Ferromagnesian
Describes minerals or mineral-rich rocks containing significant iron and magnesium; commonly applied to dark silicates such as olivine, pyroxene, amphibole and biotite.
Language and route: Modern English chemical compound. English ferro- from Latin ferrum, ‘iron’, combined with magnesian relating to magnesium; not an inherited single word from classical Latin.
Literal gloss: iron-and-magnesium-bearing
The name and its history
Not established · English petrographic term; precise first geological use unverified
- What does the word literally mean?
- Contains iron and magnesium; this is a modern chemical compound.
- What did the original author mean?
- The term described mineral constituents identified through chemical mineralogy, especially dark silicates that figure prominently in igneous and metamorphic petrology.
- Why was the name needed?
- Rock describers needed to distinguish Fe/Mg-rich silicates from quartz and feldspars and describe their changing proportions.
- What evidence supported the original concept?
- Mineral identification and iron–magnesium chemistry; USGS publications give examples and distinguish ferromagnesian composition from a rock-colour class.
- How has geology changed the concept?
- Modern classification may specify mafic minerals or a formal colour index; those schemes are not identical to simply adding the iron and magnesium content of a bulk rock.
Further historical context
This is an instructive counterexample to folk etymology: ferro- is a chemical prefix; the magnesium part reflects modern element nomenclature.
Not every dark constituent qualifies as ferromagnesian, and a separate compositional or modal scheme may use the word more narrowly.
Editorial qualification
First-use priority has not been established; modal thresholds differ between mapping and igneous-classification schemes.
Related names
Leucocratic · Colour index · Igneous · Metamorphic · Plagioclase
Sources
- USGS, Magnesium: Geochemical and Mineralogical Information — Modern ferromagnesian mineral examples
- USGS, Science Language for Geologic-Map Databases: composition qualifiers — Modern operational ferromagnesian and composition definitions
- Streckeisen (1976), To each plutonic rock its proper name — IUGS modal classification and colour index as a modifier