TURQUOISE
Turquoise is a famous and valuable hydrous copper
aluminum hydroxy-phosphate mineral, CuAl6(PO4)4(OH)8·5H2O.
It is the definition of the color term "turquoise", but the mineral
actually ranges from blue to bluish-green to green to greenish-gray. It
has a nonmetallic luster (dull or waxy luster on massive, cryptocrystalline
specimens & glassy luster on macrocrystalline samples), a pale
greenish-blue streak, is moderately hard (H = 5 to 6), has two cleavage
directions, and conchoidal fracture. It rarely forms decent crystals and
typically occurs in massive form or as finely-crystalline crusts or veinlets in
rocks.
Turquoise forms in the alteration zone of some
"porphyry copper" deposits and can be found as a vein-filling mineral
in some volcanic rocks and some phosphate-rich sedimentary rocks.
Turquoise from the Turquoise Chief Mine, St. Kevin-Independence, Lake County,
Colorado, USA (CMS # CSHS.9932, Colorado School of Mines Geology Museum,
Golden, Colorado, USA).
Turquoise from the Cripple Creek District, Teller County, Colorado, USA (ER #
2533, Ed Raines loan to the Colorado School of Mines Geology Museum, Golden,
Colorado, USA).