W-bearing rutile formed during alteration of jarosite by resurgent hydrothermal fluids in the oxide zone of the Kori Kollo gold deposit. The rutile shows sector zoning in basal sections and well developed multiple growth zones, both defined in backscatter electron images by variations in the W content. The maximum WO3 content is 5.3 wt.% and W substitutes for Ti with double substitution of Fe to maintain charge balance. The causes of multiple growth bands are considered to be changes in externally controlled variables occurring in a shallow hydrothermal system. Whereas Ti is probably leached from biotite in dacitic rocks, the W is introduced by hydrothermal fluids.

Synthetic andradite (Ca3Fe2Si3O12) has been compressed to loading pressures >21 GPa and heated to ∼1000°C by a YAG laser in a Diamond Anvil Cell (DAC). After quenching to room temperature, X-ray diffraction of the sample, still held at 21 GPa, showed that andradite had transformed to a cubic perovskite type polymorph with a = 3.460(4) Å. Upon decompression the perovskite phase transformed into an amorphous phase. The density of the perovskite polymorph (Ca3Fe2Si3O12) is ∼13.6% greater than that of isochemical andradite at 21 GPa. Ferric iron replaces Ca2+ and Si4+ in the perovskite structure (Fe3+ + Fe3+ = Si4+ + Ca2+), giving a formula unit: (Ca,Fe3+)(Si,Fe3+)O3. The new Fe3+-rich Ca-perovskite may provide new insight into the controls on the electrical conductivity of the lower mantle.