Pyroxene Composition, Crystallization & Structure
Composition Pyroxene is a metasilicate, varying in its composition. It contains as bases chiefly calcium and magnesium, with smaller amounts of ferrous iron. In some varieties, however, molecules are introduced in which are the alkalies (chiefly sodium), aluminum and ferric iron. The more important varieties of pyroxene with the formulas assigned to them follow.
- Diopside, CaMg(DSiO3)2.
- Common pyroxene, Ca(M, Fe) (SiO3)2
- Augiye, CaMg(SiO3)2 with MgAl2SiO6 and NaAiSi2O6; with iron isomorphs with both the magnesium and the aluminum .
These varieties form an isomorphous series, and all gradations between them appear. Other varieties of less common occurrence are hedenbergite, café (SiO3)2; scheffrite, a manganese pyroxene; jeffersonite, a manganese – zinc pyroxene.
Crystallization. Monocline Crystals prismatic in habit; prism faces make angles of 87° and 93° with each other. The prism zone commonly shows the prism faces truncated by the faces of both vertical pinacoids, so that the crystals show, when viewed parallel to the vertical axis a rectangular cross section with truncated corners. The interfacial angles in the prism zone are either exactly or very closely 90° and 45. The terminations vary, being made up frequently of a combination of the basal plane with pyramids both in front and behind (Figs.303-305).
Structure.-in crystals. Of the lamellar. Coarse to dine granular.
Physical Properties. Prismatic cleavage sometimes good, often interrupted. Sometimes basal parting observer, often shown by twinning lamellae (see Fig. A, pl. x). H. = 5-6. G. = 3.2-3.6. Vitreous luster. Color varying from white and light green in diopside, to green in pyroxene, through dark green to black in augite. Color deepens with increase in the amount of iron preset. Transparent to opaque.
Tests. Fusible from 4 to 4.5. Insoluble in hydrochloric acid. To test for bases: fuse with sodium carbonate; dissolve in nitric acid; evaporate to dryness; notice the formation of silica jelly; moisten residue with water and hydrochloric acid; boil and filter from insoluble silica; add ammonium hydroxide in excess, precipitate of aluminum and ferric hydroxide in excess, precipitate of aluminum and ferric hydroxide; to boiling filtrate add ammonium oxalate, precipitate of ammonium magnesium phosphate. Recognized usually by its characteristic crystals.
Occurrence. The pyroxenes are common and important rock-making minerals, being found chiefly in the dark colored igneous rocks, especially those whose magmas were rich in iron, calcium and magnesium. They are seldom to be found in rocks that contain much quartz. Augite is found in basaltic lavas, and in the dark colored intrusions known generally as trap, in gabbros and peridotites. Diopside and common pyroxene are found sometimes in syenites and similar rocks; also as metamorphic minerals in impure recrystallized dolomitic limestones, pyroxene also occurs in some gneisses. In the limestones, pyroxene is often associated with tremolite, scapolite, vesuvianite, garnet, titanite phlogopite, etc. In igneous rocks it is found with orthoclase, the plagioclase feldspar, nephelite, chrysolite, leucite amphibole, magnetite, etc. Some of the notable localities, particularly for fine crystals, are the following: For diopside, Ala and Traversella in Piedmont; Nordmark, Sweden; on various localities in Orange Country, New York; for augite, in the lavas of Vesuvius; at Val dig Fassa, Trentino, Italy; Bilin, Bohemia; hedembergite from Sweden; shefferite form Swede; jeffersonite from Franklin, New Jersey.
Names. The name pyroxene, stranger to fire, is a misnomer, and was given to the mineral because it was thought that it did not occur in igneous rocks. Diopside comes form two Greek words meaning double appearance. Augite comes from a Greek word meaning luster.
Use. Clear green diopside or common pyroxene is occasionally used as a gem material.
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