Delafossite, CuFeO2, was found from the oxidation zone of the Besshi ore deposit which is embedded in Sambagawa metamorphic rocks, and is situated at the central Shikoku, Japan. It occurs as botryoidal aggregates in small cavities and as crusts filling cracks of oxidated ore in association with the supergene minerals such as native copper, cuprite, malachite, brochantite, chrysocolla, cuproallophane, hematite and goethite. It is black with submetalic luster, opaque, and pale yellowish white color in reflected light. The unit cell parameters are a = 3.020 (5) and c = 17.17 (3) Å. Delafossite from Besshi is chemically characterized by relatively high Zn content up to 9.9 wt% as ZnO. The empirical formulae with highest and lowest Zn content are (Cu0.89Zn0.11)Σ1.00 (Fe0.88Zn0.07Al0.03Ti0.01)Σ0.99O2 and (Cu.0.91Zn0.09)Σ1.00(Fe0.95Zn0.01Al0.01)Σ0.97O2 respecrively. The general formula of delafossite can be expressed as (Cu1+1-xZn2+x)(Fe3+1-xZn2+x )O2 or (Cu1+1-xZn2+x)(Fe3+1-xx/3)O2. Delafossite was formed by the decomposition of chalcopyrite ore associated with a small amount of sphalerite under the supergene environments.
Partial melting experiments of migmatitic pelitic gneiss were performed at 850 °C and 200 MPa with varied run duration up to 1008 hrs. Granitic melts form at almost all grain boundaries between various minerals including alkali-free pairs such as sillimanite-quartz. The melts exhibit compositional heterogeneity, particularly in SiO2 and Al2O3. Relatively voluminous melts form at feldspar-quartz boundaries, where the melt thickness increases with the run duration. In low-SiO2 domains located relatively far from quartz grains, Al2O3 mineral grains form around and in feldspar grains. The melts surrounding feldspars are enriched in Al2O3. In addition to the Al2O3 mineral, euhedral An-rich plagioclase newly forms around alkali feldspar, typically at the alkali feldspar-plagioclase boundaries. On the other hand, pre-existing plagioclase is partially modified to form ‘reaction zones’ composed of fine-grained An-rich plagioclase and interstitial melt along twin surfaces and cracks. The An-rich plagioclase at the outermost part of these reaction zones subsequently dissolves in the melts. These results indicate that the dissolution processes of alkali feldspar and plagioclase in peraluminous granitic melts are controlled not only by diffusive supply of SiO2 from quartz but also by the growth of Al-rich minerals and the interface reactions.