The green color in a type of rock known as greenstone (meta) primarily comes from specific minerals present within the rock.
Understanding Greenstone (Meta)
According to the AGI Glossary of Geology, greenstone (meta) is a field term used to describe any compact, dark-green altered or metamorphosed basic igneous rock. Examples include rocks like spilite, basalt, gabbro, and diabase that have undergone metamorphic changes.
Minerals Responsible for the Green Hue
The distinctive green color of this rock is a direct result of the minerals that form during the metamorphic process.
As stated in the geological definition, greenstone owes its color to the presence of chlorite, actinolite, or epidote.
Here's a look at these key minerals:
- Chlorite: Often gives rocks a dark to light green color.
- Actinolite: A mineral in the amphibole group, typically fibrous and green to dark green.
- Epidote: A mineral common in metamorphosed rocks, frequently displaying a yellowish-green or pistachio-green color.
Mineral Contribution to Color
These minerals are formed when the original minerals in basic igneous rocks (like pyroxenes or feldspars) are altered under conditions of low-grade metamorphism.
| Mineral | Typical Color Contribution |
|---|---|
| Chlorite | Green (various shades) |
| Actinolite | Green to dark green |
| Epidote | Yellowish-green, Pistachio-green |
Where Greenstone is Found
Greenstone (meta) is derived from:
- Altered
- Metamorphosed
Basic igneous rocks such as:
- Spilite
- Basalt
- Gabbro
- Diabase
Through geological processes, these parent rocks are transformed, leading to the formation of the green minerals that give greenstone its characteristic look.
