Bityite

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.[4] Bityite has a close association with beryl, and it generally crystallizes in pseudomorphs after it, or in cavities associated with reformed beryl crystals.[5] The mineral is considered a late-stage constituent in lithium bearing pegmatites,[6] and has only been encountered in a few localities throughout the world. The mineral was named by Lacroix[7] after Mt. Bity, Madagascar from where it was first discovered.

Bityite
General
CategoryPhyllosilicate
Mica group
Formula
(repeating unit)		CaLiAl2(AlBeSi2)O10(OH)2
Strunz classification9.EC.35
Dana classification71.02.02c.03
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/c
Identification
ColorPearly white, grayish blue, greenish blue, light brown
Crystal habitDense, micaceous aggregates or rosettes and encrustations
CleavagePerfect micaceous on {001}
FractureUneven
Mohs scale hardness3
LusterVitreous, pearly on cleavages
DiaphaneityTransparent to translucent
Specific gravity5.5
Optical propertiesBiaxial (-)
Refractive indexnα = 1.651 nβ = 1.659 nγ = 1.661
Birefringenceδ = 0.010
2V angleMeasured: 35° to 52°
DispersionStrong
References[1][2][3]

Geologic occurrence

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]

Chemical composition

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 Ca1.19K0.03Na0.02(Li1.19Al3.68Mg0.35Fe0.13)5.35(Al1.53Be2.21Si4.26)8O19.30(OH)4.54F0.16.[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.

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.[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]

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;[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 Å3.[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]

References
  1. http://rruff.geo.arizona.edu/doclib/hom/bityite.pdf Handbook of Mineralogy
  2. http://webmineral.com/data/Bityite.shtml Webmineral data
  3. http://www.mindat.org/min-689.html Mindat.org
  4. Strunz, H. (1956) Bityit, ein berylliumglimmer. Zeitschrift für Kristallographie, 107, 325-330.
  5. Lahti, S. I. and Saikkonen, R. (1985) Bityite 2M1 from Eräjärvi compared with related Li-Be brittle micas. Bulletin of the Geological Society of Finland, 57, 207-215.
  6. Lin, J-C. and Guggenheim, S. (1983) The crystal structure of a Li,Be-rich brittle mica: a dioctaheral-trioctahedral intermediate. American Mineralogist, 68, 130-142.
  7. Lacroix, A. (1908) Les minéraux de felons de pegmatite à tourmaline lithique de Madagascar. Bulletin de la Société de Française et de Minéralogie, 31, 218-247
  8. Rowledge, H.P. and Hayton, J.D. (1947) Two new beryllium minerals from Londonderry. Journal and Proceedings of the Royal Society of Western Australia, 33, 45-52.
  9. Gallagher, M.J. and Hawkes, J.R. (1966) Beryllium minerals from Rhodesia and Uganda. Bulletin of the Geological Survey of Great Britain, 25, 59-75.
  10. Anthony, J.W., Bideaux, R., Bladh, K., and Nichols, M. (2003) Bityite CaLiAl2(AlBeSi2)O10(OH)2 Handbook of Mineralogy, Mineral Data Publishing (Republished by the Mineralogical Society of America).*link to bityite
  11. Deer, W.A, Howie, R. A., and Zussman, J. (1963) Rock-Forming Minerals, Volume 3, Sheet Silicates. Wiley, New York.
  12. Guggenheim, S. (1984) The brittle micas. Reviews in Mineralogy, 13, 61-104.
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