Identifying Light-Colored Non-Metallic Mine...

## Starting Point: Light Color + Non-Metallic Luster

White, colorless, and pale minerals are among the most common and most frequently confused. This flowchart uses three key tests to separate the major species.

## Step 1: Acid Test

Apply a drop of dilute hydrochloric acid (10% HCl) to the specimen.

| Reaction | Mineral | Formula | Mohs |
|----------|---------|---------|------|
| Vigorous fizzing | **Calcite** | CaCO₃ | 3 |
| Fizzes only in powder | **Dolomite** | CaMg(CO₃)₂ | 3.5–4 |
| No reaction | Continue to Step 2 | | |

## Step 2: Hardness

### Very Soft (Mohs 1–2)

| Mineral | Formula | Diagnostic Feature |
|---------|---------|-------------------|
| Talc | Mg₃Si₄O₁₀(OH)₂ | Soapy feel, pearly luster |
| Gypsum | CaSO₄·2H₂O | Scratched by fingernail, three cleavage directions |

### Soft (Mohs 2.5–4)

| Mineral | Formula | Diagnostic Feature |
|---------|---------|-------------------|
| Halite | NaCl | Salty taste, cubic cleavage |
| Calcite | CaCO₃ | Rhombohedral cleavage, double refraction |
| Barite | BaSO₄ | Very heavy for a non-metallic (SG 4.5) |
| Fluorite | CaF₂ | Octahedral cleavage, often fluorescent |

### Medium (Mohs 5–6)

| Mineral | Formula | Diagnostic Feature |
|---------|---------|-------------------|
| Apatite | Ca₅(PO₄)₃(F,Cl,OH) | Hexagonal prisms, scratched by knife |
| Orthoclase | KAlSi₃O₈ | Two cleavages at 90°, Carlsbad twinning |
| Plagioclase | (Na,Ca)(Al,Si)₄O₈ | Two cleavages near 90°, striations on cleavage face |

### Hard (Mohs 7+)

| Mineral | Formula | Diagnostic Feature |
|---------|---------|-------------------|
| Quartz | SiO₂ | No cleavage, conchoidal fracture, hexagonal prisms |
| Topaz | Al₂SiO₄(F,OH)₂ | Perfect basal cleavage, orthorhombic |
| Beryl | Be₃Al₂Si₆O₁₈ | Hexagonal prisms, no cleavage |

## Step 3: Cleavage Pattern

Cleavage is the most diagnostic property for white non-metallic minerals.

| Cleavage | Mineral |
|----------|--------|
| None (conchoidal fracture) | Quartz, opal |
| 1 perfect plane | Topaz, mica |
| 2 planes at ~90° | Orthoclase, plagioclase |
| 2 planes not at 90° | Amphibole (56°/124°) |
| 3 planes (rhombohedral) | Calcite (75°/105°) |
| 3 planes at 90° (cubic) | Halite, galena |
| 4 planes (octahedral) | Fluorite |

## Quick Decision Summary

- Fizzes in acid → Calcite or dolomite
- Soapy feel → Talc
- Tastes salty → Halite
- Very heavy → Barite
- Conchoidal fracture + hard → Quartz
- Rhombohedral cleavage → Calcite
- Octahedral cleavage → Fluorite

Identifying Light-Colored Non-Metallic Minerals