The Fantastic Gneiss Rock: A Wonderful Metamorphic Marvel

What type of rock is described as a black and white banded rock with coarse crystals of biotite, quartz, and garnet?

A. Basalt

B. Gneiss

C. Marble

D. Slate

Answer:

The correct name for the rock described is gneiss.

The black and white banded rock with coarse crystals of biotite, quartz, and garnet, and foliation associated with the folds is known as gneiss. Gneiss is a high-grade metamorphic rock with bands of light (quartz and feldspar) and dark (biotite and garnet) minerals. The alignment of mineral crystals and the presence of folds are a result of high temperatures and pressures during metamorphism.

The rock you are describing is a type of metamorphic rock known as gneiss. This rock is characterized by its alternating bands of light and dark minerals, with evidence of foliation associated with folds. The light-colored bands are rich in quartz and feldspar, while the dark-colored bands are rich in biotite mica and garnet minerals.

Some specific types of gneiss, such as augen gneiss, have large, eye-shaped quartz grains. The black and white banding, along with the coarse crystals of biotite, quartz, and garnet, are crucial identifiers for gneiss.

Foliation in gneiss is represented through gneissic foliation, where the mineral crystals in the rock are aligned with each other creating alternating light and dark macroscopic bands. This alignment may be displayed as parallel planes along which the rock splits, by overlapping sheets of platy minerals. The process leading to the formation of gneiss typically involves intense metamorphism of originally igneous or sedimentary rocks under conditions of high temperatures and pressures.