Natrojarosite and silver halides from the El Criollo pegmatite, Córdoba Province (Argentina)

COLOMBO, FERNANDO; KAMPF, ANTHONY R.

San Juan

Congreso; XI Congreso de Mineralogía y Metalogenia; 2013

Asociación Mineralógica Argentina y Universidad Nacional de SanJuan

We describe here three new minerals for the El Criollo pegmatite, Córdoba province (Argentina). Natrojarosite occurs as orangish yellow crystals showing the {102} and {01-2} pyramids, along with the {001} and {00-1} pedions. Crystals are transparent, with vitreous luster, and occur in a vug in massive white quartz associated to an undetermined Fe-Cu phosphate. EDS spectra show S, O, Fe, Na and K, with traces of P and possibly Pb as well. Aluminum is present in some points. Usually Na>K, but a few spot analyses showed only K. Therefore, there is a compositional range from natrojarosite to (rare) jarosite. The X-ray diffraction pattern (Gandolfi) shows a better match with natrojarosite. Unit-cell dimensions are a = 7.313(2), c = 16.637(6) Å. Iodargirite and chlorargirite were found as euhedral crystals up to 20 micrometers long implanted on vugs covered with brochantite crystals associated to malachite and quartz. They were identified using EDS, which showed I, Cl and Ag, with variable Cl:I ratios. These minerals are an alteration product of Ag-bearing copper sulfides that precipitated during a hydrothermal stage related to the pegmatite crystallization.