Physical ID of Minerals
Crystal Form, Cleavage & Fracture · Lustre, Colour & Streak · Hardness & Specific Gravity (28 Micro-Realms)
The Diagnostic Mineralogical Toolkit
Every mineral's internal atomic lattice and chemical bonding express themselves externally through eight diagnostic physical indicators—from Crystal Form, Cleavage, and Fracture to Lustre, Colour, Streak, Transparency, Mohs Hardness (1–10), and Specific Gravity.
I. Crystal Form, Structure, Cleavage & Fracture
External Crystal Form
- Internal atomic arrangement
- Cube, Octahedron, Prism
- Hexagonal pyramid faces
Internal Structure
- Particular molecule arrangement
- 3D repeating unit cell
- X-ray diffraction pattern
Crystal Habit
- Tabular, Acicular (needle)
- Bladed, Fibrous, Dendritic
- Botryoidal grape clusters
Cleavage Mechanics
- Breaks in given directions
- Produces flat planar surfaces
- Weakest atomic bond planes
Cleavage Directions
- 1D Basal: Mica sheets
- 3D Cubic ($90^\circ$): Halite/Galena
- 3D Rhombohedral: Calcite
Fracture Mechanics
- No cleavage plane present
- Equal bond strength all ways
- Breaks irregularly
Conchoidal & Splintery
- Conchoidal: Curved shell-like
- Seen in Quartz & Obsidian
- Earthy, Fibrous, Splintery
II. Optical Properties: Lustre, Colour, Streak & Transparency
Lustre Definition
- Appearance without regard to colour
- Surface light reflection quality
- Distinctive for each mineral
Metallic, Silky, Glossy
- Metallic: Pyrite, Galena, Gold
- Silky: Satin-spar Gypsum
- Glossy / Vitreous: Quartz
Adamantine & Pearly
- Adamantine: Diamond brilliance
- Pearly: Talc & Muscovite
- Resinous (Amber) · Earthy (Kaolin)
Intrinsic Colour
- Idiochromatic molecular structure
- Malachite (always green)
- Azurite (blue) · Sulphur (yellow)
Impurity Colour
- Allochromatic trace impurities
- Quartz: White, Green, Red, Violet
- Corundum: Ruby (Cr) vs Sapphire
Streak Powder Test
- Colour of ground powder
- Unglazed porcelain plate ($H\approx 7$)
- More reliable than surface colour
Streak Contrasts (NCERT)
- Malachite: Green body & green streak
- Fluorite: Purple/green $\to$ White streak
- Pyrite (Gold) $\to$ Greenish-black
Transparency Classes
- Transparent: Clear image passes
- Translucent: Diffused light glow
- Opaque: Light cannot pass
III. Mohs Relative Hardness Scale (1 Talc to 10 Diamond — NCERT)
Mohs Hardness Scale
- Friedrich Mohs (1812)
- Relative abrasion resistance
- 10 reference minerals
1. Talc & 2. Gypsum
- 1. Talc: Softest soapy feel
- 2. Gypsum: Hydrated sulphate
- Scratched by human fingernail
3. Calcite & 4. Fluorite
- 3. Calcite ($\text{CaCO}_3$)
- 4. Fluorite ($\text{CaF}_2$)
- Scratched by copper coin/nail
5. Apatite & 6. Feldspar
- 5. Apatite (phosphate)
- 6. Feldspar (Orthoclase)
- Feldspar scratches window glass
7. Quartz & 8. Topaz
- 7. Quartz: Scratches steel
- 8. Topaz: Fluoro-silicate gem
- Resists ordinary weathering
9. Corundum & 10. Diamond
- 9. Corundum ($\text{Al}_2\text{O}_3$)
- 10. Diamond (3D covalent C)
- 4x harder than Corundum
Field Scratch Benchmarks
- Fingernail: 2.5 · Penny: 3.5
- Pocket knife / Glass: 5.5
- Streak plate: 6.5–7.0
IV. Specific Gravity, Tenacity & Special Diagnostics
Specific Gravity ($G$)
- Weight vs equal water volume
- Weighed in air & water
- Dimensionless unitless ratio
Specific Gravity Spectrum
- Quartz/Feldspar: $\sim 2.65$
- Galena: $7.5$ · Barite: $4.5$
- Pure Gold: $19.3$ (panning)
Tenacity Classes
- Brittle: Quartz shatters
- Malleable/Ductile: Gold, Copper
- Elastic (Mica) vs Flexible (Talc)
Acid Effervescence
- Cold dilute $\text{HCl}$ test
- Calcite fizzes vigorously ($\text{CO}_2$)
- Dolomite fizzes when powdered
Magnetism & Fluorescence
- Magnetite ($\text{Fe}_3\text{O}_4$) lodestone
- Pyrrhotite weak magnetism
- Fluorite & Scheelite UV glow
Double Refraction
- Birefringence optical split
- Iceland Spar clear Calcite
- Produces double text image
Core Distinctions
Cleavage: Tendency to break smoothly along flat planar surfaces of weak atomic bonding (e.g., Mica 1D sheets, Halite 3D cubes). Fracture: Irregular or curved breakage when atomic bonds are equally strong in all directions and no cleavage planes exist (e.g., Quartz conchoidal fracture).
2. Colour vs. Streak (Fool's Gold Test)Surface Colour varies widely due to trace impurities (Quartz can be white, green, red, or violet). Streak (powder colour) remains constant: Malachite is green with a green streak, Fluorite is purple/green with a white streak, and brassy Pyrite ("Fool's Gold") leaves a greenish-black streak whereas real Gold leaves a golden-yellow streak.
3. Mohs Scale 1–10 Mnemonic"Tall Girls Can Fight And Other Queer Things Can Do": 1. Talc · 2. Gypsum · 3. Calcite · 4. Fluorite · 5. Apatite · 6. Orthoclase Feldspar · 7. Quartz · 8. Topaz · 9. Corundum · 10. Diamond.
Mohs Hardness Ladder (1–10)
NCERT Chapter 5 Reference Sequence