Orthopyroxenes

The orthopyroxenes are an important series of rock-forming minerals. They are the orthorhombic members of the pyroxene group of silicates and consist essentially of a simple chemical series of(Mg,Fe)SiO₃ minerals, in contrast with the larger subgroup of monoclinic pyroxenes ( clinopyroxenes , q.v.) which have a very wide range of chemical composition. Orthopyroxenes form a solid-solution series of general formula (Mg,Fe²⁺)SiO₃ in which Mg and Fe²⁺ are mutually replaceable between Mg₁₀₀Fe²⁺ and approximately Mg₁₀Fe²⁺ ₉₀. They occur commonly in mafic and ultramafic plutonic rocks, in mafic and intermediate volcanic rocks, and in high-grade thermally and regionally metamorphosed rocks of both igneous and sedimentary origin. The orthopyroxenes are particularly characteristic constituents of norites and charnockites.

Structure

MgSiO₃ and the magnesium-rich members of the series occur in at least three polymorphic forms, the low-temperature orthorhombic enstatite (the form almost universally found in rocks); the high-temperature protoenstatite, which also has orthorhombic symmetry; and clinoenstatite .

The essential features of the structure of enstatite and the other orthopyroxenes is the linkage of SiO₄ tetrahedra by sharing two of the four corners to form continuous chains of composition (SiO₃) _n , as in all pyroxenes. These chains are linked laterally by the octahedrally coordinated cations Mg or (Mg,Fe) to produce an orthorhombic cell with approximately double the a dimension of the endmember clinopyroxene, diopside : an idealized projection on (001) of the structure of enstatite is given in Fig. 1. There are two nonequivalent cation positions M ₁ and M ₂ in the structure, and research is currently being carried out to determine the distribution of Mg and Fe²⁺ between these sites. Present indications (from Mössbauer spectroscopy) are that orthopyroxenes from high-grade metamorphic rocks have a high degree of ordering, with Fe²⁺ preferring the M₂ position(see Order-Disorder). A detailed report was given by Virgo and Hafner (1970); more recent work has been summarized by Wood and Banno (1973).

FIGURE 1

Idealized projection on (001) of the structure of enstatite (compare Fig. 3 of Order-Disorder) (from W. A. Deer, R. A. Howie, and J. Zussman, Rock Forming Minerals, vol. IIA, Single Chain Silicates, Longman Group Limited, London, 1978, p. 6).

FIGURE 3

The relationship of the optical and physical properties to the chemical composition of orthopyroxenes (from W. A. Deer, R. A. Howie, and J. Zussman, Rock Forming Minerals, vol. IIA. Single Chain Silicates, Longman Group Limited, London, 1978, p. 109).

The relationship between cell parameters and composition is somewhat complex in that, in addition to the major increase in all three dimensions due to the substitution of the larger Fe²⁺ ion for Mg across the series, minor amounts of Ca, Mn, and Al also have an appreciable effect. In particular, the entry of Al causes a considerable contraction in the unit cell. The end-member values are a=18.228, b=8.805, c 5.185Å for MgSiO₃ and a= 18.433, b = 9.060, c = 5.258Å for (Fe²⁺, Fe³⁺, Mn)SiO₃.

Chemistry

Although, theoretically, an ideal series from MgSiO₃ to FeSiO₃, cations other than Mg and Fe²⁺ are almost invariably present in the orthopyroxenes. These commonly include Ca, Mn, Fe³⁺, Al, Ti, and Cr; in most specimens, however, the sum of these other constituents does not exceed 10 mol%. High contents of manganese are found in the more iron-rich orthopyroxenes, whereas chromium occurs mainly in the magnesium-rich orthopyroxenes of ultramafic igneous rocks. High aluminum contents occur chiefly in orthopyroxenes of high-grade metamorphic rocks, e.g., pyroxene granulites. In most analyses of material carefully separated from clinopyroxene, the CaO values are less than 1.0 wt% (equivalent to approximately 2% of the wollastonite composition).

Enstatite is stable to 985° C and above this temperature inverts to protoenstatite..The inversion temperature is, however, pressure sensitive, and the inversion curve has a slope of 84°C/kilobar. Protoenstatite can be quenched and studied at room temperature, but on cooling it usually undergoes a metastable inversion to clinoenstatite which, thus, forms in the stability field of enstatite. Protoenstatite is the stable MgSiO₃ polymorph between 985° and 1385°C; the polymorphic relationships at higher temperatures are uncertain. Pure protoenstatite melts incongruently to forsterite and liquid at 1557°C, while protoenstatite solid solution En₇₆Di₂₄ melts incongruently to forsterite and a more siliceous liquid at 1386°C. At 985°C, the maximum solubility of diopside in enstatite is 5 wt%, but this decreases at lower temperatures and is only 2% at 800°C. In many magnesium-rich orthopyroxenes of plutonic rocks, the bulk of the Ca(Mg,Fe)Si₂O₆ initially in solid solution at the time of crystallization is exsolved on cooling and forms lamellae of a Ca-rich monoclinic pyroxene as thin, parallel-sided sheets or as rows of flattened blebs in the (100) plane of the orthorhombic host (pyroxenes of Bushveld type). Similar lamellae in the more iron-rich orthopyroxenes of plutonic rocks are inverted pigeonites , and the exsolved plates of the Carich monoclinic pyroxene are oriented parallel to a plane that represents (001) of the original pigeonite (pyroxenes of Stillwater type). The Ca content of the latter is about three times greater than that of the Bushveld type. Orthopyroxenes are sometimes altered to serpentine ; where alteration is complete, the pseudomorphs show a characteristic bronze-like metallic luster or schiller and are known as “bastite.”

In the synthetic system MgO-SiO₂-H₂O, enstatite crystallizes at temperatures above 650°C at water vapor pressures greater than 350 bars and is stable in the presence of water vapor below about 850°C. Various approaches have been made to the development of a pyroxene geothermometer (see Thermometry, Geologic). An empirical enstatite geothermometer was produced using the ratio Ca/ (Ca + Mg) based on temperatures of equilibration estimated for coexisting diopsides (Boyd and Nixon, 1973), whereas a geothermometer using the ratio Ca/(Ca+Mg+Fe) of orthopyroxene coexisting with clinopyroxene was developed by Mysen (Fig. 2). No clear pressure dependence was observed over the range 10–30 kilobars. Similarly, an empirical approach to take account of the effect of Fe²⁺ on the orthopyroxene-clinopyroxene miscibility gap in natural systems has made possible the calculation of equilibration temperatures for twopyroxene assemblages (Wood and Banno, 1973). Experimental work on the use of the amount of MgSiO₃ dissolved in diopside to estimate temperature of equilibration of coexisting diopside clinopyroxene and enstatitic orthopyroxene was reported (Warner and Luth, 1974; Nehru and Wyllie, 1974).

FIGURE 2

Orthopyroxene geothermometry (from Mysen, 1973). Ca/(Ca + Mg + Fe) in orthopyroxene coexisting with clinopyroxene versus temperature.

Work on the MgSiO₃-Al₂O₃, system led to the calculation of equilibration pressures of natural orthopyroxene- garnet assemblages provided temperatures are known (Wood and Banno, 1973). A similar approach to pressure determination has been reported by Macgregor (1974) using the Al₂O₃ content of orthopyroxene coexisting with an aluminous phase such as spinel or garnet. These methods mostly involve the magnesian range of compositions; and, thus, extrapolations of values to natural phases-often containing Ti, Fe³⁺, and Mn as well as Fe²⁺ and Al–may lead to ambiguous results (see Barometry, Geologic). Nevertheless one may quote Boyd (1973) who, after plotting a fossil geotherm using estimated temperatures and pressures, concluded, “Although crude, these estimates appear sufficiently accurate to be useful.”

Optical and Physical Properties

There is very good correlation between the optical properties and chemical composition of the orthopyroxene Series (Fig. 3). Deviations from the linear variation for the refractive indices are mainly due to variable contents of Al. Because many crushed fragments lie on a {210} cleavage plane, the γ index can be measured more readily than the Α and β indices; and this is the most precise optical method of determining the Mg/Fe ratio of an orthopyroxene.

The optic axial angles of the series vary continuously and symmetrically with increasing replacement of Mg by Fe²⁺. Enstatite is optically positive as is the iron end member orthoferrosilite , whereas in the center of the series the optic sign is negative and the optic axial angle varies from 90° to approximately 50° and rises again to 90°. When all available analyses are used, there is no consistent difference of 2V between volcanic orthopyroxenes and plutonic and metamorphic orthopyroxenes (Leake, 1968). The nomenclature used to describe the compositional variations in the series is also shown in Fig. 3. The divisions between enstatite and bronzite, and between eulite and orthoferrosilite , are at 88 and 12 mol% enstatite, respectively, where the optic sign changes.

Many orthopyroxenes, particularly those of bronzite and hypersthene composition, display a characteristic pleochroism, with Α pink, γ green. This pleochroism depends upon the simultaneous substitution of Fe³⁺ and Al in the structure, and the appropriate conditions appear to be most frequently found in the relatively high T and P conditions of granulite facies metamorphism.

The members of the orthopyroxene series can be distinguished from the clinopyroxenes by their lower birefringence and their straight extinction in all [001] zone sections. Also, the commoner members, bronzite, hypersthene, ferrohypersthene, and eulite are all optically negative, whereas the augite series is positive. The optic orientation of hypersthene is shown in Fig. 4. Their color in hand specimen varies from almost colorless to grey, yellow, brown, and black with increasing iron content. In thin section, they are colorless to reddish brown or greenish.

FIGURE 4

The optic orientation of hypersthene (from W. A. Deer, R. A. Howie, and J. Zussman, Rock Forming Minerals, vol. IIA, Single Chain Silicates, Longman Group Limited, London, 1978, p. 20).

Occurrence

The more magnesium-rich orthopyroxenes are common constituents of some ultramafic igneous rocks such as pyroxenites and picrites, in which they are commonly associated with olivine , augite, and a magnesian spinel (see Rock-Forming Minerals). Orthopyroxenes occur also in the cumulate rocks of many layered intrusions, e.g., Bushveld, Skaergaard; and Stillwater, where they range in composition from bronzite to ferrohypersthene. They are essential minerals in norites, where they often result from the assimilation of aluminumrich sediments by mafic magma; this reaction increases the amount of orthopyroxene and the anorthite content of the plagioclase at the expense of calciferous clinopyroxene:

Orthopyroxene is the most characteristic and important ferromagnesian mineral of the rocks of the charnockite series and is a typical mineral of the granulite facies of regional metamorphism. It is also produced in medium-grade, thermally metamorphosed, argiliaceous rock by the breakdown of chlorite . In argillaceous hornfels of higher grades it is derived from the breakdown of biotite . The more iron-rich varieties, eulite and the rare orthoferrosilite, are found associated with fayalite , grunerite , and almandine - spessartine garnet in eulysite, a regionally metamorphosed iron-rich sediment. Orthopytoxenes are an important phase in chondritic meteorites (see Meteoritic Minerals).

Further reading

Ganguly J., Domeneghetti M.C., 1996. Cation ordering of orthopyroxenes from the Skaergaard intrusion: Implications for the subsolidus cooling rates and permeabilities. Contrib Mineral Petr 122 (4): 359–367.

Kroll H., Schlenz H., Phillips M.W., 1994. Thermodynamic modeling of non–convergent ordering in orthopyroxenes – a comparison of classical and landau approaches. Phys Chem Miner 21 (8): 555–560.

Kirfel A., 1996. Cation distributions in olivines and orthopyroxenes – An interlaboratory study. Phys Chem Miner 23 (8): 503–519.

HughJones D., Chopelas A., Augel R., 1997. Tetrahedral compression in (Mg,Fe)SiO3 orthopyroxenes. Phys Chem Miner 24 (4): 301–310.

Camara F., Doukhan J.C., Domeneghetti M.C., et al., 2000. A TEM study of Ca–rich orthopyroxenes with exsolution products: implications for Mg–Fe ordering process. Eur J Mineral 12 (4): 735–748.