Bronzite
Encyclopedia
Bronzite is a member of the orthopyroxene group of mineral
s, belonging with enstatite
and hypersthene
to the orthorhombic series of the group. Rather than a distinct species, it is really a ferriferous variety of enstatite, which owing to partial alteration has acquired a bronze-like sub-metallic luster
on the cleavage
surfaces.
Enstatite is magnesium silicate, MgSiO3, with the magnesium partly replaced by small amounts (up to about 12%) of Fe+2. In the bronzite variety, (Mg,Fe)SiO3, the iron(II) oxide ranges from about 12 to 30%, and with still more iron there is a passage to hypersthene
. The ferriferous varieties are liable to a particular kind of alteration, known as schillerization, which results in the separation of the iron as very fine films of oxide
and hydroxide
s along the cleavage cracks of the mineral. The cleavage surfaces therefore exhibit a metallic sheen or schiller, which is even more pronounced in hypersthene than in bronzite. The color of bronzite is green or brown; its specific gravity
is about 3.3–3.4, varying with the amount of iron present. The refractive indices
and optic angle increase with iron content. The enstatite endmember has a positive optic sign, whereas bronzite and hypersthene both show a negative optic sign.
Like enstatite, bronzite is a constituent of many mafic
to ultramafic igneous rocks, such as, norite
, gabbro
, and especially peridotite
, and of the serpentinite
s which have been derived from them. It also occurs in some crystalline schist
. Bronzitite, a pyroxenite
of bronzite composition, is noted in the cumulate rock
s of the Stillwater igneous complex
of Montana
.
in the Fichtelgebirge
, and in the serpentine of Kraubat near Leoben in Styria
. In this connection mention may be made of an altered form of enstatite or bronzite known as bastite or schiller spar. Here, in addition to schillerization, the original enstatite has been altered by hydration and the product has the approximate composition of serpentine. In color bastite is brown or green with the same metallic sheen as bronzite. The typical locality is Baste in the Radauthal, Harz
, where patches of pale greyish-green bastite are embedded in a darker-colored serpentine. This rock when cut and polished makes an effective decorative stone, although little used for that purpose.
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s, belonging with enstatite
Enstatite
Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...
and hypersthene
Hypersthene
Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Many references have formally abandoned this term, preferring to categorise this mineral as enstatite or ferrosilite. It is found in igneous and some metamorphic rocks as well as in stony and...
to the orthorhombic series of the group. Rather than a distinct species, it is really a ferriferous variety of enstatite, which owing to partial alteration has acquired a bronze-like sub-metallic luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
on the cleavage
Cleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
surfaces.
Enstatite is magnesium silicate, MgSiO3, with the magnesium partly replaced by small amounts (up to about 12%) of Fe+2. In the bronzite variety, (Mg,Fe)SiO3, the iron(II) oxide ranges from about 12 to 30%, and with still more iron there is a passage to hypersthene
Hypersthene
Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Many references have formally abandoned this term, preferring to categorise this mineral as enstatite or ferrosilite. It is found in igneous and some metamorphic rocks as well as in stony and...
. The ferriferous varieties are liable to a particular kind of alteration, known as schillerization, which results in the separation of the iron as very fine films of oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
and hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
s along the cleavage cracks of the mineral. The cleavage surfaces therefore exhibit a metallic sheen or schiller, which is even more pronounced in hypersthene than in bronzite. The color of bronzite is green or brown; its specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
is about 3.3–3.4, varying with the amount of iron present. The refractive indices
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
and optic angle increase with iron content. The enstatite endmember has a positive optic sign, whereas bronzite and hypersthene both show a negative optic sign.
Like enstatite, bronzite is a constituent of many mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
to ultramafic igneous rocks, such as, norite
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...
, gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
, and especially peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
, and of the serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
s which have been derived from them. It also occurs in some crystalline schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
. Bronzitite, a pyroxenite
Pyroxenite
Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...
of bronzite composition, is noted in the cumulate rock
Cumulate rock
Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks.-Formation:Cumulate rocks are the...
s of the Stillwater igneous complex
Stillwater igneous complex
The Stillwater igneous complex is a large layered intrusion located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles of the north flank of the Beartooth Mountain Range. The complex has extensive reserves of chromium ore and has a history of...
of Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
.
Ornamental usage
Bronzite is sometimes cut and polished, usually in convex forms, for small ornamental objects, but its use for this purpose is less extensive than that of hypersthene. It often has a more-or-less distinct fibrous structure, and when this is pronounced the sheen has a certain resemblance to that of cats-eye. Masses sufficiently large for cutting are found in the norite of the KupferbergKupferberg
Kupferberg is a municipality in the district of Kulmbach, in Bavaria, Germany. It is situated in the Frankenwald, 9 km northeast of Kulmbach....
in the Fichtelgebirge
Fichtelgebirge
The Fichtelgebirge is a mountain range in northeastern Bavaria, Germany. It extends from the valley of the Red Main River to the Czech border, a few foothills spilling over into the Czech Republic. It continues in a northeastern direction as the Ore Mountains, and in a southeastern direction as...
, and in the serpentine of Kraubat near Leoben in Styria
Styria (state)
Styria is a state or Bundesland, located in the southeast of Austria. In area it is the second largest of the nine Austrian federated states, covering 16,401 km². It borders Slovenia as well as the other Austrian states of Upper Austria, Lower Austria, Salzburg, Burgenland, and Carinthia. ...
. In this connection mention may be made of an altered form of enstatite or bronzite known as bastite or schiller spar. Here, in addition to schillerization, the original enstatite has been altered by hydration and the product has the approximate composition of serpentine. In color bastite is brown or green with the same metallic sheen as bronzite. The typical locality is Baste in the Radauthal, Harz
Harz
The Harz is the highest mountain range in northern Germany and its rugged terrain extends across parts of Lower Saxony, Saxony-Anhalt and Thuringia. The name Harz derives from the Middle High German word Hardt or Hart , latinized as Hercynia. The legendary Brocken is the highest summit in the Harz...
, where patches of pale greyish-green bastite are embedded in a darker-colored serpentine. This rock when cut and polished makes an effective decorative stone, although little used for that purpose.