Feldspar (Orthoclase)
“The most common mineral you've never noticed”
Mineral
Orthoclase is a potassium-rich feldspar and part of the most abundant mineral group in Earth's crust — feldspars make up about 60% of all crustal rocks. Its name means 'straight fracture' in Greek, referring to its two cleavage planes that meet at a 90-degree angle.
Composition
potassium aluminum silicate (KAlSi₃O₈)
Where Found
Worldwide in granites and gneisses; notable crystals from Baveno (Italy), Madagascar, Eifel (Germany), Colorado (USA)
Mohs Hardness
6
Fun Fact
Orthoclase is number 6 on the Mohs hardness scale — it's the standard reference mineral that a steel file can just barely scratch.
Test Your Knowledge
Why does orthoclase feldspar weather to form clay minerals that are vital for soil fertility?
Photo Attribution
Related Specimens
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