Overview
This lecture covers the key physical properties used to identify minerals, including naming conventions, crystal shape, appearance, mechanical properties, and diagnostic tests like acid reaction and magnetism.
Mineral Naming and Identification
- Mineral names may reflect appearance, chemistry, origin, or honor a scientist.
- Identification typically uses properties observable in hand specimens: luster, color, diaphaneity (transparency), shape, streak, hardness, and cleavage.
- Optical properties, seen with special microscopes, are covered in other chapters.
Crystal Shape and Mineral Habit
- Crystal shape relates to atomic arrangement; "euhedral" means well-formed faces, "anhedral" means no faces, "subhedral" is intermediate.
- "Form" refers to identically-shaped faces on a crystal, tied to symmetry.
- "Habit" describes the overall appearance, including shape and how crystals grow together (e.g., blocky, fibrous, platy).
- Aggregates of crystals may be termed granular, radiating, fibrous, or massive.
Mineral Appearance: Luster, Color, and Transparency
- Luster: Metallic, submetallic, or nonmetallic (vitreous, pearly, silky, etc.).
- Diaphaneity: Transparent (see through), translucent (light passes, can't see through), or opaque.
- Color: May aid identification, but is often ambiguous due to various causes (composition, impurities, defects).
- Streak: Color of powdered mineral, more reliable than surface color for many minerals.
Additional Optical Effects
- Luminescence: Some minerals emit visible light when energized (fluorescence, phosphorescence, thermoluminescence).
- Play of colors: Special effects like iridescence, opalescence, chatoyancy (cat's-eye), and asterism (star-like) in certain minerals.
Mechanical Properties: Tenacity, Cleavage, Fracture, Hardness
- Tenacity describes resistance to breaking (brittle, malleable, ductile, flexible, elastic).
- Cleavage: Tendency to break along planes of weak bonding; described as perfect, good, poor, and by geometric terms (e.g., cubic, basal).
- Fracture: Breaks not along cleavage planes (e.g., conchoidal, hackly, splintery).
- Hardness: Measured by Mohs scale (1=talc, 10=diamond), tested by scratch test.
Density and Specific Gravity
- Density (Ο) is mass per unit volume; most minerals: 2β8 g/cmΒ³.
- Specific gravity is density relative to water, used to "heft" minerals for identification.
Other Diagnostic Properties
- Magnetism: Strongest in minerals like magnetite (ferromagnetic).
- Electrical properties: Metals and some sulfides conduct electricity; some minerals can hold static charge.
- Acid reaction: Calcite effervesces (fizzes) with dilute HCl; useful for carbonate identification.
Key Terms & Definitions
- Luster β way a mineral reflects light (metallic, vitreous, etc.)
- Diaphaneity β ability to transmit light (transparent, translucent, opaque)
- Streak β color of mineral in powdered form
- Cleavage β tendency to break along flat planes due to atomic structure
- Fracture β irregular breakage not along cleavage planes
- Hardness β resistance to scratching, measured by Mohs scale
- Tenacity β mineralβs resistance to breaking, bending, or deformation
- Habit β characteristic external shape and crystal aggregation of minerals
- Specific Gravity β ratio of mineral's density to that of water
- Effervescence β bubbling when reacting with acid (e.g., calcite with HCl)
Action Items / Next Steps
- Practice identifying minerals using luster, streak, cleavage, and hardness.
- Watch recommended videos for visual examples of mineral properties.
- Try scratch, streak, and acid tests on mineral samples if available.