GNEISS
Gneiss
is a high-grade, foliated metamorphic rock. It differs from lower grade
foliated metamorphics, such as slate, phyllite, and schist, by having
alternating bands/stripes of light & dark minerals. Metamorphism has
been so intense that minerals have self-segregated into discrete zones.
Most gneisses form by metamorphism of schists or granites.
Gneiss
(6.8 cm across) - this is a quartz-biotite gneiss; the whitish-gray bands are
quartz (SiO2) & the black bands are biotite mica.