Synthesis of beryllian sapphirine in the system MgO-BeO-Al 2 O 3 -SiO 2 -H 2 O and comparison with naturally occurring beryllian sapphirine and khmaralite, part 2: A chemographic study of Be content as a function of P , T , assemblage and FeMg -1 exchange

Beryllium is a significant constituent in sapphirine in some metamorphic and pegmatitic rocks, and thus could have a major effect on its stability relationships. Using the stoichiometries of reactions involving sapphirine and associated phases in the MgO-BeO-Al2O3-SiO2 (MBeAS) system in conjunction with molar volume data, we have plotted maps of the sapphirine solid-solution field in both μ-μ and μ-P space, where μ is the chemical potential of an exchange component such as (BeSi)(AlAl)-1. These maps give a pressure sequence of stable MBeAS univariant reactions and divariant assemblages that are consistent with experimental data, e.g., they show how Be stabilizes sapphirine + forsterite, which is rare in nature but readily synthesized over a wide P-T range in the presence of Be. We generate a MBeAS petrogenetic grid for sapphirine-bearing assemblages over the approximate range T = 700-900 °C, P = 0-2.5 GPa, identify divariant and univariant assemblages containing sapphirine with maximum Be, and determine the sense of variation of maximum Be content with P. At lower T, maximum Be occurs at the low-P limit of surinamite stability, ca. 0.5 GPa. At higher T, maximum Be increases with P, following the MBeAS univariant reactions involving (sapphirine + surinamite + orthopyroxene + chrysoberyl + forsterite or spinel). Natural assemblages containing sapphirine and its Be-rich near-analog khmaralite from the Napier Complex, Enderby Land, East Antarctica formed at higher T (900-1100 °C) than the experiments and in bulk compositions containing substantial Fe. Associated minerals include garnet, sillimanite, quartz, and magnesiotaaffeite-6N′3S ("musgravite"), whereas forsterite is absent and cordierite is a local, late phase. μ(BeSi)(AlAl)-1-μFeMg-1 diagrams show that the stability of magnesiotaaffeite-6N′3S causes the maximally beryllian khmaralite to shift from a magnesian composition in equilibrium with orthopyroxene + surinamite + forsterite + chrysoberyl, as in the MBeAS subsystem, to a more Fe-rich composition associated with garnet + surinamite + magnesiotaaffeite-6N′3S + chrysoberyl. Khmaralite associated with sillimanite + garnet + surinamite + magnesiotaaffeite-6N′3S or chrysoberyl in a Napier Complex pegmatite from Khmara Bay is predicted to be the most Be-rich possible in the presence of sillimanite, whereas the sillimanite + quartz ± orthopyroxene ± garnet associations in quartz granulites requires a sapphirine much lower in both Be and Fe: analyses are roughly in accord with these predictions. The shape of the sapphirine/khmaralite solid-solution field is such that there is a positive correlation between high Be and high Fe2+, a chemographic effect independent of any crystal chemical effects due to the clustering of Fe and Be in the crystal structure of khmaralite. The diagram for FMBeAS shows that sapphirine + quartz, which is often cited as evidence for ultrahigh temperatures (e.g., ≥ 1040 °C), is stabilized to lower T and higher P than in the corresponding Be-free system. Hence, this minimum T may be valid only in rocks with relatively abundant sapphirine and/or very low bulk Be content so that what Be is present in the system is not concentrated in sapphirine

The crystal-chemistry of aenigmatite revisited: electron microprobe data, structure refinement, and Mössbauer spectroscopy of aenigmatite from Vesterøya (Norway)

The structure of an untwinned aenigmatite crystal from Vesterøya, Vestfold district, Norway, having a composition (Na3.73 Ca0.27)∑=4.00 (Fe2+8.55 Ti2.10 Mg0.46 Fe3+0.40 Mn0.40 Ca0.06 Zn0.01 Zr0.01) ∑=11.99 (Si11.10 Al0.64 Fe3+0.26) ∑=12.00 O40 was refined in space group P1 to give R1 0.0277 for 5515 unique reflections with Fo > 4σ(Fo) and 0.0324 for all 6145 unique data; GooF 1.124. Cell parameters are a = 10.415(1), b = 10.840(1), c = 8.931(1) Å, α = 105.107(4), β = 96.610(5), γ = 125.398(4)° and V = 746.8(1) Å3. Mössbauer spectroscopy of a bulk sample gave Fe3+/Fetot = 0.12(1) whatever was the initial fitting model, and IVFe3+/Fetot = 0.04(2), which is consistent with IVFe3+/Fetot = 0.018(1) obtained from occupancy of T3, the only T site to have Fe. Ti is fully ordered at M7, the site with the most shared edges, whereas VIFe3+ is mostly ordered at the smallest M sites (M1 and M2), consistent with what is reported in related minerals of the sapphirine group. Ca is ordered at M8, one of the two 8-coordinated sites. Bond lengths and bond-valence sums suggest that T2 and T4 are occupied by Si only, and that T1 and T3 have the highest proportion of Al. Ordering of Ai3+ and Fe3+ at T3 could be related to Ca being ordered at M8, because M8 is linked with T3 through four oxygen anions, more than M8 with any other T site. Thus, one of the four substitutions proposed for aenigmatite, Ca + IVAl ⇔ Na + IVSi, could be operating at a local scale. Since (IVAl + IVFe3+) exceeds Ca, there must be other substitutions for incorporating Al and Fe3+. Two of the four proposed substitutions, VIFe2+ + IVSi ⇔ VIFe3+ + IVAl and VIFe2+ + IVSi ⇔ VIFe3+ + IVFe3+, are consistent with the compositional and Mössbauer data, but the Ti content of 2 per 28 cations rules out incorporation of VIFe3+ in a wilkinsonite component, Na4Fe2+8Fe2+4Si12O40

Recommended nomenclature for the sapphirine and surinamite groups (sapphirine supergroup)

Minerals isostructural with sapphirine-1A, sapphirine-2M, and surinamite are closely related chain silicates that pose nomenclature problems because of the large number of sites and potential constituents, including several (Be, B, As, Sb) that are rare or absent in other chain silicates. Our recommended nomenclature for the sapphirine group (formerly aenigmatite group) makes extensive use of precedent, but applies the rules to all known natural compositions, with flexibility to allow for yet undiscovered compositions such as those reported in synthetic materials. These minerals are part of a polysomatic series composed of pyroxene or pyroxene-like and spinel modules, and thus we recommend that the sapphirine supergroup should encompass the polysomatic series. The first level in the classification is based on polysome, i.e. each group within the supergroup corresponds to a single polysome. At the second level, the sapphirine group is divided into subgroups according to the occupancy of the two largest M sites, namely, sapphirine (Mg), aenigmatite (Na), and rhonite (Ca). Classification at the third level is based on the occupancy of the smallest M site with most shared edges, M7, at which the dominant cation is most often Ti (aenigmatite, rhonite, makarochkinite), Fe3+ (wilkinsonite, dorrite, høgtuvaite) or Al (sapphirine, khmaralite); much less common is Cr (krinovite) and Sb (welshite). At the fourth level, the two most polymerized T sites are considered together, e.g. ordering of Be at these sites distinguishes høgtuvaite, makarochkinite and khmaralite. Classification at the fifth level is based on XMg = Mg/(Mg + Fe2+) at the M sites (excluding the two largest and M7). In principle, this criterion could be expanded to include other divalent cations at these sites, e.g. Mn. To date, most minerals have been found to be either Mg-dominant (XMg > 0.5), or Fe2+-dominant (XMg Si at the two most polymerized T sites vs. Al < Si in sapphirine. Further splitting of the supergroup based on occupancies other than those specified above is not recommended

Boron- and phosphate-rich rocks in the Larsemann Hills, Prydz Bay, East Antarctica: tectonic implications

Granulite-facies paragneisses enriched in boron and phosphorus are exposed over c. 15 × 5 km2 in the Larsemann Hills, Antarctica. The most widespread are biotite gneisses containing centimetre-sized prismatine crystals, but tourmaline metaquartzite and

Chopinite, [(Mg,Fe)(3)square](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 Mg3(PO4)2- II, a high-pressure polymorph of the meteoritic mineral farringtonite. A representative electron-microprobe analysis is SiO2 0.32, P2O5 47.32, Al2O

Synthesis of beryllian sapphirine in the system MgO-BeO-Al 2 O 3 -SiO 2 -H 2 O and comparison with naturally occurring beryllian sapphirine and khmaralite. Part 1: Experiments, TEM, and XRD.

Beryllian sapphirine synthesized from starting compositions with y ≤ 1 at x = 0 and y ≤ 0.5 at x = 0.5, P = 0.1-3 GPa, T = 700-1350 °C. Electron diffraction shows the sapphirines are dominantly the 1A polytype but lamellae of a 2M phase are consiste