Deep lagoon-blue crystals of cavansite occur in cavities in agglomerated breccia associated with Deccan basalts at Wagholi, western Maharashtra. X-ray, thermal and infrared studies, reported in this communication, suggest that cavansite is a 3-dimensional vanadosilicate network analogous to gismondine. Water molecules occur in the mineral both as loosely held and tightly bound crystal water.
Rb-Sr and Sm-Nd isotope compositions of seven garnet pyroxenite xenoliths from Udachnaya and Mir kimberlites, Siberia, are presented. Five of them are websterites, and the other two are clinopyroxenites. The websterite suite is divided into two groups (Group A and Group B) according to their mineral chemistry. Group B websterites are characterized by presence of jadeite-rich clinopyroxene and pyrope-poor garnet. The Garnet-orthopyroxene geobarometry indicates that Group A were produced in deep positions of the cratonic mantle (up to diamond stability field) and Group B websterites are formed at lower pressure (ca. 19 kb). Probably, the Rb-Sr isotope system was re-equilibrated at the time of kimberlite emplacement at 367 Ma. The initial Sr isotope compositions differs from one group to the other. The initial ratio 87Sr/86Sr (0.70272-0.70338) for Group A is lower than that of Group B (0.70485-0.70737). The Sm-Nd isotope system in all samples gives apparent isochron ages considerably older than that of the emplacement of host kimberlites. Group A websterites and clinopyroxenites are apparently younger (1223-582 Ma) than those of Group B (1550-1465 Ma). The origin of garnet pyroxenites is suggested as crystal accumulation from LREE-enriched melts which episodically added into peridotite-dominated lithospheric mantle. These melts played also a role of metasomatic agent to the host peridotites. A delaminated lower crustal protolith has probably been involved in Group B origin. The Sm-Nd isotope study reveals a systematic increase of initial 143Nd/144Nd with time. A vairable Sm-Nd isotope data suggest that peridotites and pyroxenites in lithospheric mantle beneath the Udachnaya area were in isotopic equilibrium, but not beneath the Mir area where peridotites depleted in initial Nd isotope composition.
A drill core from Tari-Misaka ultramafic complex, southwestern Japan, shows a one meter interval in which a decimeter scale stratification of dunite/harzburgite is well preserved. This part was petrologicaly examined in detail. Special attention was focused on the morphology and chemistry of chromian spinel. Chromian spinel is more abundant, more euhedral and higher in Ti and Fe3+ contents, and slightly lower in Cr/(Cr+Al) ratio in dunite than in harzburgite. Harzburgite becomes orthopyroxene-poor, and its chromian spinel tends to be similar to dunite spinel near (usually within 5 cm) the lithological between dunite. It was presumed that the dunite was formed as veins as a result of a reaction between harzburgite and an exotic melt: olivine crystals was formed by a reaction between hartzbergite and the melt. The petrological and chemical features of this part of the drill is basically similar to those in dunite with large chromitite pods, and harzburgite widely distributed in the Wakamatsu mine area. It is suggested that the dunites within harzburgite have the same origin irrespective of their dimension and the presence of chromite pods or not. The lower Cr# of spinel in the drill-core rocks than those in widely distributed dunite is possibly due to the difference in amount of orthopyroxene reacted with the melt. A large amount of orthopyroxene reacted and turbulency of melt current may be indispensable for the formation of podiform chromitite. The large melt conduit represented by dunite of semi-regional distribution in the Wakamatsu mine area possibly fulfilled the all of the conditions for the formation through this process of chromitite pods.
Kinoite was found in a dump at the Fuka mine, Okayama Prefecture, Japan. It occurs as aggregates of flaky crystals, and rarely as subhedral platy crystals up to 1 mm wide in a vein cutting into crystalline limestone. Associated minerals are stringhamite, calcite and an unidentified mineral. The empirical formula of the mineral is (Ca2.00Mg0.02)Σ2.02(Cu1.92Fe0.04Co0.04)Σ2.00Si2.98O10·2.25H2O on the basis of O=10 (anhydrous). The unit cell parameters are a=6.989 (1), b=12.904 (2), c=5.659 (1)Å, and β=96.15 (2)°. The mineral is optically biaxial negative with refractive indices α=1.642 (2), β=1.662 (2), and γ=1.675 (2). The Vickers microhardness is 536 kg mm−2 (50 g load) and the density is 3.14 g cm−3. It is likely that kinoite at the Fuka mine was formed as a primary mineral by a reaction of Cu- and Si-bearing fluids with limestone.