Mineralogy

Mineralogy is the study of minerals - naturally occurring, inorganic, crystalline solids with definite chemical compositions. Minerals form the fundamental building blocks of rocks and are essential for understanding Earth processes, ore deposits, and materials for human use.

What Defines a Mineral?

A substance must meet five criteria to be classified as a mineral:

1. Naturally occurring: Formed by natural geological processes
2. Inorganic: Not formed by living organisms (with some exceptions)
3. Solid: Not liquid or gas at Earth's surface
4. Crystalline: Has an ordered internal atomic arrangement
5. Definite composition: Chemical formula that varies within specific limits

Examples of True Minerals vs. Non-Minerals

• True minerals: Quartz (SiO₂), feldspar (KAlSi₃O₈), calcite (CaCO₃)
• Not minerals: Amber (organic), water (liquid), synthetic diamonds (not naturally occurring)

Crystallography

Crystal Systems

Crystals are classified into six systems based on their axial lengths and angles:

1. Cubic (Isometric): a = b = c, α = β = γ = 90°
2. Tetragonal: a = b ≠ c, α = β = γ = 90°
3. Orthorhombic: a ≠ b ≠ c, α = β = γ = 90°
4. Hexagonal: a = b ≠ c, α = β = 90°, γ = 120°
5. Monoclinic: a ≠ b ≠ c, α = γ = 90° ≠ β
6. Triclinic: a ≠ b ≠ c, α ≠ β ≠ γ ≠ 90°

Crystal Structure and Bonding

The internal arrangement of atoms determines external crystal shape:

Types of Bonding in Minerals

• Ionic bonds: Electrons transferred (e.g., halite, NaCl)
• Covalent bonds: Electrons shared (e.g., diamond, SiO₂)
• Metallic bonds: Delocalized electrons (e.g., native copper)
• Van der Waals: Weak secondary bonds (e.g., graphite layers)

Physical Properties of Minerals

Crystal Habits

The typical form a mineral assumes under favorable growth conditions:

• Prismatic: Elongated, rectangular cross-section
• Tabular: Flattened, rectangular, thin in one direction
• Acicular: Needle-like crystals
• Botryoidal: Grape-like clusters
• Drusy: Coating of small crystals

Cleavage and Fracture

Cleavage

The tendency to break along specific crystallographic planes:

Quality of cleavage:

• Perfect: Clean, smooth surfaces (mica)
• Good: Usually smooth (feldspar)
• Poor: Incomplete (quartz - no cleavage)
• None: No preferred breaking direction

Fracture

How a mineral breaks when not along cleavage planes:

• Conchoidal: Shell-like (quartz)
• Hackly: Jagged, metallic appearance (native copper)
• Uneven: Irregular (olivine)
• Fibrous: Striated (asbestos)

Hardness

The Mohs scale measures relative scratch resistance:

1. Talc (Mg₃Si₄O₁₀(OH)₂)
2. Gypsum (CaSO₄·2H₂O)
3. Calcite (CaCO₃)
4. Fluorite (CaF₂)
5. Apatite (Ca₅(PO₄)₃(F,Cl,OH))
6. Orthoclase feldspar (KAlSi₃O₈)
7. Quartz (SiO₂)
8. Topaz (Al₂SiO₄(F,OH)₂)
9. Corundum (Al₂O₃)
10. Diamond (C)

The relationship between scratch resistance and atomic structure:

Luster

How light reflects from a mineral's surface:

• Metallic: Opaque, reflective like metal
• Non-metallic:
  • Glassy/Vitreous: Like broken glass (quartz)
  • Pearly: Like pearl surface
  • Silky: Fibrous minerals
  • Dull/Earthy: Rough surface

Color and Streak

Color

May be caused by transition metal ions (Fe, Cu, Mn) or structural defects:

• Idiochromatic: Intrinsic color due to essential elements (malachite's Cu = green)
• Allochromatic: Color due to trace impurities (quartz variety colors)

Streak

Color of mineral in powdered form (obtained by rubbing on unglazed porcelain):

• More reliable than color for identification
• Eliminates surface color and impurities

Other Diagnostic Properties

Specific Gravity

Values range from 1.5 (organic minerals) to 23 (native osmium).

Magnetism

• Magnetic: Attracted to magnets (magnetite, Fe₃O₄)
• Weakly magnetic: Under special conditions
• Non-magnetic: No magnetic response

Reaction with Acid

• Effervescence: Carbonates react with HCl to produce CO₂
• Calcite: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Major Mineral Groups

Silicate Minerals (Most Common)

Silica tetrahedron (SiO₄⁴⁻) is the basic building block:

Silicate Subclassifications

1. Nesosilicates (Isolated): Independent SiO₄⁴⁻

   • Examples: Olivine, garnet
   • Formula: A₂SiO₄ or A₃B₂(SiO₄)₃

2. Sorosilicates (Double tetrahedra): Si₂O₇⁶⁻

   • Examples: Epidote, zoisite
   • Share 1 oxygen between tetrahedra

3. Cyclosilicates (Rings): (SiO₃)ₙ²ⁿ⁻

   • Examples: Beryl, tourmaline
   • Usually 6-membered rings

4. Inosilicates (Chains): Single (SiO₃)ₙ²ⁿ⁻ or double chains

   • Single chain: Pyroxenes (e.g., enstatite, diopside)
   • Double chain: Amphiboles (e.g., hornblende)

5. Phyllosilicates (Sheets): Si₄O₁₀²⁻

   • Examples: Micas (muscovite, biotite), clays
   • Perfect basal cleavage

6. Tectosilicates (Frameworks): SiO₂

   • Examples: Quartz, feldspars, zeolites
   • Most polymerized, most stable

Non-Silicate Groups

Carbonates

• Formula: ACO₃ or A₂B(CO₃)₃
• Examples: Calcite (CaCO₃), dolomite (CaMg(CO₃)₂)
• Characteristics: React with acid, often form limestone

Oxides

• Formula: AO or A₂O₃
• Examples: Hematite (Fe₂O₃), magnetite (Fe₃O₄), corundum (Al₂O₃)
• Characteristics: Often metallic in appearance

Sulfides

• Formula: AX
• Examples: Pyrite (FeS₂), galena (PbS), sphalerite (ZnS)
• Characteristics: Metallic luster, important ore minerals

Sulfates

• Formula: A(SO₄)
• Examples: Gypsum (CaSO₄·2H₂O), barite (BaSO₄)
• Characteristics: Often formed by evaporation

Halides

• Formula: AX
• Examples: Halite (NaCl), fluorite (CaF₂)
• Characteristics: Soluble in water, ionic bonding

Mineral Identification

Systematic Approach

1. Observe: Color, luster, crystal form
2. Test hardness: With known objects (fingernail=2.5, glass=5.5)
3. Check cleavage: Number of directions, angles between faces
4. Test other properties: Streak, magnetism, acid reaction
5. Consult identification charts: Cross-reference properties

Diagnostic Tests

Acid Test for Carbonates

Streak Test Procedure

Rub mineral on unglazed porcelain streak plate with firm pressure.

Hardness Test Tips

• Scratch glass (H=5.5) → H > 5.5
• Scratched by glass → H < 5.5
• Scratched by fingernail (H=2.5) → H < 2.5

Real-World Application: Ore Deposit Mineralogy

Understanding mineral associations is crucial for economic geology and resource exploration.

Hydrothermal Deposit Example

# Common mineral associations in hydrothermal systems
hydrothermal_minerals = {
    'high_temperature': ['magnetite', 'pyrrhotite', 'chalcopyrite', 'bornite'],
    'medium_temperature': ['pyrite', 'galena', 'sphalerite', 'chalcocite'],
    'low_temperature': ['hematite', 'realgar', 'orpiment', 'cinnabar']
}

# Temperature estimates from mineral stability
temperature_ranges = {
    'magnetite': (300, 600),      # Celsius
    'pyrite': (200, 500),         # Celsius
    'galena': (150, 400),         # Celsius
    'sphalerite': (100, 350),     # Celsius
    'hematite': (50, 300)         # Celsius
}

# Calculate temperature range of deposit based on mineral assemblage
mineral_assemblage = ['magnetite', 'pyrite', 'galena', 'sphalerite']
min_temp = max([temperature_ranges[mineral][0] for mineral in mineral_assemblage])
max_temp = min([temperature_ranges[mineral][1] for mineral in mineral_assemblage])

print(f"Mineral assemblage found: {mineral_assemblage}")
print(f"Estimated formation temperature: {min_temp}-{max_temp}°C")
print(f"Deposit classified as medium-temperature hydrothermal")

# Economic value assessment
metal_values = {
    'Fe': 0.08,    # $/kg for iron
    'Cu': 6.5,     # $/kg for copper
    'Pb': 2.2,     # $/kg for lead
    'Zn': 2.5,     # $/kg for zinc
    'Au': 60000    # $/kg for gold (if present as trace)
}

print("This deposit likely contains: iron, copper, lead, and zinc - valuable combination")

Alteration Minerals as Indicators

Different mineral assemblages indicate the conditions during ore formation.

Your Challenge: Mineral Identification

Identify an unknown mineral sample based on observed physical properties.

Goal: Use systematic mineral identification techniques to determine the identity of an unknown sample.

Sample Observations

# Observed properties of unknown sample
sample_properties = {
    'color': 'pink to white',
    'streak': 'white',
    'luster': 'vitreous',
    'hardness': 6,  # Scratches glass but not steel needle
    'cleavage': [90, 90, 90],  # Three directions intersecting at right angles
    'crystal_system': 'monoclinic',
    'specific_gravity': 2.56,
    'acid_reaction': False,
    'magnetism': False
}

# Common mineral database for comparison
mineral_database = {
    'orthoclase': {
        'color': ['white', 'pink', 'gray'],
        'streak': 'white',
        'luster': 'vitreous',
        'hardness': 6,
        'cleavage': [90, 90],
        'system': 'monoclinic',
        'sg': 2.56,
        'acid': False,
        'magnetic': False
    },
    'plagioclase': {
        'color': ['white', 'gray', 'green'],
        'streak': 'white', 
        'luster': 'vitreous',
        'hardness': [6, 6.5],
        'cleavage': [90, 90],  # Often shows albite twinning
        'system': ['triclinic'],
        'sg': [2.6, 2.7],
        'acid': False,
        'magnetic': False
    },
    'quartz': {
        'color': ['clear', 'white', 'purple', 'pink', 'gray'],
        'streak': 'white',
        'luster': 'vitreous',
        'hardness': 7,
        'cleavage': None,
        'system': ['trigonal'],
        'sg': 2.65,
        'acid': False,
        'magnetic': False
    },
    'calcite': {
        'color': ['white', 'colorless', 'yellow', 'orange'],
        'streak': 'white',
        'luster': 'vitreous',
        'hardness': 3,
        'cleavage': [75, 75, 75],  # Three directions
        'system': 'trigonal',
        'sg': 2.71,
        'acid': True,  # Reacts with HCl
        'magnetic': False
    }
}

# Calculate match scores between sample and database minerals
possible_matches = []
for mineral, props in mineral_database.items():
    score = 0
    total_props = 0
    
    for prop, value in sample_properties.items():
        if prop in props:
            total_props += 1
            if isinstance(value, list) and isinstance(props[prop], list):
                # Check if values overlap
                if set(value) & set(props[prop]):
                    score += 1
            elif isinstance(value, list):
                # Check if single value is in list
                if props[prop] in value:
                    score += 1
            elif isinstance(props[prop], list):
                # Check if value is in list
                if value in props[prop]:
                    score += 1
            else:
                # Direct comparison
                if value == props[prop]:
                    score += 1
    
    match_percentage = (score / total_props) * 100 if total_props > 0 else 0
    possible_matches.append((mineral, match_percentage))

# Sort by best match
possible_matches.sort(key=lambda x: x[1], reverse=True)

Use the systematic approach to identify the unknown mineral based on its properties.

Hint:

• Consider all properties together, not individually
• Some properties are more diagnostic than others
• Cleavage pattern and hardness are often key identifiers
• Crystal system can further narrow down the possibilities

# TODO: Calculate the most likely mineral identification
most_likely_mineral = ""  # Identify the mineral with best match
confidence_level = 0      # Percentage confidence in identification
secondary_possibility = "" # Second best match
key_diagnostic_properties = []  # Properties that confirm ID

# Print results
print(f"Most likely identification: {most_likely_mineral}")
print(f"Confidence level: {confidence_level:.1f}%")
print(f"Secondary possibility: {secondary_possibility}")
print(f"Key diagnostic properties: {key_diagnostic_properties}")

# Additional tests that could confirm identity
additional_tests = []  # Suggest tests to confirm identification
print(f"Additional tests to confirm: {additional_tests}")

What other properties or tests might help confirm your mineral identification?