The Mineral augite
Augite is isomorphous with the minerals Diopside and Hedenbergite. It is an intermediary member between these minerals, forming a series, but contains additional sodium and aluminum within its chemical structure. Strictly speaking, because of the variables in its chemical structure, Augite is really more of a group then a single mineral, but it is still classified a single mineral species by the IMA.
Augite is an important rock-forming mineral, and large crystals are fairly common. It is the most widespread member of the pyroxene group, and it frequently alters to many other minerals, including Hornblende, Chlorite, and Epidote. When altered to Actinolite, it is often called Uralite.
Augite usually occurs in dull crystals that are ugly and uninteresting. Occasionally, though, it is found in large lustrous crystals which are sought after by mineral collectors. The name Augite is derived from the Greek word augites, "brightness", in reference to the bright luster this mineral occasionally exhibits.
Chemical Formula
(Ca,Na)(Mg,Fe,Al)(Al,Si)2O6
Color
Green, grayish-green, greenish brown, dark brown, black
Crystal System
Monoclinic
Properties
Streak
Light green to colorless |
Hardness
5 - 6 |
Transparency
Opaque. Translucent in thin sections. |
Specific Gravity
3.2 - 3.6 |
Luster
Vitreous, dull |
Cleavage
1,2 - prismatic at cleavage angles of 87º and 93º (Characteristic of minerals in the pyroxene group). May also exhibit parting in one direction. |
Fracture
Uneven to splintery |
Tenacity
Brittle |
Crystal Habits
Often as prismatic crystals with a rectangular or octagonal cross section. Also in short, stubby crystals with a flattened slightly pyramidal termination. Other forms are columnar, grainy, massive, fibrous, and in disordered aggregates of rectangular crystals. May also be in penetration twins with v-shaped saddles. Crystals from certain localities have partially hollow etchings.
Additional Information
Composition
Silicate of calcium, sodium, magnesium, iron, and aluminum. Occasionally with zinc, manganese, and titanium impurities. |
In Group
Silicates; Inosilicates; Pyroxene Group |
Striking Features
Color, crystal habits, cleavage, and environment |
Environment
An important constituent of many igneous rocks, including basalt, diabase, and gabbro. Also in carbonatite and nepheline syenite pegmatites, and in metamorphic Serpentine deposits.
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Rock Type
Igneous, Metamorphic |
Varieties
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Variety of Augite originally described from the Val D'Fassa region in Italy which has a low iron content. This is usually responsible for this variety having a lighter green color and increased translucency then other most Augite.
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Variety of Augite rich in manganese and zinc, found in the Franklin District, Sussex Co., New Jersey and surrounding areas in the Franklin marble. Its chemical formula is Ca(Mn,Zn,Fe)Si2O6
Uses
Augite is only of interest to mineral collectors. Geologists study this mineral to learn more about mineral environments.
Noteworthy Localities
Some of the most well-known Augite occurrences are in the Italian volcanoes at Vesuvius (Monte Somma), Stromboli, Etna, and Lazio. Also in Italy is the Val D'Fassa, in Trento Province, where the Fassaite variety is found.
Other good European occurrences are the Eifel Mountains of Germany; Vlčí Hora in Chernoschin, Bohemia, Czech Republic; and the alluvial gravels of the Puy-de-Dome region in Auvergne, France.
Two volcanic islands in the Atlantic ocean have also produced good crystals, namely Agua Retorta, San Miguel Island, in the Azores Islands; and Mount Teide, Tenerife Island, in the Canary Islands.
In the U.S., good crystals are found in Diana, Lewis Co., New York; and Cedar Butte, Tillamook Co., Oregon. Large crystals of the Jeffersonite variety are found in Franklin and Ogdensburg, Sussex Co., New Jersey.
In Canada, lustrous black crystals Crystals come from the Yates mine, Otter Lake, Quebec; and good crystals from Diamond Lake, Bancroft, Hastings Co., Ontario.
Distingushing Similar Minerals
Tourmaline - Different cleavage, harder, crystals usually striated.
Epidote - Different cleavage, more translucent, usually heavily striated.
Diopside and Hedenbergite - Lighter in color (otherwise difficult to distinguish).
Enstatite - Difficult to distinguish.
Hornblende - Has slightly different cleavage angles (otherwise difficult to distinguish).
Aegirine - Crystals are usually more elongated.