These calcite crystal twins from Santa Eulalia in the Mun. de Aquiles Serdan, Chihuahua, Mexico. This district encompasses a large area that contains over a dozen mines. Mining activities are primarily for the extraction of silver and other metals. There a over 150 different types of minerals in the district including calcite. It is a carbonate that contains the elements ca and C. This is specimen has a heavy gossan matrix that is covered with numerous calcite crystals and most have twinned habits. The calcites have inclusions of iron oxides which color them with brown phantoms. They have vitreous luster. This piece measures 4 1/4 x 3 3/8 x 1 7/8 inches or 108 x 86 x 48 mm.