Anorthosite

Part of speech: noun

Definitions

  1. A coarse-grained intrusive igneous rock mainly composed of plagioclase feldspar with minor amounts of mafic minerals, often found in large plutonic bodies
  2. A type of plutonic rock characterized predominantly by nearly pure calcium-rich plagioclase feldspar and a minor presence of dark minerals like pyroxenes or olivine
  3. An igneous rock formed deep within the Earth’s crust, consisting chiefly of anorthite-rich plagioclase crystals interspersed with small quantities of ferromagnesian minerals

Etymology: This geological term entered English usage in the early 20th century, following advances in petrology and mineralogy that required precise names for newly identified rock types. It describes a type of intrusive igneous rock composed predominantly of plagioclase feldspar, with very little to no quartz content. The name was coined to reflect its distinctive mineral composition and crystalline structure. The word is formed from the prefix "an-" meaning "without" in Greek, combined with "orthos," meaning "straight" or "correct," and "-ite," a common suffix used in mineral and rock names. The "an-" here negates "orthos," indicating that the plagioclase crystals in this rock are not of the orthoclase variety (a potassium feldspar), but rather of a different, calcium-rich composition. Hence, the term literally means "rock without orthoclase," signifying its unique mineralogy. This naming convention aligns with the practice in geology of differentiating rocks based on their feldspar content, especially important during the early 1900s when the classification of igneous rocks was becoming more nuanced. The specificity of "anorthosite" helped geologists communicate clearly about rock types that otherwise might have been lumped together, aiding in mapping and understanding Earth's crustal composition. The term’s Greek roots connect it etymologically to other mineral names while emphasizing the negative prefix to highlight its distinctiveness. Its introduction reflects the evolution of scientific language, where classical elements are combined to create precise technical vocabulary, bridging ancient linguistic heritage and modern scientific discovery.

Synonyms: plagioclase-rich rock, igneous rock