Model Answer
0 min readIntroduction
Pyroxenes and amphiboles are both significant groups of silicate minerals commonly found in igneous and metamorphic rocks. They are crucial in understanding the petrogenesis and geological history of various terrains. Both mineral groups share a common building block – the silicate tetrahedron – but differ significantly in their crystal structures and resultant properties. These differences stem from variations in their chemical compositions and the arrangement of silicon, oxygen, and other cations within their structures. Understanding these distinctions is fundamental to accurate mineral identification and geological interpretation.
Crystal Structures
The fundamental difference lies in their crystal structures. Pyroxenes crystallize in the monoclinic or orthorhombic crystal system, characterized by chains of silicate tetrahedra. These chains are linked by cations like magnesium, iron, calcium, and sodium. Amphiboles, on the other hand, crystallize in the monoclinic system and possess a double-chain silicate structure. This double-chain arrangement is formed by linking two single chains of silicate tetrahedra together.
Pyroxene Structure
Pyroxenes have a general formula of XY(Si,Al)2O6, where X represents divalent cations (Ca, Mg, Fe2+) and Y represents divalent or trivalent cations (Mg, Fe2+, Al3+, Na, K). The chains are relatively rigid, leading to specific cleavage angles.
Amphibole Structure
Amphiboles have a more complex formula, generally represented as A0-1B2C5T8O22(OH,F)2, where A, B, C, and T represent different cation sites. The presence of hydroxyl (OH) or fluorine (F) in their structure is a defining characteristic. The double-chain structure allows for greater flexibility and a wider range of cation substitutions.
Physical Properties
The differences in crystal structure directly influence the physical properties of these minerals.
Cleavage
Pyroxenes exhibit two distinct cleavage planes intersecting at nearly 90 degrees, resulting in prismatic or columnar habits. This is due to the relatively strong bonds along the chain direction and weaker bonds perpendicular to it. Amphiboles display two good to excellent cleavage planes intersecting at angles of approximately 60 and 120 degrees, leading to a characteristic bladed or prismatic habit. This is a consequence of the double-chain structure and the arrangement of cations.
Hardness
Pyroxenes generally have a hardness ranging from 5.5 to 6.5 on the Mohs scale, while amphiboles typically range from 5 to 6. This slight difference is related to the bond strength within their respective structures.
Specific Gravity
The specific gravity of pyroxenes varies from 3.2 to 3.6, depending on their chemical composition. Amphiboles generally have a lower specific gravity, ranging from 2.9 to 3.3, due to the presence of lighter elements like hydroxyl and fluorine.
Luster
Both pyroxenes and amphiboles exhibit vitreous (glassy) luster, but amphiboles can sometimes appear pearly on cleavage surfaces due to the presence of hydroxyl ions.
Optical Properties
Optical properties also differ significantly between the two mineral groups.
Birefringence
Pyroxenes generally exhibit moderate to high birefringence (0.015-0.030), while amphiboles show lower birefringence (0.005-0.020). Birefringence is the difference between the two refractive indices of a mineral.
Pleochroism
Many pyroxenes display distinct pleochroism (change in color with different viewing angles), particularly those containing iron. Amphiboles also exhibit pleochroism, but it is often less pronounced than in pyroxenes.
Extinction Angle
Pyroxenes typically exhibit parallel extinction, meaning that the mineral appears dark when aligned with the polarizer. Amphiboles usually show oblique extinction, with an extinction angle between 0 and 60 degrees.
Optical Character
Both are generally biaxial minerals, but the optical character and the position of the optic axis differ based on their composition and crystal structure.
| Property | Pyroxene | Amphibole |
|---|---|---|
| Crystal System | Monoclinic/Orthorhombic | Monoclinic |
| Chain Structure | Single Chain | Double Chain |
| Cleavage | ~90° | ~60° & ~120° |
| Hardness (Mohs) | 5.5 - 6.5 | 5 - 6 |
| Specific Gravity | 3.2 - 3.6 | 2.9 - 3.3 |
| Birefringence | Moderate to High | Low |
| Extinction | Parallel | Oblique |
Conclusion
In conclusion, pyroxenes and amphiboles, while both silicate minerals, exhibit fundamental differences in their crystal structures, physical properties, and optical characteristics. These differences arise from the single versus double-chain silicate structure and variations in chemical composition. Accurate identification of these minerals is crucial for understanding the geological processes that formed them and the rocks in which they occur. Further research into the subtle variations within each group continues to refine our understanding of Earth’s dynamic systems.
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.