Model Answer
0 min readIntroduction
Pyroxenes are a significant group of rock-forming inosilicate minerals, abundant in both igneous and metamorphic rocks worldwide. Their name, derived from Greek words meaning 'fire' and 'stranger,' reflects their common occurrence as crystals within volcanic lavas. These minerals are crucial for understanding Earth's petrological processes, as their presence and composition provide insights into the temperature, pressure, and chemical conditions of rock formation. Pyroxenes are characterized by a distinctive crystal structure based on single chains of silica tetrahedra, which dictates many of their physical and optical properties, making them identifiable under a polarizing microscope.
Structure of the Pyroxene Group of Minerals
The pyroxene group of minerals shares a common fundamental crystal structure based on single chains of SiO4 tetrahedra. These chains extend indefinitely parallel to the c-crystallographic axis. Each silicon ion is surrounded by four oxygen ions, forming a tetrahedron. Within the chain, each SiO4 tetrahedron shares two of its oxygen atoms with adjacent tetrahedra, forming a repeating unit of (SiO3)n.
These tetrahedral chains are bonded together by layers of octahedrally coordinated cations. The pyroxene structure contains two distinct non-equivalent octahedral cation sites, designated M1 and M2:
- M1 Site: This site is smaller and more regular, usually occupied by smaller cations like Mg2+, Fe2+, Al3+, Cr3+, and Fe3+. These cations are coordinated to six oxygen atoms. The M1 octahedra link the apices of adjacent tetrahedral chains.
- M2 Site: This site is larger and more distorted, accommodating larger cations such as Ca2+, Na+, Mg2+, and Fe2+. The coordination number for M2 can range from 6 to 8, depending on the cation size. The M2 sites typically link the bases of opposite tetrahedral chains. The size and charge of cations occupying the M2 site are crucial in determining whether a pyroxene crystallizes in the orthorhombic or monoclinic system.
The arrangement of these tetrahedral chains and octahedral cation layers forms a repeating unit often referred to as an "I-beam" or tetrahedral-octahedral-tetrahedral (T-O-T) strip. This structural arrangement is responsible for the characteristic prismatic cleavage of pyroxenes, which typically intersects at approximately 87° and 93° (nearly 90 degrees) on the basal plane (001).
Diagrammatic Representation of Pyroxene Structure:
While a detailed 3D diagram cannot be directly inserted as an image, here's a conceptual description for a diagram:
Figure 1: Schematic of Pyroxene Single Chain Structure
- Show a linear chain of interconnected SiO4 tetrahedra, extending horizontally (representing the c-axis). Each tetrahedron should be depicted sharing corners with its neighbors.
- Above and below this chain, illustrate the M1 and M2 cation sites as octahedra, linking these chains laterally. The M1 site (smaller, regular octahedron) would be central, bridging the apices of the tetrahedral chains. The M2 site (larger, less regular polyhedron) would connect the bases of the chains.
- Highlight the approximate 90° angle of cleavage planes that result from the bonding weaknesses between these I-beam units.
Figure 2: Pyroxene Quadrilateral for Chemical Classification
- A ternary diagram (triangle) with corners representing MgSiO3 (enstatite, En), FeSiO3 (ferrosilite, Fs), and CaSiO3 (wollastonite, Wo).
- The bottom line connects En and Fs, representing the orthopyroxene series.
- A field for clinopyroxenes (diopside-hedenbergite series) would be above this line, reflecting higher Ca content.
- This diagram helps visualize the solid solution series and nomenclature within the pyroxene group.
Chemical Compositions of Orthopyroxene
Orthopyroxenes (Opx) are a subgroup of pyroxenes that crystallize in the orthorhombic crystal system. Their general chemical formula is (Mg,Fe)2Si2O6, or more simply (Mg,Fe)SiO3. They form a solid solution series between two ideal end-members:
- Enstatite (En): Mg2Si2O6 (or MgSiO3) – The magnesium-rich end-member.
- Ferrosilite (Fs): Fe2Si2O6 (or FeSiO3) – The iron-rich end-member.
The composition of natural orthopyroxenes varies along this series, with intermediate compositions often termed bronzite or hypersthene depending on the Fe2+ content. For instance, hypersthene is an orthopyroxene with significant iron content.
While primarily composed of Mg and Fe, natural orthopyroxenes can also contain small amounts of calcium (CaSiO3 component, often less than 5 mol%), aluminium (Al), titanium (Ti), and other minor elements. The presence of these minor elements can influence their optical properties and stability conditions. For example, orthopyroxenes from high-temperature microscopic veins in olivine grains may exhibit lower CaO, TiO2, and Al2O3 contents, likely reflecting the composition of their precursor mineral.
The substitution of Al for Si in the tetrahedral site is limited in most orthopyroxenes, distinguishing them from amphiboles and some clinopyroxenes which can accommodate more Al in their tetrahedral sites.
Optical Properties of Orthopyroxene (under a polarizing microscope)
Orthopyroxenes exhibit several characteristic optical properties crucial for their identification in thin sections:
In Plane Polarized Light (PPL):
- Color: Typically colorless to pale pink, pale green, or reddish-brown. The color often deepens with increasing iron content.
- Pleochroism: Weak to moderate, often seen as a change in color from pale pinkish to pale greenish or reddish-brown as the stage is rotated. This pleochroism is a key diagnostic feature, especially in iron-rich varieties (e.g., hypersthene).
- Relief: Moderate to high positive relief, meaning the mineral grains appear to stand out from the surrounding mounting medium due to a significant difference in refractive index.
- Habit: Prismatic crystals, often with good cleavage. Basal sections may appear square or eight-sided.
- Cleavage: Two distinct sets of prismatic cleavage planes intersecting at approximately 87° and 93°. These are generally well-developed and visible.
In Crossed Polarized Light (XPL):
- Birefringence: Low, typically ranging from 0.007 to 0.020. This results in first-order interference colors, usually grey to yellow, sometimes extending to second-order purple in thicker sections or with higher iron content.
- Extinction: Straight extinction (or parallel extinction). The mineral extinguishes (turns black) when its long crystallographic axis is parallel or perpendicular to the polarizer direction.
- Twinning: Simple and lamellar twins are sometimes observed, and polysynthetic twinning is rare but possible.
- Optic Sign: Can be either optically positive (+) or negative (-).
- 2V Angle: Moderate to large, and variable depending on composition, generally decreasing with increasing Fe content.
| Optical Property | Description for Orthopyroxene |
|---|---|
| Color (PPL) | Colorless, pale pink, pale green, reddish-brown (Fe-dependent) |
| Pleochroism (PPL) | Weak to moderate (pink-green or reddish-brown hues) |
| Relief (PPL) | Moderate to high positive |
| Cleavage (PPL) | Two sets at ~87° and ~93° |
| Birefringence (XPL) | Low (0.007-0.020), 1st order grey to yellow/purple |
| Extinction (XPL) | Straight or parallel |
| Twinning (XPL) | Simple and lamellar twins may be present |
| Optic Sign (XPL) | Biaxial positive (+) or negative (-) |
Conclusion
The pyroxene group, characterized by its distinctive single-chain silicate structure, represents a vital component of igneous and metamorphic rocks, offering crucial insights into Earth's geological processes. Orthopyroxenes, specifically, exemplify this structure through their orthorhombic crystallization, forming a solid-solution series primarily between enstatite and ferrosilite. Their chemical variations, influenced by Mg-Fe substitutions and minor element incorporation, dictate subtle yet significant differences in their optical properties, such as pleochroism and birefringence. Understanding these structural, chemical, and optical attributes is fundamental for petrologists and mineralogists in deciphering rock origins and evolution.
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.