TERNESITE, CA-5(SIO4)(2)SO4, A NEW MINERAL FROM THE ETTRINGER BELLERBERG EIFEL, GERMANY/

The new mineral ternesite, Ca-5(SiO4)(2)SO4, has been found at the Ett ringer Bellerberg near Mayen, Eifel, Germany. The crystal structure, a lready known from the synthetic analogue, was refined from single crys tal X-ray data: orthorhombic, space group Pnma with a=6.863(1) Angstro m, b=15.387(2) Angstrom, c=10.181(1) Angstrom, Z=4, R=0.058, R-w=0.046 for 820 unique reflections with F-0> 3 sigma(F-0) and 96 variable par ameters. The strongest peaks in the powder pattern are (d-value (Angst rom), I, hkl): 2.830, 100, (033)/2.853, 63, (230)/2.565, 55, (060)/3.1 98, 42, (132)/1.892, 39, (035) + (125). The mineral is optically biaxi al negative with refractive indices n(x) = 1.630(1) (parallel [100]), n(y) = 1.637(2) (parallel [001]), and n(z) = 1.640(1) (parallel [010]) . The optical angle 2V(x) was measured as 63.5(5)degrees.