Bityite

The first description of bityite was by Lacroix in 1908.[7] and it was discovered on Mt. Bity, Madagascar within a pegmatite named Sahatany field . It was later found in a feldspar quarry from Londonderry, Western Australia,[8] and further occurrences have been found from the Middle Urals, and three pegmatites in Zimbabwe.[9] And most recently, occurrences from the Pizzo Marcio, Val Vigezzo area in Piedmont, Italy have been discovered.[6] The most recent analysis for bityite found in the literature is for a sample from the Maantienvarsi pegmatite dyke in the Eräjärvi area in Orivesi , southern Finland.[5] The sample from Maantienvarsi occurs in close association with beryl; either in cavities with altered beryl crystals, or as a pseudomorph after beryl.[5] The mineral has been found in cavities with perthic microcline, albitic plagioclase, muscovite and tourmaline; the pseudomorphs filled with bityite have been found to contain amounts of fluorite, bertrandite, fluorapatite, quartz and beryl.[5] The mineral substitutes into portions of beryl crystals, and is either a hydrothermal alteration product or a late stage magmatic mineral.[6]

The current chemical formula for bityite is CaLiAl₂(AlBeSi₂)O₁₀(OH)₂.[10] The mineral was analyzed by Lacroix, and concluded to be a new mineral rich with concentrations of lithium and beryllium.[7] In 1947, Rowledge and Hayton discovered a new mineral from Londonderry, Western Australia with a similar chemical composition; they named it bowleyite.[8] However, mineralogical studies performed by Strunz later confirmed that the chemical composition and properties for bowleyite were actually bityite.[6] A recent chemical analysis found in the literature was performed with heavy liquids on a sample of bityite from the Maantienvarsi dyke to derive a computed formula for bityite based on 24 oxygens; the computed chemical formula is Ca_1.19K_0.03Na_0.02(Li_1.19Al_3.68Mg_0.35Fe_0.13)_5.35(Al_1.53Be_2.21Si_4.26)₈O_19.30(OH)_4.54F_0.16.[5]

The samples from Mt. Bity, Maantienvarsi, and Londonderry, Western Australia show similar chemical compositions as compared to the computed composition for bityite;[10] the chemical analysis for the three samples and the computed composition are tabulated in the adjacent table.

The atomic structure derived by X-Ray powder and optical analysis of bityite is that of a two layer modification that also exhibits a complex affinity to twinning.[6] From studies done on mica flakes from the Maantienvarsi sample, the mineral is a two layer-type modification of polytype 2M₁.[5] Bityite has a mica structure, shown in adjacent figure, which consists of tetrahedral and octahedral sheets separated by an interlayer cation. The mineral is considered a brittle mica, and it can be distinguished from the true micas by a layer charge per unit of approximately -2.0; in consequence, their interlayer cation is usually calcium or barium.[11] Bityite’s structure consists of a coupled substitution it exhibits between the sheets of polyhedra; the coupled substitution of beryllium for aluminium within the tetrahedral sites allows a single lithium substitution for a vacancy without any additional octahedral substitutions.[6] The transfer is completed by creating a tetrahedral sheet composition of Si₂BeAl.[12] The coupled substitution of lithium for vacancy and the beryllium for the tetrahedral aluminium maintains all the charges balanced; thereby, resulting in the trioctahedral end member for the margarite sub-group of the phyllosilicate group.[12]

Bityite exhibits a strong pearly luster, and occurs as a fine scaled white yellowish mass which is usually smaller than 0.3mm in diameter;[5] and, its opacity is transparent to translucent.[10] Physical properties analyses conducted with precision photographs using zirconium-filtered molybdenum radiation indicates that bityite exhibits monoclinic symmetry, and is part of the C2/c space group.[5] The unit cell dimensions are a = 4.99 Å, b = 8.68 Å, c = 19.04 Å, β=95.17°, with a volume of 821.33 ų.[5] The refraction indices measured by the immersion method are α = 1.650, β = 1.658, γ = 1.660 with 2V calculation of 52.9°.[5] Bityite’s specific gravity is 3.14, and it has a hardness of 4−4.5 based on Mohs scale of hardness.[10] Bityite’s luster is vitreous and pearly on cleavages, and it has a perfect micaceous cleavage on the {001} miller index.[10] Bityite’s crystal habit can display thin and pseudohexagonal platy crystals.[10]