| Bityite | |
| Category: | Phyllosilicate Mica group |
| Formula: | CaLiAl2(AlBeSi2)O10(OH)2 |
| Imasymbol: | Bty[1] |
| Strunz: | 9.EC.35 |
| Dana: | 71.02.02c.03 |
| System: | Monoclinic |
| Class: | Prismatic (2/m) (same H-M symbol) |
| Symmetry: | C2/c |
| Color: | Pearly white, grayish blue, greenish blue, light brown |
| Habit: | Dense, micaceous aggregates or rosettes and encrustations |
| Cleavage: | Perfect micaceous on |
| Fracture: | Uneven |
| Mohs: | 3 |
| Luster: | Vitreous, pearly on cleavages |
| Diaphaneity: | Transparent to translucent |
| Gravity: | 5.5 |
| Opticalprop: | Biaxial (-) |
| Refractive: | nα = 1.651 nβ = 1.659 nγ = 1.661 |
| Birefringence: | δ = 0.010 |
| 2V: | 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. and it was discovered on Mt. Bity, Madagascar within a pegmatite named Sahatany field http://www.mindat.org/loc-57020.html. It was later found in a feldspar quarry from Londonderry, Western Australiahttp://www.mindat.org/loc-17479.html,[9] and further occurrences have been found from the Middle Uralshttp://www.mindat.org/loc-2645.html, and three pegmatites in Zimbabwehttp://www.mindat.org/loc-21891.html.[10] And most recently, occurrences from the Pizzo Marcio, Val Vigezzo area in Piedmont, Italyhttp://www.mindat.org/loc-25849.html 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 http://www.mindat.org/loc-16490.html, 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. In 1947, Rowledge and Hayton discovered a new mineral from Londonderry, Western Australia with a similar chemical composition; they named it bowleyite. 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. The samples from Mt. Bity, Maantienvarsi, and Londonderry, Western Australia show similar chemical compositions as compared to the computed composition for bityite; 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. 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. 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; and, its opacity is transparent to translucent. 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. 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. The refraction indices measured by the immersion method are α = 1.650, β = 1.658, γ = 1.660 with 2V calculation of 52.9°. Bityite’s specific gravity is 3.14, and it has a hardness of 4−4.5 based on Mohs scale of hardness. Bityite’s luster is vitreous and pearly on cleavages, and it has a perfect micaceous cleavage on the miller index. Bityite’s crystal habit can display thin and pseudohexagonal platy crystals.