Under the influence of heat, pressure and chemically active fluids, rocks will change or METAMORPHOSE into new rocks with minerals that are in equilibrium with these altered environmental factors.
Two basic types of metamorphic rocks are recognized: FOLIATED and NON FOLIATED. Foliated metamorphic rocks have some parallel structure induced by the pressures attendant upon metamorphism. This foliation can be observed in the field by parallel bands within the rocks or it can often be observed on a smaller scale in the hand specimen. This foliation can also be seen microscopically by the alignment of platy minerals.
NON FOLIATED as the name implies, does not have any parallel orientation of the grains within the metamorphic rock. Non foliated rocks have recrystallized without producing parallel structures. This can be done in the absence of pressure but more commonly by the lack of elongate or tabular grains. For example sandstone is metamorphosed into quartzite by the normal agents of metamorphism (heat and pressure), but because of the equidimensional nature of the quartz grains, no alignment or parallel structure can take place.
Following is the basic classification of metamorphic rocks.
| FOLIATED | |
SLATE |
Rock breaks into very thin layers, usually looks smooth but not polished, no crystals visible, harder than shale and commonly black, gray or reddish (sometimes greenish) |
PHYLLITE |
Like slate, "shinier" (Phyllitic sheen - similar to satin) |
SCHIST |
Very shiny, you can see the crystals (usually mica) |
GNEISS |
|
| NON FOLIATED | |
MARBLE |
|
QUARTZITE |
|
METACONGLOMERATE |
|
SERPENTINE |
|