Summary: A garnetite that occurs locally in phyllitic schists exposed by the Santorini caldera consists mainly of euhedral garnet crystals showing alternating isotropic and birefringent lamellae. X-ray diffraction revealed two garnets with unit cell dimensions a of 12·052 ± 0·002 and 11·984 ± 0·005 Å, corresponding respectively to almost pure andradite and a granditic phase. Mean compositions determined by microprobe analysis are An97Sp1Alm2 for the isotropic and about An63Gr33Sp1Alm3 for the birefringent layers.
The garnets are deduced to have been produced mainly by thermal metamorphism of the schists. Although cyclic variations in the composition of circulating fluids could have brought about the zoning, this may just as well be the result of temperature fluctuations, individual garnet compositions being indicative of the respective formation temperature.