|AURICHALCITE ON LIMONITE. SPECIMEN ~26 mm.|
The 79 Mine, Arizona, is a well-known mining area northeast of Tucson (north of the Hayden-Winkleman area) that has produced a wide variety of spectacular mineral specimens since its discovery in 1879. MinDat (http://www.mindat.org/loc-3322.html) states: the underground mine formally produced lead, zinc, copper, gold, silver, molybdenum, antimony, vanadium and iron until the early 1950’s and mineral specimens until the 1990’s. The oldest rock exposed in the Hayden area is the Proterozoic Precambrian Mescal Formation of the Apache Group. Above this unit are several thousand feet of Paleozoic rocks (Cambrian to Pennsylvanian). The major ore body is over 300 meters long and a dozen meters wide and occurs as replacements in the Naco Limestone (Pennsylvanian) and a dike of rhyolite porphyry. The mineralization is most likely Laramide (late Cretaceous and early Tertiary) in age.
One of the “different” sort of mineral that is found at the 79 Mine is aurichalcite, a zinc copper carbonate hydroxide, (Zn,Cu)5(CO3)2(OH)6. Aurichalcite is a secondary copper mineral that usually is found in alteration zones of large hydrothermal replacement bodies. Its friends seem to be azurite, malachite, smithsonite, and limonite. I consider it “different” since it occurs as very delicate, feathery acicular crystals only a few mm long. The crystals run from pale green to blue-green to sky blue and are wonderful to examine under a scope or hand lens.
The specimen pictured here is from an old collection (I guess old is relative) procured in the 1970’s. I purchased it at the Tucson 2012 show.
|PHOTOMICROGRAPH OF AURICHALCITE.|
|PHOTOMICROGRAPH OF AURICHALCITE ON LIMONITE.|