1. MINERALS Gypsum crystals: Naica Mountain, MX

2. What is a mineral? • Naturally Occurring • Inorganic Solid • Specific Chemical Composition • Characteristic Crystal Structure • Regular geometric arrangement of atoms

3. Mineral Properties • Crystal form/Growth habit • Luster • Hardness • Specific Gravity • Streak • Cleavage • Fracture • Color • Other Properties

4. Crystal Form/Growth Habit • Shape that occurs when a mineral grows freely • Acicular – needle-like • Globular – hemispherical masses • Fibrous – extremely slender prisms • Octahedral – eight sided • Radiating – radiating outward from a central point 

5. Luster • A description of the way light interacts with the surface of a crystal or mineral • Metallic: This has a shiny surface, resembling polished metal. Reflective. • Non-metallic: This as a surface that does not shine or reflect light well.

6. Hardness • A measure of how much a mineral resists being scratched. • We use Mohs scale of hardness to rate the hardness of minerals. • Devised by German mineralogist, Frederich Mohs • Scale is arbitrary

9. FOR THIS LAB: You can use the following materials and the list of the hardnesses of them to determine hardness for each mineral.

10. Specific Gravity (SG) • The ratio of the density of a given solid or liquid substance to the density of water at a specific temperature and pressure, typically at 4°C (39°F) and 1 atm (14.7 psi). • Value is dimensionless, there are no units

11. Streak • The color of the powder left when scratched across a porcelain plate

12. Cleavage • Description of how a mineral breaks • To determine what is a cleavage plane look for the following: • A smooth break along a crystal plane • A reproducible face—the mineral is broken along the same faces multiple times in one sample • Describe cleavage using: perfect, imperfect, good, distinct, indistinct, and poor

13. Cleavage

14. Fracture • If there is no plane of weakness, then the breaks will not be the same • Conchoidal fracture swirls • Non-Conchoidal fracture has no pattern

15. What’s the difference between cleavage and fracture? • Cleavage: breakage along planes of atomic weakness within a crystal. Look for a distinct “pattern” of breakage. Don’t confuse breakage pattern with crystal form (how it grows). • Fracture: Irregular mineral breakage.

16. Color (Occasionally distinctive, but can be deceiving) Hey look five different minerals! Wrong, all calcite

17. Other Properties • Reacts with Acid • Salty • Magnetic • Labradorescence

18. Labradorescence (Note twinning on right sample)