Identifying Minerals by Crystal Shape

## Crystal Shape as an ID Tool

When a mineral displays well-formed crystals, the crystal shape (habit) is often the fastest route to identification. Each crystal system produces characteristic forms that reflect the internal atomic arrangement.

## Identification by Crystal Form

### Cubic Forms

| Crystal Form | Common Minerals | Additional Tests |
|-------------|----------------|------------------|
| Cube | Pyrite (FeS₂), halite (NaCl), galena (PbS), fluorite (CaF₂) | Pyrite: striations on faces; Halite: salty taste |
| Octahedron | Magnetite (Fe₃O₄), fluorite, diamond (C), spinel | Magnetite: magnetic; Diamond: adamantine luster |
| Dodecahedron | Garnet, pyrite | Garnet: vitreous luster; Pyrite: metallic |
| Pyritohedron | Pyrite | Striated faces, brass-yellow color |

### Hexagonal/Trigonal Forms

| Crystal Form | Common Minerals | Additional Tests |
|-------------|----------------|------------------|
| Hexagonal prism | Quartz (SiO₂), beryl (Be₃Al₂Si₆O₁₈), apatite | Quartz: Mohs 7, no cleavage; Beryl: Mohs 7.5–8 |
| Rhombohedron | Calcite (CaCO₃), dolomite, siderite | Calcite: fizzes in HCl |
| Scalenohedron | Calcite ("dogtooth spar") | Pointed, elongated rhombohedra |
| Trigonal prism | Tourmaline | Striated, triangular cross-section |

### Orthorhombic Forms

| Crystal Form | Common Minerals | Additional Tests |
|-------------|----------------|------------------|
| Tabular | Barite (BaSO₄), celestine (SrSO₄) | Barite: heavy (SG 4.5) |
| Prismatic | Topaz (Al₂SiO₄(F,OH)₂), olivine | Topaz: perfect basal cleavage |
| Bladed | Stibnite (Sb₂S₃), kyanite* | Stibnite: metallic, soft (Mohs 2) |

### Monoclinic Forms

| Crystal Form | Common Minerals | Additional Tests |
|-------------|----------------|------------------|
| Prismatic (stubby) | Augite, diopside | Augite: dark, 87° cleavage angle |
| Prismatic (elongated) | Epidote, gypsum | Epidote: pistachio green |
| Tabular (swallow-tail twin) | Gypsum | Mohs 2, twin crystals |

## Habit vs. Form

| Term | Meaning | Example |
|------|---------|--------|
| Habit | Overall shape tendency | Quartz is prismatic |
| Form | Specific set of crystal faces | Quartz prism + rhombohedron termination |
| Twinning | Intergrown crystals | Staurolite cross (60° or 90°) |

## Diagnostic Twins

- **Staurolite**: Cruciform (cross-shaped) penetration twins
- **Gypsum**: Swallow-tail contact twins
- **Fluorite**: Interpenetrant cubes
- **Orthoclase**: Carlsbad twins
- **Plagioclase**: Albite polysynthetic twinning (visible as striations)
- **Aragonite**: Pseudo-hexagonal trillings

## When Crystals Are Absent

Massive, granular, or cryptocrystalline specimens cannot use this flowchart. Proceed to hardness, streak, and chemical tests instead.