Photomicrographs of tiny caledonite crystals collected Rowley Mine, Arizona. Width FOV ~3 mm.
Saturday, March 14, 2015
The Painted Rock Mountains lay southwest of Phoenix, Arizona, approximately 75 miles west of Casa Grande, Arizona, along I-8 and near Gila Bend. It is a small range, as mountains go, perhaps 12 miles long and up to 4 miles wide. It is one of those typical Arizona Basin and Range chains with high hills containing both intrusive and extrusive igneous rocks, including basalt flows and thick pyroclastic beds, surrounded by fans of eroded alluvium. There appears to be numerous small faults but perhaps not the large bounding normal faults that I am familiar with in the Utah ranges.
The name comes from numerous petroglyph sites created by members of the Hohokam Culture (mostly). The Hohokam evidently lived along the Gila River (~0-1450 AD), now usually dry or intermittent, but at one time a nice stream. The Gila heads in the mountains of western New Mexico and flows west toward Gila Bend finally emptying into the Colorado River at Yuma. Rumor has it that at one time fairly large boats could navigate all the way from the mouth to Phoenix!
The mountains contain one of the best known specimen mines in Arizona—the Rowley Mine. Now inactive for mining (last produced in 1943) the Rowley produced copper (covellite, malachite, chalcocite), lead (anglesite, galena), iron (hematite) and minor amounts of gold, silver, molybdenum, vanadium, and fluorite, and even smaller amounts of barium and vanadinite. However, what the Rowley really produces, or did produce, are wulfenite crystals and “balls” of red to orange to yellow mimitite. Jones (2011) reported that the crystals of both mimitite and wulfenite are found in the cracks and crevices of shattered barite veins. MinDat lists 47 valid minerals from the Rowley including caledonite (lead and copper sulfate-carbonate).
Caledonite [Cu2Pb5(OH)6CO3(SO4)3] is another one of those brightly colored (blue to green) minerals that is found in the secondary oxidized zone of lead-copper deposits. The crystals are usually quite small, often striated prismatic, but sometimes aggregates or tabs. It is rather soft at 2.5-3.0 (Mohs), translucent, a vitreous luster and has a green-white to blue-green streak. Caledonite is a rare mineral; its classic locality in Arizona is the Mammoth-St. Anthony (Tiger) Mine near the north end of the Santa Catalina Mountains.
Jones, B, 2011, The Frugal Collector, Volume 1: Miller Magazines, Ventura, CA.