Brüggenite, Ca(I0 3 )2 - H2 0, is found in veins of high-purity soda niter in rhyolite tuff at Pampa Pique III, Oficina Lautaro, Chile, as long columnar anhedral crystals, as prismatic crystals, and as irregular anhedral crystals or encrusting masses. The mineral is colorless to bright yellow, and transparent to translucent, has vitreous luster and a hardness of about 3 1/2, is brittle, and has conchoidal fracture. It is biaxial and has variable optic angle and refractive indices, which were determined by the spindle-stage method. Refractive indices are as follows: nα =1.772-1.779, nβ=1.795-1.802, and nγ=1.817-1.824 (all ± 0.003); optic angle shows a range of 2VZ = 86°-96°; Z = b and X∧c = -47°; dispersion (Z) is r<v, moderate. Brüggenite is monoclinic, space group P21/c, with a=8.509±0.001 A, b = 10.027±0.002 A, c = 7.512±0.001 A, β = 95°16.00'±0.55', volume 638.2 A3 , Z = 4, G(calc) = 4.244, G(meas)=4.24±0.01. The X-ray diffraction powder pattern of brüggenite has the following strong lines (hkl, dhkl1 ): 031, 3.051 A (100); 220, 3.238 A (90); 200, 4.235 A (80); 002, 3.739 A (60); 012, 3.503 A (60); 231, 2.522 A (60). The composition was confirmed by chemical analyses.
