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Quartz vs Feldspar: Cleavage, Fracture and Hardness

Quartz and feldspar are the two most abundant-looking light minerals in many granites, pegmatites, and metamorphic rocks. Both are hard enough to scratch common glass, so the glass test alone does not separate them. Quartz lacks cleavage and commonly breaks with curved conchoidal surfaces; feldspar has two good cleavage directions near right angles and usually looks blockier. Look for repeated flat break surfaces. Feldspar commonly shows two cleavage directions meeting near 90 degrees, while quartz has no cleavage and breaks with curved, shell-like fracture. Quartz is Mohs 7; feldspar is about Mohs 6, but both can scratch glass. Rotate a fresh surface under light and trace repeated planes. Two flat directions near 90 degrees strongly favour feldspar. Look for curved conchoidal fracture or irregular glassy grains with no cleavage; these features favour quartz. Use a hand lens to check feldspar cleavage faces for fine parallel striations, especially in plagioclase. Use hardness only as supporting evidence. Both minerals commonly scratch glass, although quartz is harder. Confirm several grains in a rock because quartz and feldspar commonly occur together. A quartz grain attached to feldspar can make the specimen scratch glass, so test a clean representative grain rather than the whole mixed rock. Colour is not diagnostic: both minerals may be white, gray, pink, clear, or weathered. Do not break a valuable crystal to expose cleavage; use an existing damaged edge or a loose expendable grain.

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  • Granite vs Diorite — Compare granite and diorite using quartz, potassium feldspar, plagioclase, dark-mineral abundance, texture, and classification limits.
  • Gneiss vs Granite — Tell gneiss from granite using mineral banding, foliation, grain alignment, interlocking texture, cross-cutting contacts, and metamorphic context.
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