Showing papers in "Canadian Mineralogist in 1977"

Homogenization of zoned garnets from pelitic schists

Abstract: ABsrRAcr Garnets from low-variatrce pelitic assemblages in the garnet and staurolite zones of the Mt. Rabfuh area, 8.C., generally show strong compositional zoning. A decnease in Mn oufwards from the oore is balanced by an increase in Fe and Mg. Zoning in garnets from the cordierite zone is weak or absent. Mn oommonly insreases.sligbtly from core to rim; Mg shows a corresponding decrease. Ihere is little or no Ca zoning in eitler high or low-grade rocks. At all metamorphic grades, the garnet rims are in partition equilibrium with adjacent ferromagnesian minerals. Ilmenite inclusions in low-grade rocks are in eguilibrium with immediately adjacent garqet, but inclusions in cordierite-zone ganrets are not in equilibrium with €ianet. The data suggest: (1) gxowth of zoned garnets by a fractionation-dopletion process aX lower grades; (2) homogenization of zoned garnets at temperatures abovo about 600'C; (3) modification of the bulk composition of homogeneous garnets by exchange equilibria involving garnet and cordierite; and (4) formation of Mn-rich rims on both high and low-grade garnets during retrogression of the garnet. Volume diffusion was the main process which created and modified the zoning in the cordierito zone, but was of only minor importance in lower grade rocls. The onset of significant volume diffusion may have initiated changes in the equilibria with other minerals.

Progressive homogenization of metamorphic garnets, South Morar, Scotland; evidence for volume diffusion

Abstract: AssrRAcr The distribution of Fe; Mn, Ca and Mg has been examined in a suite of optically and texturally zoned garnets from regionally metamorphosed rocks, South Morirr, Inverness-shire, Scotland. At the garnet grade of metamorphism, smaller garnets (2-3 mm) are strongly zoned and have Mn-rich cores. Larger garnets from the same rocks (3-4.5 mm) havo homogeneous cores with lower concetrtrations of Mn. With increasing gade across the kyanite and sillimanite zotres, optical and textural zones gradually disappear, and garnetS of all sizes become progressively more homogeneous. Volume diffusion, during or after growth, provides the simplest explanation for the occurrence of the homogeneous garnets in both situations.

The tanco pegmatite at bernic lake, manitoba. ix. beryl*

Abstract: Beryl occurs in 4 out of 9 zones and replacement units of the Tanoo pegmatite. The major accumula­ tions are in the wall zone and centml intermediate zone, and they are concentrated in the footwall parts of the pegm atite. Six paragenetic varieties of primary beryl are recognized, but most have the same range of chemical composition and may be genetically related. The alkali contents of the ma­ jority of primary beryl are am1Jng the higest known (lR.O about 6 to 7 wt. %, including 2.5 to 3.75 wt.

The structural role of ferric iron in silicate melts

Abstract: fron is the major cation idvolved in oxidationre,drrti,* processes in magmatic silicate liquids. The rnolecular structural confillurations assumed by ferric and fi:rrous iron arJ important in deterrnining their activities in the$e liquids. They, in turn, affect both crystril-fractionation trends a1d the compositions of precipitated iron-titaniurn phases on cooling. rn discussing the structural characteristics of iron in silicate rrelts, it is useful to consider the general structural model which has emerged through investigation in the past two decades. In explaining ma:ry physical properties of these liqtti4s, it has been useful to regard acid cations such as Sia+ as \"network-formels,, (in four-fold oxygen coordination), and basic cations such as Na, K. Ca, Mg Fe2+ as \"network-modifiers,' (in six-fold or higher coordination). On the basis of sirnple geometric packing arguments, aluminum has an ionic radius intermediate between that preferring fourand six-fold soordination. It can thus assume either structural role. To exist in tetrahedral coordination, however, it must have art \"associated\" charge balancing cation (i.e. Na, K Ca'a, Mga, etc.). This simplistic structural model finds extensive support in studies of alumino-silicate melts in which singularities in physical properties have been observed when tfe iatio of Al8+ to monovalent*Zzdivalent cations = I (i.e. stochiometric composition). Such singularities occur in refractive indices (Schairei & Bowen 1956), and electrical conductivitv Osard f 959). The viscosity model of Bottinga t W\"itt (1972), which is also based on this structural model,-has been very successful in accurately predicting the viscosities of silicate melts over a wide range of composition and temperature. The high-pressure extension of this model (Waff 1975) has successfully predicted a viscosity decrease in magmas with increased pressure, as verified by Kushiro et aI. (1966). It also offers a consistent explanation of the electrical conductivity data of Waff & Weill (1975) for magmatic liquids.

Platinum-group minerals from Onverwacht; I, Pt-Fe-Cu-Ni alloys

Abstract: Ferroan platinum and two different types of FtFe{u-Ni alloys occur in a sample from tle former Onverwacht mine, Transvaal. Alloy t has a composition near PtFeo.rNt.r"Cuo.ro and is tetragonal, P$/mmm, a 2.741(3J, c 3.6a4Q)4,, D(meas.) 14,3 g/cms, D(calc.) 15.80 g/cm8. It may represent a copper- and nickel-rich variety of tetraferroplatinum. Should that bo the case, the unit cell of type tetraferroplatinum from Mooihoek must be a 2.724(6), c 3.702Q)4. A[oy I is especially characterized by perfect {001} cleavage and is very sectile. Alloy 2 has the composition (Pt,Pd) l.oaFeo.zgCuo.zeNio.zsSbo.ro but its X-ray powder pattern could not be indexed.

The composition and properties of some native platinum concentrates from different localities

Abstract: AsstrRAgT Tho mineralogy, microhardness, and reflectivif of Pt-Fe alloy concentrates from four localities are dessibed. Illicrohardness does not give any useful iqdication of the chemical composition of Pt'Fe alloys, but tle reflectivity is shown to increase strongly with the iridium content and decrease weakly with the other solid-solution elements. In the platinum-rich grains examined here, the mineralogical constitution depends critically on the ratio of iridium to osmium. This has been explained satisfactorily in terms of a pseudo-ternary phase diagram sec.tion, an important featur€ of which is a large tlree-phase field. This phase diagram, which may be generally applicable to all native-metal min' erais compos,ed of Pt, Ir, Os, Ru and Fe, shows the limits of tbe terminal solid solutions and predicts the compoeitions of contiguous phases. The mineralogiel studies indicate tlat suffides of the platinoid metals other than platinum are formed in preference to cooperite, in agreement witl previous work. Calculated free energies of formatioo are used to explain these observations. All tle platinoid-metal sulfides apparently harn formed at comparatively low temperatures.