Martin G. Yates

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.

Boralsilite (Al 16 B 6 Si 2 O 37 ); a new mineral related to sillimanite from pegmatites in granulite-facies rocks

Single crystals of MnSiO(SiO4) with the titanite structure together with MnSiO3 clinopyroxene were synthesized from a MnO-SiO2 oxide mixture at 1000 8C and 9.2 GPa in a multi-anvil press. The crystal structure of MnSi2O5 [space group C2/c, a 5 6.332(1) Å, b 5 8.161(1) Å, c 5 6.583(1) Å, b 5 114.459(3)8, and V 5 309.66 Å3] was refined at room temperature from single-crystal X-ray data to R1 5 2.23%. The monoclinic MnSi2O5 phase has the titanite aristotype structure and is similar to the monoclinic Ca-analogue CaSi2O5. Si occurs in compressed octahedral coordination, replacing Ti in titanite, and in tetrahedral coordination as an orthosilicate group. Mn has a distorted sevenfold coordination with MnO distances between 2.086 and 2.365 Å.

Boromullite, Al9BSi2O19, a new mineral from granulite-facies metapelites, Mount Stafford, central Australia: a natural analogue of a synthetic "boron-mullite"

Boromullite is a new mineral corresponding to a 1:1 polysome composed of Al5BO9 and Al2SiO5 modules. Electron-microprobe analysis of the holotype prism is SiO2 19.01(1.12), TiO2 0.01(0.02), B2O3 6.52(0.75), Al2O3 74.10(0.95), MgO 0.07(0.03), CaO 0.00(0.02), MnO 0.01(0.04), FeO 0.40(0.08), Sum 100.12 wt.%, which gives Mg0.01Fe0.03Al8.88Si1.93B1.14O18.94 (normalised to 12 cations), ideally Al9BSi2O19. Overall, in the type specimen, it ranges in composition from Mg0.01Fe0.03Al8.72Si2.44B0.80O19.20 to Mg0.01Fe0.03Al9.22Si1.38B1.35O18.67. Single-crystal X-ray diffraction gives orthorhombic symmetry, Cmc21, a 5.7168(19) Å, b 15.023(5) Å, c 7.675(3) Å, V 659.2(7) Å3, calculated density 3.081 g/cm3, Z = 2. The refined structure model indicates two superimposed modules present in equal proportions in the holotype prism. Module 1 has the topology and stoichiometry of sillimanite and carries all the Si, whereas module 2 is a type of mullite defect structure in which Si is replaced by B in triangular coordination and by Al in tetrahedral coordination, i.e., Al5BO9. The strongest lines in the powder pattern [d in Å, (Imeas.), (hkl)] are 5.37(50) (021), 3.38(100) (022, 041), 2.67 (60) (042), 2.51(60) (221, 023), 2.19(80) (222), 2.11(50) (043), 1.512(80) (263). Boromullite is colourless and transparent, biaxial (+), nx 1.627(1), ny 1.634(1), nz 1.649(1) (589 nm). 2Vz (meas) = 57(2)◦, 2Vz (calc) = 69(12)◦. In the type specimen boromullite tends to form prisms or bundles of prisms up to 0.4 mm long, typically as fringes or overgrowths on aggregates of sillimanite or as narrow overgrowths around embayed werdingite prisms. In other samples boromullite and sillimanite are intergrown on a fine scale (from < 1 μm to > 10 μm). Sekaninaite-cordierite, potassium feldspar, biotite, werdingite and its Fe-dominant analogue, hercynite, and ilmenite are other commonly associated minerals, whereas ominelite-grandidierite, plagioclase, andalusite, and tourmaline are much subordinate. The most widespread accessories are monazite-(Ce), an apatite-group mineral and zircon. Boromullite formed during anatexis of B-rich pelitic rocks under granulite facies conditions (810 ◦C ≈ T ≥ 775−785 ◦C, P = 3.3–4 kbar), possibly due to a shift in bulk composition to lower SiO2 and B2O3 contents associated with melt extraction. The assemblage boromullite + cordierite + sillimanite lies at lower SiO2 and B2O3 contents than the assemblage werdingite + cordierite + sillimanite and thus a decrease in SiO2 and B2O3 leads to the replacement of werdingite by boromullite, consistent with textural relations. Key-words: boron, new mineral, Australia, electron microprobe, crystal structure, granulite facies, anatexis, boromullite.

Stornesite-(Y), (Y, Ca)□2Na6(Ca,Na)8(Mg,Fe)43(PO4)36, the first terrestrial Mg-dominant member of the fillowite group, from granulite-facies paragneiss in the Larsemann Hills, Prydz Bay, East Antarctica

Abstract Stornesite-(Y), end-member formula Y⃞2Na6(Ca5Na3)Mg43(PO4)36, is a new Y-dominant analog of the meteoritic mineral chladniite. A representative electron microprobe analysis is SiO2 = 0.02, P2O5 = 48.11, SO3 = 0.05, MgO = 23.16, MnO = 0.24, FeO = 15.55, Na2O = 5.04, CaO = 5.66, SrO = 0.02, Y2O3 = 1.43, Yb2O3 = 0.24, UO2 = 0.01, Sum = 99.53 wt%, which gives Y0.68Yb0.06Na8.69Ca5.40Sr0.01Mg30.71Fe11.56 Mn0.18Si0.02S0.04P36.22O144. Overall, Y + REE range from 0.542 to 0.985 atoms per formula, and atomic Mg/(Mg + Fe) ratio from 0.684 to 0.749. Single-crystal X-ray diffraction gives trigonal symmetry, R3̅, a = 14.9628(27) Å, c = 42.756(11) Å, V = 8290(4) Å3, calculated density = 3.196 g/cm3, Z = 3. The mineral is isostructural with synthetic chladniite, but the (0, 0, 0) site is dominantly occupied by Y instead of Ca. Bond lengths are considerably shorter than for Ca sites; Y and Yb are fully ordered at this site, which is our rationale for recognizing stornesite-(Y) as a distinct species. The strongest lines in the powder pattern [d in Å, (I), (hkl)] are 3.67 (40) (0 3 6, 3 0 6), 3.52 (40) (0 0 12, 3 1 2, 1 3 2̅), 2.94 (60) (0 1 14, 3 2 2̅, 2 3 2), 2.73 (100) (2 0 14, 0 3 12, 3 0 12), 1.84 (40) (1 5 14, 5 11̅4̅ , 0 6 12, 6 0 12). The mineral is optically uniaxial +, nω = 1.6215(10) and nε = 1.6250(10) at 589 nm. Its color is pale yellow in standard thin sections. Stornesite-(Y) is found as inclusions in fluorapatite nodules in two paragneiss specimens from Johnston Fjord, Stornes Peninsula (whence the name) and in a third from Brattnevet, Larsemann Hills. Associated minerals are wagnerite, xenotime-(Y), monazite-(Ce), P-bearing K-feldspar, biotite, sillimanite, quartz, and pyrite; it is commonly altered to rusty material and secondary phosphates. Grains are anhedral, subhedral, or locally euhedral with hexagonal or rhombic outlines; maximum dimensions are 1 × 0.25 mm. It is inferred to have formed at 800.860 °C, 6.7 kbar by reaction of biotite with an anatectic melt locally enriched in P by interaction with fluorapatite.

Multi-technique geochemical characterization of the Alca obsidian source, Peruvian Andes

We report results from comprehensive mapping and multi-technique geochemical characterization of obsidian from the Alca source in the Peruvian Andes (15.3°S, 72.7°W), aimed at understanding patterns of extraction and trade in one of the world’s centers of complex civilization. Alca obsidian was among the most economically important and widely distributed volcanic glasses used for stone tool making in South America from ca. 13 ka until recently, yet the geologic source of this material has never been studied comprehensively. Our work establishes Alca as one of the largest obsidian sources in South America and the only Peruvian source known to have patterned intrasource geochemical variability. There are six geochemically distinct Alca subsources exposed from 2710 to 5165 m elevation over >330 km 2 of the highlands, an area nearly seven times larger than previously known. Our results now permit provenance determination of artifacts to specific outcrops. We analyzed 252 geologic samples using energy-dispersive X-ray fluorescence, neutron activation analysis, wavelength-dispersive X-ray fluorescence, and nondestructive, portable energy-dispersive X-ray fluorescence. All techniques distinguish the same six Alca subsources, establishing analytical comparability between geochemical methods. Discrimination of Alca obsidian to the subsource level using portable X-ray fluorescence represents a major advance in nondestructive provenance analysis. Further nondestructive analysis of robust sets of obsidian artifacts within Peru will allow high-resolution study of the evolution of central Andean exchange systems.

Chopinite, [(Mg, Fe)3□](PO4)2, a new mineral isostructural with sarcopside, from a fluorapatite segregation in granulite-facies paragneiss, Larsemann Hills, Prydz Bay, East Antarctica

Chopinite, the Mg-dominant analogue of sarcopside, is a new mineral corresponding to synthetic Mg 3 (PO 4 ) 2 -II, a high-pressure polymorph of the meteoritic mineral farringtonite. A representative electron-microprobe analysis is SiO 2 0.32, P 2 O 5 47.32, Al 2 O 3 0.05, MgO 30.35, MnO 0.15, FeO 20.99, CaO 0.35, F 0.02, Cl 0.01, Sum 99.54 wt%, which gives Ca 0.02 Mg 2.20 Fe 0.86 Mn 0.01 Si 0.02 P 1.95 O 8 . Single-crystal X-ray diffraction gives monoclinic symmetry, P 2 1 /c, a = 5.9305(7) A, b = 4.7583(6) A, c = 10.2566(10) A, = β 90.663(9)°, V 289.41(6) A 3 , calculated density 3.34 g/cm 3 , Z = 2. Chopinite is of the olivine structure type, but with ordered vacancies and strongly distorted octahedra due to the valence 5+ for P, which results in marked ordering of Mg at M2, whereas Fe 2+ concentrates at M1, most likely because of its axial symmetry. The strongest lines in the powder pattern [ d in A, ( I calc ), ( hkl )] are 5.92 (42) (100), 3.84(100) (102), 3.48(52) (111, 012, 111), 2.51(72) (113, 113), 2.44 (73) (211, 211). Chopinite is colorless and transparent, biaxial (−), α 1.595(2), β1.648(2), γ1.656(2) (589 nm). 2V x (meas.) = 40(2)°, 2V x (calc.) = 41°; X // b , Z ^ a ~55°. Chopinite is found as four inclusions isolated in a fluorapatite segregation in a quartz mass in a paragneiss from Brattnevet, Larsemann Hills, East Antarctica. Grains are mostly anhedral and range from 0.1 × 0.3 mm to 0.2 × 0.6 mm in size. Minerals present in the chopinite-bearing specimen include wagnerite- Ma 5 bc , xenotime-(Y), stornesite-(Y), P-bearing K-feldspar and plagioclase, Ti-rich biotite, sillimanite, orthopyroxene, sapphirine, hercynite, and corundum. It is inferred to have formed as a result of high melt P concentrations by reaction of biotite with an anatectic melt in which P/Ca ratio exceeded that buffered by apatite saturation due to the very slow diffusion of P relative to Ca in anatectic melt.

Hyalotekite, (Ba,Pb,K)(4)(Ca,Y)(2)Si-8(B,Be)(2)(Si,B)(2)O28F, a Tectosilicate Related to Scapolite: New Structure Refinement, Phase Transitions and a Short-Range Ordered 3B Superstructure

Abstract Hyalotekite, a framework silicate of composition (Ba,Pb,K)4(Ca,Y)2Si8(B,Be)2 (Si,B)2O28F, is found in relatively high-temperature (≥ 500°C) Mn skarns at Långban, Sweden, and peralkaline pegmatites at Dara-i-Pioz, Tajikistan. A new paragenesis at Dara-i-Pioz is pegmatite consisting of the Ba borosilicates leucosphenite and tienshanite, as well as caesium kupletskite, aegirine, pyrochlore, microcline and quartz. Hyalotekite has been partially replaced by barylite and danburite. This hyalotekite contains 1.29-1.78 wt.% Y2O3, equivalent to 0.172-0.238 Y pfu or 8-11% Y on the Ca site; its Pb/(Pb+Ba) ratio ranges 0.36-0.44. Electron microprobe F contents of Långban and Dara-i-Pioz hyalotekite range 1.04-1.45 wt.%, consistent with full occupancy of the F site. A new refinement of the structure factor data used in the original structural determination of a Långban hyalotekite resulted in a structural formula, (Pb1.96Ba1.86K0.18)Ca2(B1.76Be0.24)(Si1.56B0.44)Si8O28F, consistent with chemical data and all cations with positive-definite thermal parameters, although with a slight excess of positive charge (+57.14 as opposed to the ideal +57.00). An unusual feature of the hyalotekite framework is that 4 of 28 oxygens are non-bridging; by merging these 4 oxygens into two, the framework topology of scapolite is obtained. The triclinic symmetry of hyalotekite observed at room temperature is obtained from a hypothetical tetragonal parent structure via a sequence of displacive phase transitions. Some of these transitions are associated with cation ordering, either Pb-Ba ordering in the large cation sites, or B-Be and Si-B ordering on tetrahedral sites. Others are largely displacive but affect the coordination of the large cations (Pb, Ba, K, Ca). High-resolution electron microscopy suggests that the undulatory extinction characteristic of hyalotekite is due to a fine mosaic microstructure. This suggests that at least one of these transitions occurs in nature during cooling, and that it is first order with a large volume change. A diffuse superstructure observed by electron diffraction implies the existence of a further stage of short-range cation ordering which probably involves both (Pb,K)-Ba and (BeSi,BB)-BSi.

The role of Fe and cation order in the crystal chemistry of surinamite, (Mg,Fe2+)3(Al,Fe3+)3O[AlBeSi3O15]: A crystal structure, Mössbauer spectroscopic, and optical spectroscopic study

Abstract The crystal structure of surinamite from “Christmas Point,” Enderby Land, Antarctica, has been newly refined by single-crystal X-ray diffraction: VI(Mg2.26Fe2+0.74Fe3+0.39Al2.61)OIV(Al1.00Be1.00Si3.00)O15 (simplified formula), space group P2/n, a = 9.915(2), b = 11.368(2), c = 9.617(2), b = 109.30(2)°, Z = 4, wR(F2) = 0.074 for 4876 independent reflections. The refined site occupancies agree well with the chemical composition determined by electron microprobe analysis and with the Fe3+/Fe2+ ratio estimated from stoichiometry. The surinamite structure is characterized by an ordered Al/Be/Si distribution on the tetrahedral sites and by charge ordering with extensive Mg2+-Fe2+ and Al3+-Fe3+ exchange on the octahedral sites. This cation ordering is distinct from that observed in related phases of the sapphirine series and aenigmatite groups, and the difference is linked to the unique structural topology of the tetrahedral chains in surinamite. Optical and Mössbauer spectra of surinamite have been fully interpreted in terms of an octahedral distribution of Fe2+ and Fe3+ cations that agrees very well with the X-ray site populations. Both the X-ray and Mössbauer data establish the absence of significant tetrahedral Fe and the non-uniform distribution of octahedral Fe. Intense and strongly polarized absorption bands caused by IVCT-processes in clusters of Fe3+ and Fe2+ iron in edgesharing octahedral sites produce the unusual color and pleochroism observed in surinamite.

Prismatine and Ferrohogbomite-2N2S in Granulite-Facies Fe-Oxide Lenses in the Eastern Ghats Belt at Venugopalapuram, Vizianagaram District, Andhra Pradesh, India: Do Such Lenses Have a Tourmaline-Enriched Lateritic Precursor?

Abstract Fluorine-rich prismatine, (⃞, Fe, Mg)(Mg, Al, Fe)5Al4(Si, B, Al)5O21(OH, F), with F/(OH+F) = 0.36-0.40 and hercynite are major constituents of a Fe-Al-B-rich lens in ultrahigh-temperature granulite-facies quartz-sillimanite-hypersthene-cordierite gneisses of the Eastern Ghats belt, Andhra Pradesh5 India. Hemo-ilmenite, sapphirine, magnetite, biotite and sillimanite are subordinate. Lithium5 Be and B are concentrated in prismatine (140 ppm Li, 170 ppm Be, and 2.8-3.0 wt. % B2O3). Another Fe-rich lens is dominantly magnetite, which encloses fine-grained zincian ferrohögbomite-2N2S, (Fe2+,Mg, Zn, Al)6 (Al,Fe3+,Ti)16O30(OH)2, containing minor Ga2O3 (0.30-0.92 wt.%). Fe-Al-B-rich lenses with prismatine (or kornerupine) constitute a distinctive type of B-enrichment in granulite-facies rocks and have been reported from four other localities worldwide. A scenario involving a tourmaline- enriched lateritic precursor affected by dehydration melting is our preferred explanation for the origin of the Fe-Al-B-rich lenses at the five localities. Whole-rock analyses and field relationships at another of these localities, Bok se Puts5 Namaqualand, South Africa, are consistent with this scenario. Under granulite-facies Conditions5 tourmaline would have broken down to give kornerupine-prismatine (±other borosilicates) plus a sodic melt containing H2O and B. Removal of this melt depleted the rock in Na and B5 but the formation of ferromagnesian borosilicate phases in the restite prevented total loss of B.

Hyttsjoite, a new complex layered plumbosilicate with unique tetrahedral sheets from Langban, Sweden

Abstract Hyttsjöite, circa Pb18Ba2Ca5Mn22+Fe23+Si30O90Cl · 6H2O, is a new mineral from the Långban mines, Filipstad district, Värmland, Sweden. It occurs sparingly in Mn-rich skarn with andradite, hedyphane, aegirine, rhodonite, melanotekite, calcite, quartz, potassium feldspar, pectolite, and barite. It is inferred to have formed below 300°C at 2-4 kbar from the breakdown of medium-temperature metamorphic assemblages in which hedyphane was the principal reactant. The name is from Hyttsjön, which is a lake situated west of the Långban mines. The average analysis is SiO2 26.38, Al2O3 0.02, Fe2O3 2.77, CaO 4.35, MnO 2.00, PbO 58.13, BaO 4.65, Cl 0.65, H2O (calc) 1.58,O = Cl -0.15, total 100.40 wt%. Hyttsjöite grains are mostly 0.1-0.6 mm across, subequant to skeletal in outline, and colorless, and they have a prominent {001} cleavage. Optically, the mineral is uniaxial (-), ω = 1.845(4), ε ≈ 1.815.It is trigonal, space group R3̅, a = 9.865(2),c = 79A5(1)Å, v= 6695(3) Å3, and Z = 3; Dcalc= 5.10 g/cm3. The most intense X-ray (FeKa λ= 1.9373 Å) powder diffraction lines are as follows [d(Å)(I)(hkl)]: 13A(50)(006), 443(30)(0.0.18), 3.98(30)(027,1.1.12), 3.32(100)( 1.0.22,0.0.24,1.1.18), 3.11 (40)(217,128), 2.969(40)(2.1.10,0.2.19,1.0.25,1.2.11,0.0.27), and 2.671(80)(1.0.28,2.0.23,1.2.17). The crystal structure consists of composite SiO4 + PbOn layers of two kinds (L1 and L2), which are supported by column segments parallel to c, the columns being composed of facesharing CaO9, FeO6, BaO12, and MnO6 polyhedra. Each layer has two sheets of SiO4 tetrahedra in pinwheel-like modules, joined together to form a puckered planar network filled out by PbOn groups. Layer L1 is continuous (Si8O23), with intralayer Mn, whereas L2 is discontinuous (Si7O22), with intralayer Fe and Ca. L1 and L2 are distinct from layers in other layered silicates, although they show some similarities to layers in minerals of the gyro lite group.