Model Answer
0 min readIntroduction
Petrological microscopy is a fundamental technique in geology used to identify minerals in thin sections of rocks based on their optical properties. It involves observing minerals under plane-polarized light (PPL) and cross-polarized light (XPL) to reveal diagnostic features. Hornblende and augite are common rock-forming minerals belonging to the amphibole and pyroxene groups, respectively. Though they can appear similar in hand specimens, their distinct optical characteristics under the microscope allow for their precise differentiation, which is crucial for understanding rock genesis and metamorphic processes.
Characteristic Optical Properties of Hornblende and Augite
Under a petrological microscope, hornblende and augite exhibit distinct optical properties that aid in their identification. These properties are observed under both Plane-Polarized Light (PPL) and Crossed-Polarized Light (XPL).Hornblende (Amphibole Group)
- Color and Pleochroism (PPL): Hornblende typically displays strong pleochroism, meaning its color changes as the stage is rotated. It ranges from light yellow-green to dark green, or yellow-brown to dark brown [4, 10, 16]. This strong pleochroism is a key distinguishing feature.
- Relief (PPL): It exhibits moderate to high positive relief [4, 6, 15, 16], appearing to stand out against the mounting medium.
- Cleavage (PPL): Hornblende has two sets of perfect prismatic cleavages intersecting at characteristic angles of approximately 56° and 124° [4, 6, 9, 15, 16]. This produces a distinctive diamond-shaped cross-section.
- Birefringence and Interference Colors (XPL): Hornblende is anisotropic and shows second-order interference colors, which may be masked by its strong intrinsic color in thicker sections [4, 6, 9, 15, 16].
- Extinction (XPL): It exhibits oblique extinction, with maximum extinction angles ranging from about 12° to 30° in longitudinal sections [4, 6, 15]. Simple twinning is relatively common [10, 16].
Augite (Pyroxene Group)
- Color and Pleochroism (PPL): Augite is typically colorless to pale green, pale brown, or purple-brown [1, 4, 17]. It generally shows weak or absent pleochroism, distinguishing it from hornblende [1, 4, 17].
- Relief (PPL): It exhibits high positive relief [1, 4, 12].
- Cleavage (PPL): Augite has two distinct sets of prismatic cleavages intersecting at nearly right angles, specifically 87° and 93° [1, 4, 17]. This often results in a four-sided or eight-sided crystal shape in cross-section.
- Birefringence and Interference Colors (XPL): Augite is anisotropic and typically displays middle second-order interference colors, often bright yellow or green [1, 4, 17].
- Extinction (XPL): It shows inclined extinction, with maximum extinction angles ranging from 36° to 45° in longitudinal sections [1, 4, 12, 17]. Simple and polysynthetic twinning, sometimes forming a "herringbone" pattern, is common [1, 5, 12, 17].
The following table summarizes the key optical differences:
| Optical Property | Hornblende | Augite |
|---|---|---|
| Color (PPL) | Yellow-green to dark green/brown | Colorless to pale green/brown |
| Pleochroism (PPL) | Strong (distinct color change) | Weak or absent |
| Relief (PPL) | Moderate to High | High |
| Cleavage Angle (PPL) | 56° and 124° (diamond shape) | 87° and 93° (nearly right angles) |
| Interference Colors (XPL) | Second-order (may be masked) | Middle second-order (bright) |
| Extinction Angle (XPL) | 12°-30° (oblique) | 36°-45° (inclined) |
Conclusion
The petrological microscope serves as an indispensable tool for distinguishing between minerals like hornblende and augite, which are crucial components of various igneous and metamorphic rocks. Their characteristic optical properties—including distinct colors, pleochroism, relief, cleavage angles, birefringence, and extinction patterns—provide unique fingerprints for identification. Understanding these differences allows geologists to accurately classify rocks, infer their geological history, and determine the conditions under which they formed, thereby contributing significantly to the fields of petrology and mineralogy.
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.