Rocks and Minerals I - Notes

The Importance of Rocks

• Rocks provide evidence of the past by allowing researchers to reconstruct past landscapes, earth structures, and history, including plate tectonic processes.

• Principle of Uniformitarianism (James Hutton, 1785): 'The present is the key to the past is the key to the future.'

• Rocks form Earth’s landscape based on weathering and erosion.

Importance of Rocks in Geography and Environmental Sciences

• Natural Hazard Analysis: Earthquake, landslide, flooding, soil, and coastal erosion behavior depend on rock type.

• River and Glacier Reconstructions: Water- and ice-transported rocks (pebbles, boulders) give clues about origin and flow.

• Environmental Analysis: Water and soil quality and composition depends on rock type underlying the soil or in the tributary catchment area.

Classification of Rocks

• Igneous:

  • Source: Melting of rocks in hot, deep crust and upper mantle.

  • Process: Crystallization (solidification of magma or lava).

  • Example: Coarsely crystallized granite.

• Sedimentary:

  • Source: Weathering and erosion of rocks exposed at the surface.

  • Process: Deposition, burial, and lithification.

  • Example: Cross-laminated sandstone.

• Metamorphic:

  • Source: Rocks under high temperatures and pressures in deep crust and upper mantle.

  • Process: Recrystallization in solid state of new minerals.

  • Example: Gneiss.

Rock Classification: Petrology

• Origin

• Composition

• Texture

• Formation and composition of rocks

• Definition of "rock": A naturally-occurring mass of inorganic or organic material, often aggregates of minerals.

• Definition of "mineral": Naturally occurring, inorganic solid with a definable chemical composition and crystal structure. ~2500 minerals known on Earth.

Physical Properties of a Mineral

1. Crystal form/habit

2. Cleavage

3. Hardness

4. Tenacity

5. Density (specific gravity)

6. Streak

7. Colour

8. Luster

9. Optical properties

10. Magnetism

11. Effervescence in $HCl$

12. Odor and taste

Mineral Identification

• Crystal Habit: Related to the symmetry of a perfect crystal of the mineral (rare).

• Crystal Cleavage I:

  • Cleavage is described as: Perfect, Good, Poor

  • Cleavage directions include:

    • One direction (e.g., biotite)

    • Two directions at right angles (e.g., feldspar)

    • Two directions not at right angles (e.g., amphibole)

• Crystal Cleavage II:

  • Three directions at right angles (e.g., halite)

  • Three directions not at right angles (e.g., calcite)

  • Four directions (e.g., fluorite)

  • Six directions (e.g., sphalerite)

• Hardness:

  • Resistance to being scratched, measured by Mohs Scale of Hardness

  • Mohs Scale (examples):

    • Talc (1)

    • Gypsum (2)

    • Calcite (3)

    • Fluorite (4)

    • Apatite (5)

    • Orthoclase (6)

    • Quartz (7)

    • Topaz (8)

    • Corundum (9)

    • Diamond (10)

• Tenacity:

  • Resistance to breakage

  • Examples: Brittle (Quartz), Malleable (Gold), Sectile (Talc), Flexible (Gypsum), Elastic (Mica).

• Density:

  • Most major rock-forming minerals have similar densities around $2.5$ to $3.0 g/cm^3$.

  • Streak: Colour of the powder produced when it is dragged across an un-weathered surface (shows the mineral’s true colour).

  • Colour: Can vary depending on the presence of impurities in the minerals.

• Luster:

  • Describes how light is reflected:

    • Metallic: Strong reflections produced by opaque substances.

    • Vitreous: Bright, as in glass.

    • Resinous: Characteristic of resins, such as amber.

    • Greasy: The appearance of being coated with an oily substance.

    • Pearly: The whitish iridescence of such materials as pearl.

    • Silky: The sheen of fibrous materials such as silk.

    • Adamantine: The brilliant luster of diamond and similar minerals.

• Optical Properties (Petrography):

  • Opacity: light transmitted, translucent, or opaque.

  • Other optical properties: refraction, polarization, reflection, absorption, fluorescence, transmittance, diffraction, dispersion.

• Magnetism:

  • Minerals are attracted to a magnet because of the movement of electrons in their crystalline structure (e.g., Magnetite).

• Effervescence in $HCl$:

  • Carbonate minerals (e.g., calcite, aragonite) are unstable when in contact with hydrochloric acid ($HCl$).

  • They fizz (effervesce).

• Odor and Taste

Atomic Structure of Minerals

• Covalent Bonds: Hard minerals

• Ionic Bonds: Less hard minerals

• Van der Waals: Soft minerals

• Example: $NaCl$: Ionic bonds

Element Replacement in Minerals (Solid Solution)

• Cations of the same size replacing each other in structure.

• Examples:

  • Dolomite: $CaMg(CO<em>3)</em>2$

  • Ankerite: $CaFe(CO<em>3)</em>2$

Crystal Systems

• Defined by relative length of sides and angles between the sides

  • Isometric: $A = B = C$, All angles = $90°$

  • Tetragonal: $A = B ≠ C$, All angles = $90°$

  • Orthorhombic: $A ≠ B ≠ C$, All angles = $90°$

  • Monoclinic: $A ≠ B ≠ C$, Angle A and C=$90°$, Angle B and C>$90°$

  • Triclinic: $A ≠ B ≠ C$, All angles ≠ $90°$

  • Hexagonal: $A1 = A2 = A3 ≠ C$, Angle A sides =$60°$

Requirements for Crystal Growth from a Liquid (Magma or Brine)

1. Right atoms and ions

2. Right concentrations

3. Right temperature

4. Right pressure

5. Space to grow

Crystal Destruction

• Melting

• Melting/recrystallization

• Dissolution

• Chemical Alteration

Mineral Classification

1. Native Element

   • Copper $(Cu)$

   • Gold $(Au)$

2. Sulfide

   • Galena $(PbS)$

   • Pyrite $(FeS_2)$

3. Oxide

   • Hematite $(Fe<em>2O</em>3)$

4. Hydroxide

   • Gibbsite (Bauxite): $(AI(OH)_3)$

5. Halide

   • Halite $(NaCI)$

6. Sulfate

   • Gypsum $(CaSO<em>4 · 2H</em>2O)$

7. Carbonate

   • Calcite $(CaCO_3)$

   • Dolomite $(CaMg(CO<em>3)</em>2)$

   • Siderite $(FeCO_3)$

8. Phosphate

   • Apatite $(Ca<em>5(PO</em>4)_3(F,Cl,OH))$

9. Silicate (e.g., Quartz $SiO_2$)

Most Abundant Minerals in Earth's Crust

• Plagioclase Feldspar (39%)

• Quartz (12%)

• Alkali Feldspar (12%)

• Pyroxenes (11%)

• Other:

  • Amphiboles (5%)

  • Micas (5%)

  • Clays (5%)

  • Other Silicates (3%)

  • Nonsilicates (8%)

Silicates

• Silicate ion $(SiO_4^{4-})$

• Quartz structure

  • Oxygen ions $(O^{2-})$

  • Silicon ion $(Si^{++})$

Silicate Families

• Nesosilicate: 1Si : 4O

• Sorosilicate: 2Si : 7O

• Cyclosilicate: 1Si : 3O

• Inosilicate Single chain: 1Si : 3O

• Phylosilicate: 2Si : 5O

• Tectosilicate: 1Si : 2O

• Inosilicate Double chain: 4Si : 11O

Nesosilicates: Olivine Group

• General formula: $(Mg,Fe,Ca)<em>2SiO</em>4$

• Cleavage: 1 plane

• Hardness: 6.5 - 7

• Only in igneous rocks

  • Forsterite: $Mg<em>2SiO</em>4$

  • Fayalite: $Fe<em>2SiO</em>4$

  • Larnite

Inosilicates (Single Chain): Pyroxene Group

• General formula:

  • Orthopyroxene (opx): $(Mg,Fe)SiO_3$

  • Clinopyroxene (cpx): $Ca(Mg,Fe)Si<em>2O</em>6$

• Cleavage: 2 planes at $90°$

• Hardness: 5.5 - 7

• Common in igneous rocks

Inosilicates (Double Chain): Amphibole Group

• General formula: $(Na,Ca,Mg, Fe)<em>2(Mg,Fe,Al)</em>5(Al,Si)<em>8O</em>{22}(OH)_2$

• Cleavage: 2 planes at $60°$ and $120°$

• Hardness: 5.5 - 6

• Common in igneous and metamorphic rocks

  • Hornblende (black): $(Ca, Na)_{2–3}(Mg, Fe, Al)₅(Al, Si)₈O₂₂(OH)₂$

  • Tremolite (green): $Ca<em>2Mg</em>5Si<em>8O</em>{22}(OH)_2$

Phylosilicates: Mica Group

• General formula: $K(Al,Mg,Fe)<em>{2-3}(AlSi</em>3O<em>{10})(OH)</em>2$

• Biotite (black): $K(Mg,Fe^{2+})(AlSi<em>3O</em>{10})(OH)_2$

• Muscovite (white): $KAl<em>2(AlSi</em>3O<em>{10})(OH)</em>2$

• Cleavage: 1 plane

• Hardness: 2 - 3

• Common in igneous and metamorphic rocks

Phylosilicates: Clay Group

• Varied compositions

• Form by weathering of micas and feldspars

• Hardness: 1 - 2

• Common in sedimentary and metamorphic rocks

Tectosilicates: Quartz

• General formula: $SiO_2$

• Cleavage: none, conchoidal fracture is typical

• Hardness: 7

• Very durable

• In all rocks: igneous, sedimentary, metamorphic

Tectosilicates: Feldspar Group

• General formula:

  • Orthoclase (alkali) feldspar: $KAlSi<em>3O</em>8$

  • Plagioclase feldspar: $(Ca,Na)AlSi<em>3O</em>8$

  • Sanidine (volcanic): $KAlSi<em>3O</em>8$

  • Anorthite: $CaAl<em>2Si</em>2O_8$

  • Albite: $NaAlSi<em>3O</em>8$

• Cleavage: 2 planes at $90°$

• Hardness: 6

• Common in igneous rocks

Exercise 1: Mineral Identification

• List of minerals (Quartz, Sanidine, Albite, Biotite, Hypersthene, Forsterite, Fayalite, Calcite, Siderite, Gypsum) for chemical formula and hardness identification.

Igneous Rocks

• 'Igneous' derives from the Latin word "ignis" meaning "fire"

• Forms through the cooling and solidification of magma

• Subdivided into two different types based on formation location: plutonic (intrusive) and volcanic (extrusive) rocks

Plutonic or Intrusive Igneous Rocks

• Form when magma is trapped inside the Earth (below surface)

• Very slow cooling of magma over many thousands or millions of years until it solidifies

• Minerals have time to grow, hence coarse-grained texture of rocks

• Examples: Granite and Gabbro

Volcanic or Extrusive Igneous Rocks

• Magma = lava cools above (or very near) the Earth’s surface

• Form at erupting volcanoes and oozing fissures

• Almost instant cooling and solidification of lava

• Minerals do not have time to grow, hence very fine-grained, glassy (effusive lava), or vesicular texture of rocks (explosive eruptions; e.g., pumice)

• Examples: Basalt, Obsidian, and Pumice

Magma Origin

• Most magmas originate from 50 km to 250 km depth.

• Locations:

  • Island arc volcanoes (e.g., Java, Indonesia)

  • Plate divergence boundary (e.g., Mid-Atlantic Ridge, Iceland)

  • Hot-spot volcano (e.g., Volcanoes National Park, Hawaii)

  • Continental margin volcano (e.g., Mt. Rainier, Washington)

Factors Affecting Rock Melting Temperatures

Magmatic Differentiation

1. Partial melting

2. Fractional crystallization

3. Assimilation

4. Magma mixing

• Definition: Various processes by which magmas undergo bulk chemical change during the melting process, cooling, emplacement, or eruption.

Partial Melting

• Subducting oceanic crust carries water-rich sediments with it.

• The trapped water is released as the temperature increases, causing the sedimentary rocks to melt at lower temperatures.

• Minerals with lower melting points melt first, forming magma.

Fractional Crystallisation

• Systematic removal of mineral precipitates from magma gradually changes the chemical composition of the magma from an originally homogeneous, mafic melt to a more silicic magma that is relatively depleted in some elements and enriched in others.

Common Minerals of Igneous Rocks

• Different minerals have different melting/crystallization temperatures due to their distinct crystal structures.

Bowen’s Reaction Series

• As magma temperature decreases, minerals crystallize in an ordered series

• During simultaneous crystallization, plagioclase feldspar crystallizes from calcium-rich to sodium-rich form

• The composition of magma changes from ultramafic to andesitic

• Temperature: ~$1200°C$ to ~$600°C$

• Magma composition: Ultramafic to Silicic

Assimilation

• The water and molten sediments melt parts of the overlying plate.

• Molten sediments assimilate with lithospheric rock.

• Magma of intermediate composition is erupted to form arc volcanoes.

Magma Mixing

• Partial melting creates a magma of a particular composition.

• Cooling causes minerals to crystallize and settle.

• A basaltic magma chamber breaks through.

• Mixing results in andesitic magma.

• Crystals may accumulate on the sides and roof of the chamber due to turbulence.

Examples of Plate Tectonic Settings and Igneous Rock Formation

• Continental margin volcano (e.g., Mt. Rainier, Washington, USA): Volcanic arc, mafic to silicic intrusive and extrusive rocks.

• Island arc volcanoes (e.g., Java, Indonesia): Mafic to intermediate intrusive and extrusive rocks.

• Plate divergence boundary (e.g., Mid-Atlantic Ridge, Iceland): Mafic intrusive and extrusive rocks.

• Hot-spot volcano (e.g., Volcanoes National Park, Hawaii): Mafic intrusive and extrusive rocks.

Compositions of Igneous Rocks

Silicic (Felsic) Igneous Rocks

• Silica content: > 65 wt%

• Mineral content:

  • Potassium feldspar (orthoclase, sanidine) dominant

  • Rich in quartz

  • Some plagioclase feldspar

  • Biotite common, some amphibole

• Examples: Granite (Plutonic rock), Rhyolite (Volcanic rock)

Intermediate Igneous Rocks

• Silica content: 55-65 wt%

• Mineral content:

  • Na-plagioclase feldspar dominates

  • Some potassium feldspar

  • Amphibole, biotite mica common

  • Little or no quartz

• Examples: Diorite (Plutonic rock), Andesite (Volcanic rock)

Mafic Igneous Rocks

• Silica content: < 45 wt%

• Mineral content:

  • Rich in Ca-plagioclase feldspar

  • Olivine, pyroxene common

  • No quartz

• Examples: Gabbro (Plutonic rock), Basalt (Volcanic rock)

Ultramafic Igneous Rocks

• Silica content: 40-50 wt%

• Mineral content:

  • Rich in olivine

  • Some pyroxene

  • Little plagioclase feldspar

  • No quartz

• Examples: Peridotite (Plutonic rock), Komatiite (Volcanic rock) (extremely rare, only found in Archean Eon)

Classification of Plutonic (Intrusive) Rocks

• STRECKEISEN or QAPF diagram: Classification based on mineral content.

Classification of Volcanic (Extrusive) Rocks

• STRECKEISEN or QAPF diagram: Classification based on mineral content (difficult due to the small size of crystals).

• TAS (Total-Alkali-Silica) diagram: Classification based on major element chemical composition of volcanic rock.

Exercise 2: Igneous Rocks Identification

1. Describe nine igneous rocks in terms of color, mineral grain size, and textural features.

2. Sort the rocks by plutonic/volcanic origin and attribute them to compositional groups.

3. Name the rocks.