A mineral that is a magnesian variety of siderite.
Origin
Mid 19th century; earliest use found in James Nicol (1810–1879), geologist. From German Pistomesit from ancient Greek πιστός trustworthy, genuine + μέσον middle + German -it, after Mesitinmesitine, so called on account of its having been analysed as having the chemical consistency previously thought to be that of mesitine or mesitite, i.e. the exact intermediate between magnesite and siderite.