mesitite

Mesitite (from — middle, between), formerly better known as mesitine spar, ferrous magnesite or brown spar is a variety of magnesite, a carbonate mineral, one of the so-called brown spars, regarded as an iron-bearing variety of magnesite or, in other cases, breunnerite. True to its name, mesitite is the middle member of a continuous isomorphic series magnesite—siderite with the general formula (Mg,Fe)CO3, in which the iron ion content (Fe2+) is approximately 30 to 50%, and the iron to magnesium ratio ranges, accordingly, from 30:70 to 50:50.

== Name and history == Mesitine spar, later abbreviated to mesitite in accordance with the internationally accepted terminology, less commonly mesitin (from ) was an old trivial name, widely used among miners, geologists and mineralogists for some ferruginous magnesites. Until the end of the 19th century, various authors sometimes considered mesitine spar to be a synonym (variety) of breunnerite, in other cases — a synonym (variety) of pistomesite. In the most general form, mesitine was considered to be a brown spar, which in turn is a variety of bitter spar (in this case, more often magnesite), colored with manganese or iron carbonate.