# Question #a6b61

Jun 1, 2016

See below.

#### Explanation:

An earth scientist would probably look more at the crystallography, but these are the things I would look at as a chemist.

1. Fluorite and calcite

Fluorite (${\text{CaF}}_{2}$) has a Mohs hardness of 4 and fluoresces under ultraviolet light.

Calcite (${\text{CaCO}}_{3}$) has a Mohs hardness of 3 and forms bubbles of ${\text{CO}}_{2}$ when a drop of dilute $\text{HCl}$ is placed on its surface.

Calcite is softer than fluorite. If you have the two minerals together, scratch one against the other. The one that makes a scratch will be the fluorite.

2. Talc and Gypsum

Talc tends to feel slippery and form tiny cleavage fragments when scratched with your fingernail.

Gypsum does not feel slippery. It forms a powder when scratched with your fingernail.

3. Plagioclase and orthoclase

Plagioclase can be pink or blue, while orthoclase tends to be dark grey or black.

Unfortunately, both can be light in colour.

If it has striations on the cleavage surface, it is plagioclase.

If it has no striations, it may be orthoclase.

4. Hornblende and augite

Hornblende forms long black rectangular crystals with a greenish hue and with highly reflective 60°/120 ° cleavages.

Augite forms shiny, glassy, dark green to black blocky crystals with an almost metallic lustre and dull 90 ° cleavages.