicate ideal composition Al16B6Si2O37. The lines indicate ideal substitutions. Three Si per 37 O corresponds to 50% solid solution of an ideal werdingite component (Fe,Mg)2Al14B4Si4O37.