The best way to tell the difference between amphibole and pyroxene is by examining their cleavage angles, particularly when viewing the minerals in cross-section looking down their c-crystallographic axis. This distinct difference in cleavage provides the most reliable diagnostic characteristic.
Understanding Mineral Cleavage
Mineral cleavage refers to the tendency of a mineral to break along specific planes of weakness, producing smooth, flat surfaces. These planes are determined by the internal atomic structure of the mineral. The angles at which these cleavage planes intersect are highly consistent for a given mineral and serve as a crucial diagnostic property for identification.
Key Distinguishing Feature: Cleavage Angles
The most effective method for differentiating amphibole and pyroxene involves observing the angles at which their cleavage planes intersect. This is best done by examining a cross-section of the mineral crystal.
- Pyroxene Cleavage: When viewed looking down its c-crystallographic axis, pyroxene exhibits two distinct cleavage planes that intersect at angles very close to 90 degrees. Specifically, these angles are typically observed at 92-93 degrees and 87-88 degrees. This near-perpendicular intersection is a hallmark of pyroxenes.
- Amphibole Cleavage: In contrast, amphibole crystals, when viewed looking down their c-crystallographic axis, display two cleavage planes that intersect at more oblique angles. These angles are characteristic and measure approximately 56 degrees and 124 degrees. The non-perpendicular intersection (often described as "rhombic" or "diamond-shaped" in cross-section) is key to identifying amphiboles.
Visualizing the Difference
To observe these angles, geologists often look at mineral grains in hand samples or, more precisely, in thin sections under a microscope. By finding a crystal oriented with its c-axis facing the viewer, the cross-sectional shape and the angles formed by the cleavage planes become apparent. Pyroxenes will tend to show a roughly square or rectangular outline with right-angle breaks, while amphiboles will exhibit a more elongated, diamond-shaped outline with oblique breaks.
Comparative Summary: Amphibole vs. Pyroxene
The table below summarizes the primary differences, with a focus on their cleavage patterns:
| Feature | Pyroxene | Amphibole |
|---|---|---|
| Cleavage Angle | Near 90° (specifically 92-93° and 87-88°) | 56° and 124° |
| Cross-Section | Generally squarish or octagonal | Diamond-shaped or hexagonal |
| Crystal Habit | Often stubby, prismatic, or blocky | Typically bladed, prismatic, or acicular |
| Typical Colors | Dark green, black, brown (e.g., augite) | Dark green, black (e.g., hornblende) |
Additional Distinguishing Characteristics (Secondary)
While cleavage is the most reliable method, other characteristics can support the identification:
- Crystal Habit: Pyroxenes often form short, stubby, or blocky crystals, giving them a more robust appearance. Amphiboles, on the other hand, tend to form longer, bladed, or needle-like (acicular) crystals.
- Cross-Sectional Shape: In many cases, the cross-section of a pyroxene crystal appears more squarish or octagonal, reflecting its near 90-degree cleavage. Amphiboles often show a more elongated, diamond-shaped cross-section due to their oblique cleavage angles.
By carefully observing the angles of the cleavage planes, particularly in cross-section, one can confidently distinguish between amphibole and pyroxene.
