A sedimentary rock is metamorphosed into three rock types: (1) a banded gneissic rock; (2) a rock with small mica crystals with a shiny aspect; (3) a very shiny rock with large mica crystals; and (4) a dull slate with cleavage. Which of the following lists these rocks in order of increasing metamorphic grade?

A dull slate with cleavage < a rock with small mica crystals with a shiny aspect < a very shiny rock with large mica crystals < a banded gneissic rock

Explanation:

Increasing metamorphic grade comes with increase in temperature and pressure on the rock materials. Metamorphism is also accompanied by new minerological changes and growth of new minerals. The mineral sizes becomes bigger and they begin to align under the influence of stress.

From the given metamorphic rocks, our protolith might have been a clayey sandstone.

As metamorphism of the rock proceeds, slate which is a fine grained metamorphic forms. Increasing metamorphic conditions causes the tansformation of the slate into a shiny rock with growth of platy minerals such as micas from the clay. As metamorphic grade increase, the sizes of the mica increases and finally, at the highest peak of metamorphism, a banded gneiss forms. The banded gneiss gives a texture which bears alternating domains rich in quartz and those rich in mica alternating.