When we recognize the virtues, the talent, the beauty of Mother Earth, something is born in us, some kind of connection, love is born.
Thich Nhat Hanh
Sometimes, in examining a table or flat of minerals, something just reaches out and grabs you---take me home, take me home…The beckoning mineral need not be expensive nor rare nor exotic, just something of beauty in the eye of the beholder. A table of minerals at a recent Tucson Show had one of those specimens that simply reached out and grabbed me. It does not have showy crystals nor exotic minerals; however, the banding and colors commanded an allure. So, it became “mine.”
The most striking minerals in the specimen are sky blue chrysocolla, a hydrated copper aluminum hydroxy silicate [(Cu, Al)2H2Si2O5(OH)4-nH2O but with a variable composition] intermixed with tenorite, a black copper oxide [CuO]. Also present are calcite [CaCO3], a banded light blue silicate such as chalcedony or perhaps silica-infused chrysocolla, and an unknown tan-orange-green mineral. Some might call the specimen a geode or a broken vug. It was collected in the Boleo District, Mun. de Mulegé, Baja California Sur (BC Sur), Mexico.
|Reverse of specimen above: Ca=calcite, C=botryoidal chalcedony, B=banded chalcedony, T=tenorite, ?=blue chalcedony; G=green"chalcedony.|
Chrysocolla has been used as a semi-precious gemstone for centuries, often as a substitute for turquoise, but its internal structure is still not well understood. For example, most mineralogy books and web sites believe chrysocolla belongs to the Orthorhombic Crystal System (Three crystallographic axes (A,B,C) of unequal length that make angles of 90 degrees with each other). However, Frost and Xi (2013) point out that this assignment “remains uncertain.” Some geologists believe chrysocolla is crystalline in nature while others believe it “generally amorphous” and therefore not a true mineral (strict sense) (Klein, 2002). Sun (1963) reported “chrysocolla is not a definite chemical compound but a hydrogel containing mainly SiO, CuO and H2O, and minor amounts of Al2O3, CaO and MgO.” Frost and Xi (2013) stated “chrysocolla is a colloidal mineral…but questioned…whether chrysocolla is: 1) a mesoscopic [somewhere between microscopic and macroscopic, between big and small] assemblage of spertiniite, Cu(OH)2, silica, and water; 2) represents a colloidal gel; or 3) is composed of microcrystals with a distinct structure.” Their definitive study of chrysocolla, based on X-Ray Powder Diffraction, Raman Spectroscopy, and Infrared Spectroscopy studies, “concluded that chrysocolla is not based upon spertiniite but is an amorphous hydrated copper silicate…with a simplified chemical formula of CuSiO3-2H2O.”
It would seem that an amorphous substance would not be classified as a mineral but as a mineraloid, most of which do not have a definite structure on the atomic scale (chrysocolla is never in visible crystals; www.minerals.net) However, evidently some forms of chrysocolla have identifiable structures at the “nano-level” (crystals acicular; www.webmineral.com) and hence it has mineral (IMA) status rather than a mineraloid. At least that is my interpretation of the situation and any other explanation is above my pay grade.
Chrysocolla has variety of colors but usually is some shade of blue and/or green—due to coloring by the major cation, copper (an idiochromatic mineral). The streak on an unglazed porcelain ranges from pale blue to light green. Chrysocolla is quite soft at ~2.5-3.0+(Mohs) and is a good property to distinguish itself from much harder turquoise at ~5-6 (Mohs). Chrysocolla has a luster from ranging from earthy (dull) to vitreous to waxy and appears in solid and fibrous veins, in tuffs forming tiny crystals, commonly as massive or perhaps as encrustations, as rounded balls or botryoids, and even as stalactitic columns. Most forms are opaque but thin slices are translucent and are fairly brittle when “hit.”
Contrasting with the sky-blue chrysocolla is tenorite, a black copper oxide (CuO). Tenorite would not be an impressive mineral with its earthy to dull to metallic luster and generally massive habit without common accompanying friends—chrysocolla, malachite and azurite. It is opaque with a black streak and is brittle commonly with a conchoidal fracture. It is soft at ~3.5 (Mohs). I have only observed massive and botryoidal tenorite; however, some localities produce small crystals (Monoclinic Crystal System). It appears, from my reading, that visible crystals are only formed when tenorite is the product of volcanic sublimation (crystallized from gasses around volcanic vents). In fact, the type locality for tenorite is around Mt. Vesuvius in Italy.
|Photomicrograph banded chalcedony left grading into blue chalcedony or silica infused chrysocolla surrounding black tenorite. Notice green ?chalcedony encased in the blue. Width of photo ~1.2 cm.|
|Photomicrograph of calcite in center of vug. Width of photo ~ 1.2 cm.|
|Photomicrograph of banded chalcedony and tenorite. Note botryoidal calcite in upper left quadrant. Width of photo ~1.2 cm.|
|Reverse of specimen with silica rind, blue chrysocolla and black tenorite. Width of photomicrograph ~1.2.|
|Blue chalcedony and black tenorite. Width of photomicrograph ~1.2 cm.|
|Blue chalcedony and black tenorite. Width of photomicrograph ~1.2 cm.|
Both massive tenorite and chrysocolla are somewhat common minerals in the oxidized zone of hydrothermal copper deposits. It seems as if every copper mine in the western U.S and Mexico produces these two minerals; they are not uncommon. As noted above, my specimen came from the Boleo District in Baja California. The discovery locality, the Boleo Copper District located near the town of Santa Rosalia, has been the site of major sulfide mining (open pit until the 1980s) and at one time (at least in the 1950s) was the second largest producer of copper in Mexico (Wilson and Rocha, 1955). Underground mining started in 2012 with the first production in 2014 and I presume mining is still active. As best I can decipher, the copper deposits are in an uplifted belt of Neogene (Miocene or Pliocene) rocks within the El Boleo Formation (deltaic and near-shore marine claystone-siltstone-sandstone beds). The major sulfide ore minerals are chalcocite (Cu2S) accompanied by chalcopyrite (CuFeS2), bornite (Cu5FeS4), covellite (CuS), and native copper (Cu). The oxidized zone (above the sulfide deposits) has a large variety of copper oxides, copper carbonates, copper silicates, manganese oxides, and rare halide minerals.
My geode-looking specimen has cream to white massive calcite in the center surrounded by botryoidal “silica”, perhaps chalcedony, and then grades into banded chalcedony? or perhaps silica-infused chrysocolla. About all I can tell is the “silica” is much harder (Mohs) than the chrysocolla. The banded chalcedony seems to flow around the tenorite and, at times, seems to grade into the chrysocolla. The “rind” is a tan to gray “crumbly” or shattered microcrystalline “silica.” There are many “things” I don’t understand about this specimen but am trying to “learn” additional facts to satisfy my curiosity. It turns out that a specimen I purchased since it grabbed me seems to have a complex history! What more could I ask for?
Frost, R.L., and Y. Xi, 2013, Is chrysocolla (Cu,Al)2H2Si2O5(OH)4·nH2O related to spertiniite Cu(OH)2? -a vibrational spectroscopic study: http://eprints.qut.edu.au/58692/11/586
Sun, M.-S., 1963, The nature of chrysocolla from the Inspiration Mine, Arizona: American Mineralogist, v. 48.
Wilson, I.F. and V.S. Rocha, 1955, Geology and mineral deposits of the Boleo copper district, Baja, California, Mexico: U.S. Geological Survey Professional Paper 273.