perthite

This term is used exclusively within the specialized field of mineralogy and petrology. It describes a specific structural relationship where two different feldspars separate during cooling, creating a characteristic striped or lamellar appearance. Because it refers to a specific mineral variety, it is typically used in technical reports, geological surveys, and academic textbooks.

As a mass noun referring to a mineral substance, it is treated as uncountable. It describes the material itself rather than individual crystals, meaning it does not typically take a plural form in professional geological discourse.