Model Answer
0 min readIntroduction
Pyroxenes constitute a significant group of rock-forming minerals, crucial in understanding the composition and evolution of the Earth’s mantle and crust. These inosilicate minerals are characterized by their chain structure of silicate tetrahedra. They are found in a wide variety of igneous and metamorphic rocks, providing valuable insights into the geological processes that formed them. Understanding their classification, composition, structure, and specific properties like those of hypersthene is fundamental to petrological studies and geological mapping. This answer will comprehensively discuss these aspects of the pyroxene group.
Classification of Pyroxenes
Pyroxenes are broadly classified based on their chemical composition, particularly the proportions of magnesium (Mg), iron (Fe), calcium (Ca), and sodium (Na). The general formula for pyroxenes is XY(Si,Al)2O6, where X represents Ca, Na, Mg, Fe2+, and Y represents Mg, Fe2+, Al3+, Ti4+. The classification can be summarized as follows:
| Group | Chemical Composition | Key Minerals |
|---|---|---|
| Calcium Pyroxenes | Rich in Calcium (Ca) | Diopside (CaMgSi2O6), Heddenbergite (CaFeSi2O6) |
| Magnesium-Iron Pyroxenes | Rich in Magnesium (Mg) and Iron (Fe) | Enstatite (MgSiO3), Ferrosilite (FeSiO3), Hypersthene ((Mg,Fe)SiO3) |
| Sodium Pyroxenes | Rich in Sodium (Na) | Jadeite (NaAlSi2O6), Aegirine (NaFeSi2O6) |
| Omphacite | Intermediate composition (NaMgFe2+)Si2O6 | Omphacite |
Composition of Pyroxenes
The composition of pyroxenes is highly variable, leading to a continuous solid solution series between end-members. For instance, the Enstatite-Ferrosilite series (MgSiO3 – FeSiO3) exhibits a complete range of compositions. Similarly, the Diopside-Heddenbergite series (CaMgSi2O6 – CaFeSi2O6) shows compositional variation. The presence of aluminum (Al) can substitute for silicon (Si) in the tetrahedral sites, leading to variations in the mineral’s properties. Titanium (Ti) and manganese (Mn) can also occur in trace amounts, influencing color and other characteristics.
Structure of Pyroxenes
Pyroxenes crystallize in the monoclinic or orthorhombic crystal system. Their structure is based on chains of silicate tetrahedra (SiO4)4- linked together by sharing oxygen atoms. These chains run parallel to the c-axis. The chains are then linked by cations (Ca, Mg, Fe, Na) occupying M1 and M2 sites. The arrangement of these cations and the resulting chain packing determine the specific pyroxene structure. There are two main structural arrangements: clinopyroxenes (monoclinic) and orthopyroxenes (orthorhombic). Clinopyroxenes have a slight angle between the silicate chains and the c-axis, while orthopyroxenes have chains aligned almost perfectly with the c-axis.
Optical Properties and Occurrence of Hypersthene
Hypersthene, with the general formula (Mg,Fe)SiO3, is an orthopyroxene. Its optical properties are distinctive:
- Color: Typically olive-green to brownish-green, sometimes bronze-colored.
- Luster: Vitreous to resinous.
- Cleavage: Two perfect cleavages at nearly 90 degrees.
- Refractive Index: Relatively low, contributing to its low relief in thin section.
- Pleochroism: Strong pleochroism, exhibiting different colors when viewed from different crystallographic directions.
- Extinction Angle: Characteristically oblique extinction angle.
Occurrence: Hypersthene is commonly found in mafic and ultramafic igneous rocks such as gabbros, diorites, and peridotites. It is also a common constituent of metamorphic rocks like eclogites and granulites. Significant occurrences are found in the Bushveld Complex (South Africa), Stillwater Complex (Montana, USA), and various locations in Norway and Canada. It is often associated with olivine and plagioclase feldspar.
Conclusion
In conclusion, the pyroxene group represents a diverse and important suite of silicate minerals, classified primarily by their chemical composition and structural arrangements. Understanding their composition-structure relationship is crucial for interpreting the geological history of rocks in which they are found. Hypersthene, as a representative orthopyroxene, exemplifies the characteristic optical properties and occurrence patterns of this mineral group, making it a valuable indicator mineral in geological studies. Further research into pyroxene compositions and structures continues to refine our understanding of Earth’s dynamic processes.
Answer Length
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