Identifying Blue and Purple Minerals

## Blue and Purple in Mineralogy

Blue and purple are relatively uncommon mineral colors, making them easier to narrow down than greens or whites. Blue typically results from copper, iron, or cobalt, while purple is usually caused by iron, manganese, or radiation damage.

## Blue Mineral Flowchart

### Step 1: Hardness

**Hard (Mohs 7+)**:

| Mineral | Formula | Mohs | Crystal System | Diagnostic |
|---------|---------|------|----------------|------------|
| Sapphire (corundum) | Al₂O₃+Fe/Ti | 9 | Trigonal | Hexagonal barrel crystals, adamantine luster |
| Kyanite | Al₂SiO₅ | 4.5–7* | Triclinic | *Hardness varies by direction (unique!) |
| Cordierite (iolite) | (Mg,Fe)₂Al₄Si₅O₁₈ | 7–7.5 | Orthorhombic | Strong pleochroism (blue/violet/yellow) |
| Benitoite | BaTiSi₃O₉ | 6–6.5 | Hexagonal | UV fluorescent blue, extremely rare |

**Medium (Mohs 5–6.5)**:

| Mineral | Formula | Mohs | Crystal System | Diagnostic |
|---------|---------|------|----------------|------------|
| Sodalite | Na₈(AlSiO₄)₆Cl₂ | 5.5–6 | Cubic | Royal blue, massive, white streaks |
| Lazurite (lapis lazuli) | Na₃Ca(Al₃Si₃O₁₂)S | 5–5.5 | Cubic | Deep blue, pyrite inclusions |
| Hauyne | Na₃Ca(Si₃Al₃)O₁₂(SO₄) | 5.5–6 | Cubic | Vivid blue, rare, in volcanic rocks |
| Apatite | Ca₅(PO₄)₃(F,Cl,OH) | 5 | Hexagonal | Neon blue from Madagascar |

**Soft (Mohs < 5)**:

| Mineral | Formula | Mohs | Crystal System | Diagnostic |
|---------|---------|------|----------------|------------|
| Azurite | Cu₃(CO₃)₂(OH)₂ | 3.5–4 | Monoclinic | Deep azure blue, fizzes in acid, with malachite |
| Celestine | SrSO₄ | 3–3.5 | Orthorhombic | Pale blue, tabular, in geodes |
| Chrysocolla | (Cu,Al)₂H₂Si₂O₅(OH)₄ | 2–4 | Amorphous | Blue-green, earthy, with copper ores |
| Vivianite | Fe₃(PO₄)₂·8H₂O | 1.5–2 | Monoclinic | Darkens on light exposure |

## Purple Mineral Flowchart

| Mineral | Formula | Mohs | Crystal System | Diagnostic |
|---------|---------|------|----------------|------------|
| Amethyst (quartz) | SiO₂+Fe | 7 | Trigonal | Hexagonal prisms, conchoidal fracture |
| Fluorite | CaF₂ | 4 | Cubic | Octahedral cleavage, often zoned |
| Charoite | (K,Sr)(Ca,Na)₁₃Si₄O₁₀(OH,F) | 5–6 | Monoclinic | Swirling purple, only from Siberia |
| Lepidolite | K(Li,Al)₃(AlSi)₄O₁₀(OH,F)₂ | 2.5–3.5 | Monoclinic | Lilac mica, in pegmatites |
| Sugilite | KNa₂(Fe,Mn,Al)₂Li₃Si₁₂O₃₀ | 6–6.5 | Hexagonal | Purple, massive, rare (South Africa) |
| Purpurite | (Mn,Fe)PO₄ | 4–4.5 | Orthorhombic | Dark purple, earthy luster |

## Key Diagnostic Shortcuts

- Blue + fizzes in acid → Azurite
- Blue + pyrite inclusions → Lapis lazuli
- Purple + hexagonal prism → Amethyst
- Purple + micaceous → Lepidolite
- Blue + directional hardness → Kyanite