Krausite in fumaroles from Santa Barbara mine, northwestern Argentina

Krausite, monoclinic KFe(SO4)(2).H2O, has been known so far from only three localities. It was now discovered in the Santa Barbara mine, a sulphur dep osit located in northwestern Argentina, in a new paragenetic environment, f ormed by. current activity of thermal springs. The mineral occurs in a soft sulfate mass deposited around two fumarolic vents that discharge a CO2-ric h aqueous vapour phase in the floor of underground works. Associated minera ls are hexahydrite, alunogen and voltaite. Krausite forms rosettes less tha n or equal to 1 mm. in diameter, composed of short prismatic crystals of pa le lemon-yellow color. Unit-cell parameters are a 7.908(10), b 5.127(5), c 8.951(11)Angstrom, beta 102 degrees, V 354.1(5)Angstrom (3). The crystals a re transparent with perfect cleavage // to {001}, (+) 2V(calc) = 88 degrees , alpha = 1.588(3), beta = 1.645(3), gamma = 1.714(3). The chemical analysi s gave Na2O 0.70, K2O 17.36, Fe2O3 25.91, SO3 50.90, H2O 4.96, total 99.83 wt.%, leading to the formula K1.16Na0.07Fe1.02 (SO4)2 . 0.86 H2O.