Ganterite, a new barium-dominant analogue of muscovite from the Berisal Complex, Simplon Region, Switzerland

Ganterite, [Ba0.5(Na,K)0.5]Al2(Si2.5Al1.5O10)(OH)2, the barium-dominant analogue of muscovite, was discovered in the crystalline basement rocks of the Berisal Complex, Simplon Region, Switzerland. Examples of this new rock-forming mica are found in bands and lenses of white-mica schist, and in a leucocratic zoisite–celsian gneiss. Samples of the schist, and especially of the celsian-bearing gneiss, are characterized by high whole-rock Ba contents of up to 15 wt.% BaO. The mineral paragenesis consists of zoisite, quartz, plagioclase, apatite, zircon and amphibole in the schist, and zoisite, celsian, quartz, margarite ± armenite in the gneiss. Ganterite is light grey to silver, has a vitreous luster, a perfect {001} cleavage, a laminated fracture, and a flexible tenacity. Mohs hardness, determined from micro-hardness indentations, is 4–4½. The mica is biaxial (−), α 1.600 (calc.), β 1.619, γ 1.622, and 2V(meas.) equal to 42.5 ± 2°. The calculated density of the most Ba-rich ganterite is 3.11 g/cm3. Single crystals typically are 0.5 by 0.15 mm (or less), and occur in small bands, lenses or clusters 0.5 to 10 cm in thickness. Ganterite is monoclinic, space group C2/c, Z = 4, a 5.212(1), b 9.046(2), c 19.978(4) Å, β 95°48′, V 937.6 Å3, corresponding to a 2M1 polytype. The strongest seven powder-diffraction lines [d in Å(I)(hkl)] are: 2.571(100)(131,2̅02), 2.602(95)(130,1̅31), 1.5054(91)(060,2010), 3.737(77)(023), 3.887(76)(1̅13), 4.481(71)(110), and 3.495(71)(1̅14). The new mineral species is named after the geographical region in which it was found.