Brittle mica, phyllosilicate mineral
Bityite is considered a rare mineral, and it is an endmember to the margarite mica sub-group found within the phyllosilicate group. The mineral was first described by Antoine François Alfred Lacroix in 1908, and later its chemical composition was concluded by Professor Hugo Strunz.[5] Bityite has a close association with beryl, and it generally crystallizes in pseudomorphs after it, or in cavities associated with reformed beryl crystals.[6] The mineral is considered a late-stage constituent in lithium bearing pegmatites,[7] and has only been encountered in a few localities throughout the world. The mineral was named by Lacroix[8] after Mt. Bity, Madagascar from where it was first discovered.
Geologic occurrence
The first description of bityite was by Lacroix in 1908.[8] and it was discovered on Mt. Bity, Madagascar within a pegmatite named Sahatany field [1]. It was later found in a feldspar quarry from Londonderry, Western Australia[2],[9] and further occurrences have been found from the Middle Urals[3], and three pegmatites in Zimbabwe[4].[10] And most recently, occurrences from the Pizzo Marcio, Val Vigezzo area in Piedmont, Italy[5] have been discovered.[7] 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 [6], southern Finland.[6] The sample from Maantienvarsi occurs in close association with beryl; either in cavities with altered beryl crystals, or as a pseudomorph after beryl.[6] 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.[6] The mineral substitutes into portions of beryl crystals, and is either a hydrothermal alteration product or a late stage magmatic mineral.[7]
Chemical composition
The current chemical formula for bityite is CaLiAl2(AlBeSi2)O10(OH)2.[11] The mineral was analyzed by Lacroix, and concluded to be a new mineral rich with concentrations of lithium and beryllium.[8] In 1947, Rowledge and Hayton discovered a new mineral from Londonderry, Western Australia with a similar chemical composition; they named it bowleyite.[9] However, mineralogical studies performed by Strunz later confirmed that the chemical composition and properties for bowleyite were actually bityite.[7] 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 Ca1.19K0.03Na0.02(Li1.19Al3.68Mg0.35Fe0.13)5.35(Al1.53Be2.21Si4.26)8O19.30(OH)4.54F0.16.[6]
The samples from Mt. Bity, Maantienvarsi, and Londonderry, Western Australia show similar chemical compositions as compared to the computed composition for bityite;[11] the chemical analysis for the three samples and the computed composition are tabulated in the adjacent table.
Structure
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.[7] From studies done on mica flakes from the Maantienvarsi sample, the mineral is a two layer-type modification of polytype 2M1.[6] 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.[12] 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.[7] The transfer is completed by creating a tetrahedral sheet composition of Si2BeAl.[13] 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.[13]
Physical properties
Bityite exhibits a strong pearly luster, and occurs as a fine scaled white yellowish mass which is usually smaller than 0.3mm in diameter;[6] and, its opacity is transparent to translucent.[11] 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.[6] 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 Å3.[6] The refraction indices measured by the immersion method are α = 1.650, β = 1.658, γ = 1.660 with 2V calculation of 52.9°.[6] Bityite’s specific gravity is 3.14, and it has a hardness of 4−4.5 based on Mohs scale of hardness.[11] Bityite’s luster is vitreous and pearly on cleavages, and it has a perfect micaceous cleavage on the {001} miller index.[11] Bityite’s crystal habit can display thin and pseudohexagonal platy crystals.[11]
