Showing papers in "American Mineralogist in 1969"

Chemical composition of altered chromites from the Stillwater Complex, Montana

Abstract: Small amounts of very iron-rich chromite are formed as an alteration product of normal cumulus chromite accompanying the alteration of primary silicates to chlorite in the Stillrvater Complex, Montana. We have analyzed two parent chromite-ferritchromit pairs from the Stillwater Complex b1'a combination of wet and microprobe methods; both the parent chromite and ferritchromit can be expressed by the general formula (Mg, l-e,+) (Cr, Al, Fe3+)rOr, and neither contains cation deficiencies of the maghemite type. The alteration is essentially a volume-for-volume replacement of MgO and AbOr by FeO and !'erOr; content is oniy slightly different in the primary and secondary phases.

Eclogite inclusions in kimberlite pipes at Garnet Ridge, Northeastern Arizona

Abstract: Pyroxene in eclogite inclusions from kimberlite pipes at Garnet Ridge, northeastern Arizona, is essentially diopsidic jadeite (58 to 73 percent NaAlSizOo) with an appreciable proportion of acmite. The coexisting garnet is pyropic almandine (62 to 70 percent Fe3 AIzSiaOrz and 12 to 23 percent ![g3A12Si3Or:) with an appreciable content of grossularite. Chemically, the eciogite at Garnet Ridge differs markedly from those found elsewhere as inclusions in kimberlite. Although the compositions of pyroxene and garnet in eclogite from Garnet Ridge resemble those of pyroxene and garnet in eclogite occurring as lenses in glaucophane schist terrains, there is no geological evidence at Garnet Ridge suggesting derivation from such a basement. The association of sparse eclogite inclusions with abundant xenoliths of granite gneiss and granite at Garnet Ridge may suggest that the garnetpyroxene rocks were derived from eclogite lenses occurring in a migmatitic granite gneiss basement. However, the presence of similar granitic inclusions in abundance without eclogite in nearby minette diatremes does not favor this idea. 'fhe fact that soda-rich eclogite occurs at Garnet Ridge in the same general region as the Hopi Butte: soda-rich volcanic province raises the question as to whether there may be some genetic significance to this association.

The crystal structure of sapphirine

Abstract: SAPPHIRINE is a principal constituent of certain high grade silica-deficient metamorphic rocks at Fiskenaesset, West Greenland, where it occurs with cordierite, corundum, anorthite and biotite. A crystallo-chemical enigma, it is one of the few rock-forming minerals with unknown crystal structure.

Crystal structure and cation ordering of piemontite

Abstract: The crystal structure of the Mn3+-bearing epidote, piemontite, has been refined. For a crystal of composition, Cz6,Pm"PSll, refinement of the occupancies cf the two large, nominally calcium sites and the three non-equivalent octahedral sites, gives: where M = MnD. ,Feu. All of the (Mn, Fe) substitution for Al occurs in one of the two nonequivalent chains of edge-sharing octahedra. In the substituted chain, which is topologically identical with that found in the olivine structure, the larger M(3) site (symmetry-m) lying on the edge of the chain, contains most of the (Mn,Fe) with lesser amounts in the smaller M(l) site (symmetry-I) which forms the core of the chain.