To identify feldspar, focus on its distinctive physical properties, particularly its excellent cleavage, which sets it apart from many other common minerals.
Key Identification Features of Feldspar
Feldspar, a group of rock-forming minerals, can be identified by observing a combination of its physical characteristics:
Cleavage
Feldspar exhibits good cleavage in two directions, typically intersecting at or very near 90 degrees. This means the mineral breaks along smooth, flat surfaces rather than fracturing irregularly. A common indicator in mineral grains is the presence of a straight edge, which strongly suggests it is feldspar. This feature is a primary way to distinguish it from quartz, which has no cleavage and therefore breaks with a conchoidal (shell-like) fracture, lacking straight edges.
Hardness
Feldspar has a moderate hardness, ranging from 6 to 6.5 on the Mohs scale. This means it can scratch glass and a steel knife, but it can be scratched by harder minerals like quartz.
Luster
Feldspar typically displays a vitreous (glassy) to pearly luster when observed on freshly broken or cleavage surfaces.
Color and Streak
Feldspar colors can vary widely depending on the type (e.g., orthoclase, plagioclase). Common colors include:
- White
- Gray
- Pink
- Cream
- Green (less common, like amazonite)
All feldspars produce a white streak when rubbed against an unglazed porcelain plate.
Specific Gravity
Feldspar has a specific gravity generally ranging from 2.5 to 2.8, making it relatively light to moderately dense.
Crystal Habit
Feldspar commonly forms prismatic or blocky crystals. In igneous and metamorphic rocks, it often appears as well-formed, often rectangular, grains.
Striations (for Plagioclase Feldspar)
A particularly useful feature for identifying plagioclase feldspar is the presence of fine parallel lines or grooves called striations on one or more of its cleavage faces. These striations are a result of polysynthetic twinning and are usually visible with a hand lens. Potassium feldspars (like orthoclase and microcline) do not exhibit striations.
Distinguishing Feldspar from Similar Minerals
It's crucial to differentiate feldspar from other minerals that may appear similar, such as quartz.
| Feature | Feldspar | Quartz |
|---|---|---|
| Cleavage | Good cleavage (2 directions at/near 90°) | No cleavage (conchoidal fracture) |
| Hardness | 6–6.5 Mohs | 7 Mohs |
| Luster | Vitreous to pearly | Vitreous |
| Striations | Present on plagioclase | Absent |
| Crystal | Often blocky, rectangular | Often prismatic with pyramidal terminations |
When examining mineral grains, a straight, flat edge is a strong clue for feldspar, differentiating it from the typically irregular or curved fracture surfaces of quartz.
Practical Tips for Field Identification
- Hand Lens: Always carry a hand lens (10x magnification) to observe fine details like striations and subtle cleavage planes.
- Hardness Test: Use a steel nail or a piece of glass to test its hardness. If it scratches glass but isn't scratched by a nail, it's likely feldspar.
- Examine Fresh Surfaces: Look for cleavage on freshly broken surfaces as weathered surfaces can obscure these features.
- Consider the Rock Type: Feldspar is a primary component of many igneous and metamorphic rocks, such as granite, basalt, and gneiss.
By systematically evaluating these physical properties, especially its characteristic cleavage, you can confidently identify feldspar.
