METAMORPHIC INDEX MINERALS

Revealing Secrets of Earth’s Deep Crust

- BOB JONES

3D rendering of the interior structure of Earth

When you admire a nicely cut specimen of charoite, surely you wonder how and where it formed. It may come as a surprise to learn the rock consists of more than a dozen minerals. Additionally, the formation of this rock takes place deep within (as far as nine miles down) the earth’s crust.

Geologists do not have x-ray vision, nor have they been that deep into the earth, so how do they know what happens that deep in the earth’s crust? After all, the charoite rock and all others must be close to the surface so we can study them. What brought them to us is the crustal movement. There must be clues in deeply formed rocks that give away hidden secrets, telling us how far down into the earth something forms. The clues do exist and are present within minerals in such rocks. They are known as index minerals.

This is a fine example of two metamorphic index minerals kyanite and staurolite from Switzerland. GROBEN COLLECTION

PRODUCTS OF SERIOUS HEAT AND PRESSURE

Index minerals are not minerals listed alphabetically in a book. They are minerals Mother Nature forms deep in the earth, using incredible amounts of pressure and temperature so high it is hard to believe the numbers. I’ve been underground in mines where the rock temperature is nearly 150 degrees. The deep gold mines in South Africa are far hotter, but such temperatures are child’s play compared to deep crust temperatures and pressures.