Piedmont lithologies Metamorphic grade in these rocks is very high. The mineralogy indicates metamorphism took place at temperatures and pressures well within the upper amphibolite to granulite facies. Retrograde metamorphism is minor, and in most cases the mineral phases appear to be in equilibrium. Biotite Gneiss Mineralogically, the biotite gneisses are composed of plagioclase, quartz, biotite, and iron oxides. Garnet, sillimanite, and orthoclase in various combinations are also present in the samples.
Accessory minerals are zircon, sphene, and apatite.
Some of the plagioclase grains are antiperthitic. Contacts between the coarse and fine-grained phases are always sharp and may have large elongated poikiloblastic garnets concentrated along them. The hornblende and biotite may be oriented to define a weak foliation, but in many of the gneisses these minerals are not aligned. Locally, the felsic gneisses contain segregations of quartz and plagioclase with a single magnetite grain in the core. The magnetite grains range in size from 2 to 4 mm, and the plagioclase-quartz mantles range from 5 to 8 mm.