Bityite | |
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General | |
Category |
Phyllosilicate Mica group |
Formula (repeating unit) | CaLiAl2(AlBeSi2)O10(OH)2 |
IMA symbol | Bty [1] |
Strunz classification | 9.EC.35 |
Dana classification | 71.02.02c.03 |
Crystal system | Monoclinic |
Crystal class | Prismatic (2/m) (same H-M symbol) |
Space group | C2/c |
Identification | |
Color | Pearly white, grayish blue, greenish blue, light brown |
Crystal habit | Dense, micaceous aggregates or rosettes and encrustations |
Cleavage | Perfect micaceous on {001} |
Fracture | Uneven |
Mohs scale hardness | 3 |
Luster | Vitreous, pearly on cleavages |
Diaphaneity | Transparent to translucent |
Specific gravity | 5.5 |
Optical properties | Biaxial (-) |
Refractive index | nα = 1.651 nβ = 1.659 nγ = 1.661 |
Birefringence | δ = 0.010 |
2V angle | Measured: 35° to 52° |
Dispersion | Strong |
References | [2] [3] [4] |
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.
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]
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.
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]
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]