sphalerite geobarometry in the Balmat-Edwards district , New yorkl
Sphaleri te-pyri te-pyrrhoti te barometry was undertaken in the Balmat-Edwards distr ict in the NW Adirondacks to determine the pressures in the metamorphosed ore body. Metamorphic temperatures were estimated at 625 t 25"C using calcite-dolomite thermometry. Sphaleri tes with extensive exsolut ion of chalcopyrite are very inhomogeneous, even within ten microns of the po + py + sp tr iple point, and have iron contents of 16-12 mole percent FeS, corresponding to 3-7 kbar (Scott, 1973). This local variat ion in pressure is unreasonable; either the system did not equi l ibrate during metamorphism, errat ical ly reequil ibrated afterwards, or exsolved chalcopyrite has variably affected the barometer. A few sphaleri tes without microscopic chalcopyrite blebs yielded consistent FeS contents of 13-12 mole percent where touching pyrrhoti te-pyri te and correspond to pressures of 6.5-7.5 kbar. These data and the presence of si l l imanite, indicative of pressures less than 6-6.5 kbar at 600-650.C, in the adjacent paragneisses uggest hat the Balmat distr ict was subject to a regional metamorphism of 6.5 a 0.5 kbar. Incautious application of the sphaleri te barometer based on a few samples or on heterogeneous materials can lead to large errors in est imated pressures.
New Data on Phase Relations in the Cu-Fe-S System*
The phase relations in the central portion of the Cu-Fe-S "dry" system have been studied at 100oC and 600oC. The 600oC isothermal section is presented and low- temperature phase relations based on synthetic and natural assemblages are discussed. At low temperatures, the minerals chalcopyrite (CuFeS2), cubanite (CuF%Sa), talnakhite (CuoesSo), mooihoekite (Cuoeoo), and haycockire (Cu4FesSs) are stable phases. Chalcopyrite, talnakhite, and mooihoekite have been synthesized, while attempts to synthesize cubanite and haycockite have been unsuccessful. Synthetic talnakhite (/-phase), on heating,. shows transformations at ,186oC and at ,230oC to intermediate high-temperature phases, I and II, and finally transforms at 520 to 525oC to a face-centered cubic ((cc) sphalerite-like structure (iss). None of these phases are quenchable. Synthetic mooihoekite (-/-phase) transforms to intermediate high-temperature phase A at ,167oC, and this phase changes at ,236oC to the (cc sphalerite-like structure (iss). Both high-temperature forms of mooihoekite are un- quenchable. For the haycockite composition, an unquenchable high-temperature phase is formed between 20 and 200oC which appears isostructural with (cc high-temperature mooihoekite and iss. On quenching, this phase changes to a structure which can be indexed as having a primitive cubic cell with a  5.32A. Cubanite transforms fo a (cc phase at 200 to 210oC which appears isostructural The studies made to date have already shown that the central area of the Cu-Fe-S system is char- acterized by a large solid solution field at elevated temperatures which breaks up into five distinct phases at low temperatures. The picture is com- plicated by numerous phase transformations, un- quenchable phases, and phases with closely related crystal chemistry, resulting in their having similar physical appearance and X-ray diffraction powder patterns. The slow rate of most of the reactions at low temperatures also makes it difficult (in some cases impossible) to achieve equilibrium in the labo- ratory. In this study it was decided to approach an understanding of the low-temperature phase relations by a thorough study of the naturally occurring min- erals suspected to exist in the chalcopyrite composi- tion area because they have had a better chance to equilibrate and crystallize. Such mineralogical studies on talnakhite, mooihoekite, and haycockite have been reported above. With the realization that most of the low-temperature experiments would not reach equilibrium, careful electron probe analyses were
On the striation developed in cubanite by heating, a criterion for thermometamorphism
In experimenting with a microscope heating stage it was established that a striation develops in cubanite within definite limits of temperature in vacuo or under argon gas pressure. This feature remained as the sample was cooled and repolished. The X-ray studies carried out suggest a structural transition in cubanite as the striation was formed. The striation thus indicates the proximity of transition while the striation as such is a consequence of heat-expansion. The practical and genetic significance of the phenomenon is pointed out.ZusammenfassungBei Versuchen mit einem Heiztisch konnte unter dem Mikroskop festgestellt werden, daß in Cubanit innerhalb bestimmter Temperaturgrenzen im Vakuum oder in Argon-Gas (unter Druck) eine Lamellierung entsteht. Diese Erscheinung blieb beim Abkühlen und Aufpolieren erhalten. Die röntgenographische Untersuchung weist auf einen Strukturübergang in Cubanit hin. Die Lamellierung zeigt somit die Nähe eines Übergangs an, ist aber als solche eine Folge der Ausdehnung durch Erwärmen. Es wird auf die praktische und genetische Bedeutung hingewiesen.