A POTENTIALLY NEW BOROSILICATE MINERAL, CA. AL16B4SI4O38, RELATED TO BORALSILITE FROM THE MANJAKA PEGMATITE, SAHATANY VALLEY, MADAGASCAR

The system B₂O₃-Al₂O₃-SiO₂ (BAS) includes two ternary phases occurring naturally, boromullite, Al₉BSi₂O₁₉, and boralsilite, Al₁₆B₆Si₂O₃₇, as well as synthetic compounds structurally related to mullite such as Al₁₆B₄Si₄O₃₈. A potentially new anhydrous ternary BAS mineral structurally akin to boralsilite, but distinct in composition, occurs with albite and K-feldspar as a breakdown product of spodumene in the elbaite-subtype Manjaka granitic pegmatite, Sahatany Valley, Madagascar, where it forms rare intergrowths of subparallel prisms up to 100 μm long. Optically, the mineral is biaxial (–), α = 1.607(1), β = 1.634(1), γ = 1.637(1) (white light), 2V_x(calc.) = 36.4°, X ≈ c; Y ≈ a; Z = b. An averaged analysis by EMP and LA-ICP-MS (Li, Be) gives (wt%) SiO₂ 20.24, B₂O₃ 11.73, Al₂O₃ 64.77, BeO 1.03, MnO 0.01, FeO 0.13, Li₂O 1.40, Sum 99.29. Raman spectroscopy in the 3000–4000 cm⁻¹ region rules out the presence of significant OH or H₂O. The mineral is monoclinic, space group I2/m, a = 10.3832(12), b = 5.6682(7), c = 10.8228(12) Å, β = 90.106(11)°; V = 636.97(13) ų, Z = 1. In the structure, R₁ = 0.0416 for 550 Fo > 4σ(Fo), AlO₆ octahedral chains //[010] are cross-linked by Si₂O₇ disilicate groups, BO₃ triangles, and chains of AlO₄ and two AlO₅ polyhedra. The last, Al4 and Al5, cannot be occupied simultaneously; the refinement gives 54% and 20% occupancy, respectively. Bond valence sums for Al4 suggest Li is likely to be sited here, whereas Be could be at Al5. One of the 9 O sites is only 20% occupied; this O9 site completes the coordination of Al5 and also represents the 4th corner of a partially occupied BO₄ tetrahedron, in which case the B site is shifted out of the plane of the BO₃ triangle. If all sites were filled (Al4 and Al5 to 50%), the formula becomes Al₁₆B₄Si₄O₃₈, close to Li_1.08Be_0.47Fe_0.02Al_14.65B_3.89Si_3.88O_36.62 calculated from the analyses assuming cations sum to 24. The compatibility index based on the Gladstone – Dale relationship is 0.001 (“superior”). Boralsilite and the new mineral are inferred to have grown in disequilibrium conditions resulting from a combined pressure+chemical quench. Since Li contents of the new mineral average 7 times that of boralsilite, Li released by spodumene breakdown could have created a suitable environment for a second boralsilite-like mineral to crystallize.