Model Answer
0 min readIntroduction
Pyroxenes constitute a significant group of rock-forming minerals, crucial components of igneous and metamorphic rocks. They are inosilicate minerals, meaning their structure is based on chains of silicate tetrahedra (SiO4). These minerals are characterized by their prismatic or acicular (needle-like) crystals and typically exhibit a vitreous to resinous luster. Understanding the classification, composition, structure, and specific properties of pyroxenes, like hypersthene, is fundamental to interpreting the petrogenesis and geological history of rocks. This answer will delve into these aspects, providing a comprehensive overview 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 main pyroxene series are:
- Enstatite Series: (MgSiO3 – FeSiO3) – Magnesium-rich to iron-rich. Characterized by low calcium content.
- Orthopyroxene Series: (Mg,Fe)SiO3 – A solid solution series between enstatite and ferrosilite.
- Clinopyroxene Series: (Ca,Mg,Fe,Al)(Si,Al)2O6 – Calcium-rich. This series is more complex and includes several important members.
- Wollastonite Series: CaSiO3 – A calcium silicate mineral, often associated with skarn deposits.
Composition of Pyroxenes
The general chemical formula for pyroxenes is XY(Si,Al)2O6, where X represents Ca, Na, Mg, Fe2+, and Y represents Mg, Fe2+, Al3+, Ti4+. The specific composition determines the mineral's properties.
Key compositional variations include:
- Augite: (Ca,Mg,Fe)2(Si,Al)2O6 – The most common clinopyroxene, containing varying amounts of calcium, magnesium, and iron.
- Diopside: CaMgSi2O6 – A calcium magnesium silicate.
- Hedenbergite: CaFeSi2O6 – A calcium iron silicate.
- Aegirine: NaFeSi2O6 – A sodium iron silicate.
Structure of Pyroxenes
Pyroxenes possess a chain silicate structure. Silicate tetrahedra (SiO4) are linked together to form long chains. These chains are held together by cations (Ca, Mg, Fe, etc.) located in the spaces between the chains. The structure differs slightly between ortho- and clinopyroxenes.
- Orthopyroxenes: Have chains that are arranged parallel to each other, resulting in a monoclinic crystal system.
- Clinopyroxenes: Have chains that are arranged at an angle to each other, resulting in a monoclinic or triclinic crystal system.
The arrangement of cations and the presence of aluminum in the structure influence the mineral's properties, such as density and refractive index.
Hypersthene: A Detailed Commentary
Composition and Structure
Hypersthene is an orthopyroxene with a composition typically represented as (Mg,Fe)SiO3. It is an intermediate member of the enstatite-ferrosilite series, containing significant amounts of both magnesium and iron. Its structure is characterized by single-chain silicate tetrahedra linked by magnesium and iron cations.
Optical Properties
Hypersthene exhibits distinct optical properties:
- Color: Typically dark green to brownish-black.
- Luster: Vitreous to resinous.
- Cleavage: Two perfect cleavages at nearly 90 degrees.
- Refractive Index: Relatively high, contributing to its noticeable birefringence.
- Pleochroism: Strong pleochroism, meaning it exhibits different colors when viewed from different crystallographic directions. This is a key identifying feature.
Occurrence
Hypersthene is commonly found in:
- Igneous Rocks: Frequently occurs in mafic and ultramafic igneous rocks such as gabbros, diorites, and peridotites.
- Metamorphic Rocks: Found in metamorphic rocks like skarns and granulites, often formed during regional metamorphism.
- Impact Craters: Can be formed by shock metamorphism during meteorite impacts.
- Specific Locations: Notable occurrences include the Adirondack Mountains (USA), and various locations in Canada and Russia.
Conclusion
The pyroxene group represents a diverse and important suite of silicate minerals, crucial for understanding the composition and evolution of the Earth’s mantle and crust. Their classification, based on chemical composition and structural variations, allows for accurate identification and interpretation of rock origins. Hypersthene, as a representative orthopyroxene, exemplifies the characteristic optical properties and occurrence patterns of this mineral group. Continued research into pyroxene compositions and structures will further refine our understanding of planetary processes.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.