Southern Arizona has numerous sites known for their pyroxene minerals, from Twin Buttes south of Tucson, to Ajo, to the Chiricahua, Santa Rita and Patagonia mountain ranges.
Pyroxenes are classified as a group of rock-forming minerals and are often the primary constituents of igneous rocks. Pyroxenes vary in composition, including magnesium, iron, calcium, zinc and manganese along with sodium, lithium, titanium or aluminum.
They range 5 to 6 on the Mohs hardness scale.
Iron supplanting calcium and magnesium causes chemical change in pyroxene minerals, such as diopside converting to hedenbergite and enstatite transforming to hypersthene.
Additional attributes include crystal symmetry: either orthorhombic or monoclinic and prism angles and cleavages along two intersecting lines running at 87 degrees and 93 degrees from their crystal faces.
The more iron content, the darker the color.
Varieties of pyroxene found in Arizona include diopside, enstatite, spodumene, augite, hedenbergite and aegirine.
Augite is the most common and widespread among the pyroxene minerals found in dikes, lavas and sills. It is a mixture of aluminum, iron, magnesium and calcium silicates; its name is derived from the Greek term auge, meaning “luster.”
Found in igneous rocks, including basaltic lava as vitreous, monoclinic crystals and massive, it has a dark green to black coloration.
One notable locality for augite specimens in Arizona is northwest of Flagstaff at Red Mountain, a cinder cone volcano that exploded more than 750,000 years ago.
Specimens occur in volcanic tuff becoming larger and more plentiful along the 1 mile hike to the amphitheater or exposed interior of the cinder cone, which was caused by a phreatomagmatic eruption resulting from interactions between magma and water. Xenocrysts of augite appear as prismatic, monoclinic crystals.
Diopside is a mineral named after the Greek terminology “di,” meaning double, and “opsis,” meaning view. Diopside, a calcium-magnesium silicate, is classified as having short prismatic crystals that take on a different appearance when viewed from different angles.
Chrome diopside appears as green crystals due to the content of chromium impurities. Diopside occurs in crystalline limestones and seldom in volcanic rocks. Transparent crystals of diopside are popular in the gem trade.
A locality in Arizona accounted for diopside deposits in the form of yellow, elongated crystals is the El Tigre Mine in the Chiricahua Mountains. The mine was a producer of lead and silver with limited development in the 1940s, including a 500-foot tunnel with a 350- foot raise to the surface.
The El Tigre Mill, owned by the Tamco Mining and Milling Co., operated at Pinery Canyon. It was short lived due to the lack of sufficient ore from local mines, although one report mentioned 50 tons of ore from the nearby Hilltop Mine lying idle onsite.
Hedenbergite is another common rock-forming mineral found in contact metamorphic rocks. Notable for its iron content, it also contains calcium and has a vitreous luster when viewed in the sun and in sufficient artificial lighting.
Although crystals are uncommon, those that exist appear as short prisms, acicular and fibrous. The common form of hedenbergite is in aggregate, either granular or massive.
The Senator Morgan Mine, one of the early producers in the Twin Buttes Mining District south of Tucson, is a notable location for several dark green and black pyroxene minerals.
While chalcopyrite is the dominate ore, both diopside and hedenbergite are notable gangue minerals found at the site and nearby wash.
Spodumene, a lithium aluminum silicate formed in lithium-bearing granite pegmatites, is found at the Midnight Owl and Morning Star Mines in Maricopa County.
Lithium is an important industrial metal used as a compound in ceramic glazes, a bonding agent in ventilating systems of submarines and spaceships, and an explosive material in hydrogen bombs.
Enstatite, a magnesium silicate, is found in several Arizona localities including the Reagan claims near the 79 Mine in Gila County and in some andesites in the Ajo Mining district and the Santa Rita and Patagonia mountains.
Noted for a high melting point, it is used for gemstones and its crystals are used to line kilns, furnaces and ovens.
Aegirine, a sodium iron silicate, is found with diopsidic jadeite and almandine garnet in eclogite inclusions from kimberlite pipes in Northern Arizona’s Monument Valley, Garnet Ridge.
It is notable among pyroxene minerals for its long, needlelike crystals. This mineral is found in moon rocks and stony meteorites.