It is an oxide mineral belonging to with magnesiochromite (MgCr O ); substitution of aluminum occurs leading to hercynite
(FeAl O ).2 4
Careful examination may reveal more occurrences of diopside, but I recall only small subhedral to anhedral grains of what appears to be diopside associated with small hercynite
octahedrons and clintonite rosettes near the summit.
The occurrence of orthopyroxene and hercynite
, and specific chemical composition of high-grade phases, such as a high Ti content in biotite and a high Al content in orthopyroxene, are in accordance with the other geothermobarometric estimates, which suggests the peak temperature of formation over 800[degrees]C and pressure of 5-6 kbar (Fig.
They were usually classified as either pleonast or hercynite
; however always exclusively on the basis of visual microscopic evaluation.
The deoxidation products are [Al.sub.2][O.sub.3] and hercynite
The most common of these are hercynite
(FeO[Al.sub.2][O.sub.3]) and its solid solution series, and magnetite ([Fe.sub.3][O.sub.4]) (Hofmann 1962, 1966, Longworth and Tite 1979, Longworth and Warren 1975, Maggetti et al., 1981).
Associated minerals are: hematite, ilmenite, pseudorutile, magnetite and hercynite
The composition of larger lobate grains shows their pertinence to titaniferous magnetite with a higher content of ulvospinel molecule ([Fe.sub.2]Ti[O.sub.4]) and low contents of Mn, Mg, Al, thus forming very low proportions of jacobsite, magnesioferrite, and hercynite
, respectively, and sometimes V.
In the +90% alumina refractory system, iron oxide can react with the alumina to form a spinel compound FeO*[Al.sub.2][O.sub.3] (hercynite
For example, aluminum-coated or killed steel used in cast iron melting will result in alumina or alumina complex mineral deposits such as hercynite
(iron aluminate), mullite (silica alumina), etc.
PHOTO : CL micrograph of a buildup sample showing anorthite (yellow), corundum (red) and hercynite