Identifying Dark Metallic Minerals

## Starting Point: Dark Color + Metallic Luster

When you encounter a dark, shiny mineral with a metallic or submetallic luster, follow this systematic flowchart to narrow down the identification.

## Step 1: Magnetic Test

Test the specimen with a strong magnet before any other steps.

| Result | Proceed To |
|--------|------------|
| Strongly magnetic | **Magnetite** (Fe₃O₄) — Mohs 5.5–6.5, cubic, black streak |
| Weakly magnetic | Possible pyrrhotite or ilmenite — continue to Step 2 |
| Not magnetic | Continue to Step 2 |

## Step 2: Streak Color

Scratch the mineral across an unglazed porcelain tile.

| Streak Color | Candidates |
|-------------|------------|
| Black | Magnetite, ilmenite, chromite, uraninite |
| Greenish-black | Pyrite, chalcopyrite (despite gold color) |
| Dark reddish-brown | Hematite (despite sometimes appearing metallic silver) |
| Gray-black | Galena, molybdenite |
| Dark brown | Goethite, limonite |

## Step 3: Hardness

### Hard (Mohs 5.5+)

| Mineral | Formula | Mohs | Crystal System | Diagnostic Feature |
|---------|---------|------|----------------|-------------------|
| Magnetite | Fe₃O₄ | 5.5–6.5 | Cubic | Magnetic |
| Ilmenite | FeTiO₃ | 5–6 | Trigonal | Weakly magnetic, tabular |
| Chromite | FeCr₂O₄ | 5.5 | Cubic | Dark brown streak |
| Pyrite | FeS₂ | 6–6.5 | Cubic | Cubic/pyritohedral crystals |
| Arsenopyrite | FeAsS | 5.5–6 | Monoclinic | Silvery, garlic smell when struck |

### Soft (Mohs < 5.5)

| Mineral | Formula | Mohs | Crystal System | Diagnostic Feature |
|---------|---------|------|----------------|-------------------|
| Galena | PbS | 2.5 | Cubic | Very heavy (SG 7.6), perfect cubic cleavage |
| Molybdenite | MoS₂ | 1–1.5 | Hexagonal | Greasy feel, flexible sheets, writes on paper |
| Graphite | C | 1–2 | Hexagonal | Greasy feel, black streak, writes on paper |
| Chalcocite | Cu₂S | 2.5–3 | Monoclinic | Dark gray, conchoidal fracture |
| Stibnite | Sb₂S₃ | 2 | Orthorhombic | Prismatic bladed crystals |

## Step 4: Crystal Form and Habit

- **Cubic crystals or octahedra**: Magnetite, pyrite, galena
- **Prismatic/bladed**: Stibnite, arsenopyrite
- **Tabular**: Ilmenite, hematite (specular variety)
- **Massive/granular**: Chromite, ilmenite
- **Platy/micaceous**: Molybdenite, graphite

## Step 5: Specific Gravity (Heft Test)

Pick up the specimen. Does it feel heavier than expected?

- **Very heavy** (SG > 7): Galena (7.6)
- **Heavy** (SG 5–7): Magnetite (5.2), arsenopyrite (6.1), pyrite (5.0)
- **Normal** (SG 3–5): Ilmenite (4.7), chromite (4.6), hematite (5.3)
- **Light** (SG < 3): Graphite (2.2), molybdenite (4.7 but feels light due to platy habit)