The mineral with the chemical formula K(AlSi3O8) corresponds to which potassium feldspar species?

The mineral with the chemical formula K(AlSi3O8) corresponds to which potassium feldspar species?

• A. Quartz
• B. Orthoclase
• C. Microcline
• D. Sanidine

Answer: (D)

Potassium feldspars share the composition KAlSi3O8, but exist as different high- and low-temperature forms that differ in crystal structure and ordering. The high-temperature end-member is sanidine, which forms at elevated temperatures in igneous environments such as rhyolites. Even though orthoclase and microcline have the same ideal formula, they represent lower-temperature forms with varying degrees of structural ordering. Quartz, by contrast, is SiO2 and not a potassium aluminosilicate. Therefore, the given formula best corresponds to sanidine.

Potassium feldspars share the composition KAlSi3O8, but exist as different high- and low-temperature forms that differ in crystal structure and ordering. The high-temperature end-member is sanidine, which forms at elevated temperatures in igneous environments such as rhyolites. Even though orthoclase and microcline have the same ideal formula, they represent lower-temperature forms with varying degrees of structural ordering. Quartz, by contrast, is SiO2 and not a potassium aluminosilicate. Therefore, the given formula best corresponds to sanidine.