OTTRELITE (from Ottrez, Belgium, the original locality), in mineralogy, a member of a group of closely related minerals with the general composition RO. in which RO is dominantly FeO, but is often in part represented by a content of Mg0 or MnO. On account of its petrographic importance the term ottrelite is often used as the group name for this series of minerals, though the name clintonite is also in use. The chief members are chloritoid (H, Fe Al, sismondine (H, [Fe, Mg] in which the magnesia rises to 7%, and ottrelite the manganiferous variety in which the Mn0 content may rise to 8%. They are grey, green or black micaceous minerals, but in distinction from the "elastic" micas and "flexible" chlorites they are often referred to as the "brittle micas" on account of the brittleness of their laminae. Like the micas and chlorites they possess monoclinic symmetry and a perfect cleavage parallel to the flat surface (00i) of the plates. Their superior hardness (H=6.5) readily distinguishes them from both these groups of minerals. Multiple twinning on the mica
law is exceedingly common, and a zonal structure (often of hour glass type) is often apparent. The ottrelite group of minerals is confined to metamorphic rocks—particularly those developed in regional metamorphism—in slates, phyllites and schists. Note worthy occurrences of this mineral are the slates of the Ardennes, of Tintagel (Cornwall) and in the Mesozoic and Permian schists of the Swiss and Italian Alps.
The minerals margarite, xanthophyllite and kossmatite show some relations with the ottrelite group of minerals. They are distinguished by an inferior hardness and contain calcium as an essential constituent. Margarite O.) occurs in white pearly scales associated with corundum and is a common mineral of emery deposits. Xanthophyllite occurs in talc-chlorite schists at Slatoust in the Urals and in altered limestone at Riverside, California, while kossmatite is a recently described lime-rich mineral (H=2.5) occurring in the dolomite marbles of West Macedonia. (C. E. T.)