GANTERITE, A NEW BARIUM-DOMINANT ANALOGUE OF MUSCOVITE FROM THE BERISAL COMPLEX, SIMPLON REGION, SWITZERLAND

Ganterite, [Ba 0.5 (Na,K) 0.5 ]Al 2 (Si 2.5 Al 1.5 O 10 )(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 2 V (meas.) equal to 42.5 ± 2°. The calculated density of the most Ba-rich ganterite is 3.11 g/cm 3 . 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) A, β 95°48′, V 937.6 A 3 , corresponding to a 2 M 1 polytype. The strongest seven powder-diffraction lines [ d in A(I)( hkl )] are: 2.571(100)(131,202), 2.602(95)(130,131), 1.5054(91)(060,2010), 3.737(77)(023), 3.887(76)(113), 4.481(71)(110), and 3.495(71)(114). The new mineral species is named after the geographical region in which it was found.