Identifying Minerals in Sedimentary Rocks

## Minerals in Sedimentary Environments

Sedimentary rocks form through weathering, transport, deposition, and lithification. The minerals they contain fall into three categories: detrital (inherited from parent rock), authigenic (formed in situ), and biogenic (produced by organisms).

## Detrital Minerals (Surviving Transport)

Only the hardest and most chemically resistant minerals survive weathering and transport. Their presence tells you about the source rock.

| Mineral | Formula | Mohs | Resistance | Parent Rock |
|---------|---------|------|------------|-------------|
| Quartz | SiO₂ | 7 | Very high | Granite, gneiss |
| Zircon | ZrSiO₄ | 7.5 | Very high | Granite, pegmatite |
| Tourmaline | Complex borosilicate | 7–7.5 | High | Pegmatite, schist |
| Rutile | TiO₂ | 6–6.5 | High | Metamorphic rocks |
| Garnet | X₃Y₂(SiO₄)₃ | 6.5–7.5 | Moderate | Schist, gneiss |
| Magnetite | Fe₃O₄ | 5.5–6.5 | Moderate | Igneous/metamorphic |

## Authigenic Minerals (Formed in Place)

### Chemical Precipitates

| Mineral | Formula | Environment | Diagnostic |
|---------|---------|------------|------------|
| Calcite | CaCO₃ | Marine, lacustrine | Fizzes in HCl |
| Dolomite | CaMg(CO₃)₂ | Evaporitic basins | Fizzes in warm/powdered HCl |
| Halite | NaCl | Evaporite basins | Cubic crystals, salty taste |
| Gypsum | CaSO₄·2H₂O | Evaporite basins | Mohs 2, satin spar variety |
| Barite | BaSO₄ | Concretions, veins | Very heavy (SG 4.5) |
| Pyrite | FeS₂ | Reducing environments | Cubic, in shale/limestone |

### Clay Minerals (Weathering Products)

| Clay Mineral | Formula | Parent Mineral |
|-------------|---------|----------------|
| Kaolinite | Al₂Si₂O₅(OH)₄ | Feldspar (tropical weathering) |
| Illite | (K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀(OH)₂ | Feldspar (temperate weathering) |
| Smectite | (Na,Ca)(Al,Mg)₂Si₄O₁₀(OH)₂·nH₂O | Volcanic ash |

## Geodes and Concretions

Geodes are hollow spheres lined with crystals, formed in sedimentary rocks by precipitation from groundwater.

| Contents | Mineral | Where Found |
|----------|---------|-------------|
| Quartz crystals | SiO₂ | Midwest USA (Keokuk geodes), Brazil |
| Amethyst | SiO₂+Fe | Uruguay, Brazil (in basalt cavities) |
| Calcite | CaCO₃ | Worldwide in limestone |
| Celestine | SrSO₄ | Madagascar (world's largest geodes) |
| Barite | BaSO₄ | Various sedimentary basins |

## Fossils and Mineral Replacement

Fossils are often preserved through mineral replacement:

- **Silicification**: Original material replaced by quartz/chalcedony (petrified wood)
- **Pyritization**: Replaced by pyrite (ammonites, brachiopods)
- **Calcite replacement**: Original aragonite shells recrystallize to calcite
- **Phosphatization**: Replaced by apatite (vertebrate fossils, conodonts)

## Identification Flowchart

1. **Fizzes in acid?** → Calcite or dolomite (limestone/dolostone)
2. **Salty taste?** → Halite (evaporite)
3. **Very soft (Mohs 2)?** → Gypsum (evaporite)
4. **Cubic crystals in shale?** → Pyrite
5. **Translucent bands?** → Agate/chalcedony (geode or nodule)
6. **Heavy tabular crystals?** → Barite (concretion)