Metamorphic Rocks:
Metamorphic rocks form from other rocks under the high temperatures and pressures of burial in the earth's crust. Generally, their bulk chemical composition remains unchanged, but their mineralogy and texture undergoes significant changes. The metamorphic rocks are classified mainly by the presence or absence of foliation - i.e., whether or not they are layered. Highly foliated metamorphic rocks are generally not suitable for high end aggregate uses.
In the table below, only those metamorphic rocks used as aggregates are listed.
| Foliated | Non-foliated | |
| Silicates: | gneiss, schist, slate, phyllite | quartzite, granofels, amphibolite |
| Carbonates: | marble |
Schist, slate and phyllite are highly foliated; on crushing, they break down into platy fragments which, in mass aggregate, produce large voids.
Quartzite is a very hard rock, difficult to process, causing considerable wear on the equipment.
For a more in-depth discussion and classification of metamorphic rocks go to this site.
