CALAVERITE & SYLVANITE
Calaverite and sylvanite are rare precious metal telluride minerals. Calaverite is gold telluride (AuTe2) and sylvanite is gold silver telluride ((Au,Ag)2Te4). The telluride minerals volatilize at relatively low temperatures. With heating, the tellurium component of these minerals readily vaporizes, leaving behind blebs of gold or gold/silver.
Calaverite has a metallic luster, a yellowish-gold to silvery color, is moderately soft, and has a variety of crystal forms. Sylvanite has a metallic luster, a bright silver color, is fairly soft, and usually occurs in the form of elongated blades or small granular masses.
Calaverite & sylvanite occur with other telluride minerals, and are known from only a few scattered localities on Earth. They are important gold & gold/silver ore minerals in precious metal telluride deposits such as the Golden Quadrilateral in Romania, near Kalgoorlie in western Australia, and the Cripple Creek Diatreme in Colorado, USA.
Calaverite (gold) (field of view ~1.7 cm across) in a purple fluorite vein, from the Cripple Creek Diatreme (Early Oligocene, 32 million years) of central Colorado, USA.
Sylvanite (bright silver) (field of view 1.2 cm across) from Boulder County, Colorado, USA.