Calcite

This specimen is one of 4 pictured at the San Sebastian page on Mindat. It is comprised of distorted flattened crystals to nearly 5cm with a silken luster and a fine coating of limonite. There are also dark brown calcites that are trigonal prisms. A very fine Mexican specimen that I bought  in 2002 from Willis Earth Treasures.

Locality: San Sebastian Mine, Aurora Level, Charcas, San Luis Potosi, Mexico

Composition: calcium carbonate