Minerals • A mineral is a naturally occurring,  inorganic, usually non‐biologic, crystalline  solid, which is physically and chemically  distinctive. • Form in the geosphere (most minerals),  hydrosphere (e.g., halite, gypsum),  biosphere (e.g., calcite, aragonite), and  even the atmosphere (e.g., water ice, as  snow) • Consistent and recognizable physical and  chemical properties

Composition of Earth’s Crust • Common elements – Nearly 98% of the atoms in Earth’s crust are represented by the 8 most common elements

• O, Si, Al, Fe, Ca, Na, K, Mg

• Common mineral types – Most minerals are silicates (contain Si and O bonded together)

• Minerals have crystalline structures – Regular 3-D arrangement of atoms

Silicate Structures •

The Silicon-Oxygen tetrahedron

– Strongly bonded silicate ion – Basic structure for silicate minerals •

Sharing of O atoms in tetrahedra

– The more shared O atoms per tetrahedron, the more complex the silicate structure • Isolated tetrahedra (none shared) • Chain silicates (2 shared) • Double-chain silicates (alternating 2 and 3 shared) • Sheet silicates (3 shared) • Framework silicates (4 shared)

Garnet Augite (inosilicate)

Tremolite (amphibole) Biotite (mica)

Quartz

Feldsapr (albite)

Non-silicate Minerals • Carbonates – Contain CO3 in their structures (e.g., calcite - CaCO3)

• Sulfates – Contain SO4 in their structures (e.g., gypsum - CaSO4. 2H2O)

• Sulfides – Contain S (but no O) in their structures (e.g., pyrite - FeS2)

• Oxides – Contain O, but not bonded to Si, C or S (e.g., hematite - Fe2O3)

• Native elements – Composed entirely of one element (e.g., diamond - C; gold - Au)

Minerals • A mineral must meet the following criteria: – Crystalline solid

• Atoms are arranged in a consistent and orderly geometric pattern – Forms through natural geological processes – Has a specific chemical composition • May include some internal compositional variation, such as the solid solution of Ca and Na in plagioclase)

• Rock-forming minerals – Although over 4000 minerals have been identified, only a few hundred are common enough to be generally important to geology (rock-forming minerals) – Over 90% of Earth’s crust is composed of minerals from only 5 groups (feldspars, pyroxenes, amphiboles, micas, quartz)

Minerals • Ore minerals – Minerals of commercial value – Most are non-silicates (primary source of metals)

• Examples: magnetite and hematite (iron), chalcopyrite (copper), galena (lead), sphalerite (zinc) – Must be able to be extracted profitably to be considered current resources

• Gemstones – Prized for their beauty and (often) hardness – May be commercially useful

• Diamond, corundum, garnet, and quartz are used as abrasives

Mineral Properties • Physical and chemical properties of minerals are closely linked to their atomic structures and compositions •

Color

– Visible hue of a mineral •

Streak

– Color left behind when mineral is scraped on unglazed porcelain •

Luster

– Manner in which light reflects off surface of a mineral •

Hardness

– Scratch-resistance •

Crystal form

– External geometric form

Mineral Properties •

Cleavage

– Breakage along flat planes •

Fracture

– Irregular breakage •

Specific gravity

– Density relative to that of water •

Magnetism

– Attracted to magnet •

Chemical reaction

– Calcite fizzes in dilute HCl

Crystal Habit “appearance in hand specimens”

Massive, Granular, Compact

find grained

Lamellar, Micaceous, Bladed

layered

Fibrous, Acicular, Radiating

needlelike

Dendritic

branching

Banded, Concentric, Geodes

bands

Botryoidal, Globular, Stalactitic

orbs etc.

Oölitic, Pisolitic

pea like

Now to Rocks. What is the difference between a rock and a mineral?

The Rock Cycle • A rock is a naturally formed, consolidated material usually composed of grains of one or more minerals • The rock cycle shows how one type of rocky material gets transformed into another – Representation of how rocks are formed, broken down, and processed in response to changing conditions – Processes may involve interactions of geosphere with hydrosphere, atmosphere and/or biosphere – Arrows indicate possible process paths within the cycle

Imagine the first rock and the cycles that it has been  through.

The Rock Cycle and Plate Tectonics • Magma is created by melting of rock  above a subduction zone • Less dense magma rises and cools  to form igneous rock • Igneous rock exposed at surface  gets weathered into sediment Convergent plate boundary • Sediments transported to low areas,  buried and hardened into sedimentary rock • Sedimentary rock heated and squeezed at depth to form metamorphic  rock • Metamorphic rock may heat up and melt at depth to form magma

Igneous Rocks • Magma is molten rock • Igneous rocks form when magma cools and solidifies – Intrusive igneous rocks form when magma solidifies underground

Granite

• Granite is a common example

– Extrusive igneous rocks form when magma solidifies at the Earth’s surface (lava) • Basalt is a common example Basalt

Igneous Rock Textures • Texture refers to the size, shape and arrangement of grains or other constituents within a rock • Texture of igneous rocks is primarily controlled by cooling rate • Extrusive igneous rocks cool quickly at or near Earth’s surface and are typically finegrained (most crystals 1 mm)

Fine-grained igneous rock

Coarse-grained igneous roc

The basics of igneous rock textures

Fast cooling

small xtals

Slow cooling

large xtals

It is a function of viscosity of the melt,  which is controlled by composition and  temperature.

Coarse grained igneous rock

Fine grained igneous rock

Special Igneous Textures • A pegmatite is an extremely coarse-grained igneous rock (most crystals >5 cm) formed when magma cools very slowly at depth • A glassy texture contains no crystals at all, and is formed by extremely rapid cooling • A porphyritic texture includes two distinct crystal sizes, with the larger having formed first during slow cooling underground and the small forming during more rapid cooling at the Earth’s surface

Pegmatitic igneous rock

Porphyritic igneous rock

Pegmatite: Very coarse grained igneous rock

Porphyritic igneous rock: Big xtals in a fine grain matrix

Igneous Rock Identification •

Igneous rock names are based on texture (grain size) and mineralogic composition Textural classification

• – –

•

Plutonic rocks (gabbro-diorite-granite) are coarse-grained and cooled slowly at depth Volcanic rocks (basalt-andesite-rhyolite) are typically fine-grained and cooled rapidly at the Earth’s surface

Compositional classification – – –

Mafic rocks (gabbro-basalt) contain abundant dark-colored ferromagnesian minerals Intermediate rocks (diorite-andesite) contain roughly equal amounts of dark- and light-colored minerals Felsic rocks (granite-rhyolite) contain abundant light-colored minerals

Igneous Rock Identification •

Igneous rock names are based on texture (grain size) and mineralogic composition

Devil’s Tower; a volcanic neck, a feeder pipe

Sill; parallels layers in the country rock

Dike; cuts across layers in the country rock

Half Dome; part of the Sierra Nevada batholith

Bowen’s Reaction Series

Six common Igneous Rocks 1000 C

Solidifying Temperature

500 C

Increasing Grain Size

Silica Content

Minerals

Basalt

low

pyroxene, olivine, feldspar, & amphibole

Gabbro

Andesite

intermediate

feldspar, amphibole, pyroxene, biotite mica

Diorite

Rhyolite

high

feldspar, quartz, muscovite mica, & amphibole

Granite

Present (in order of abundance)

Plutonic Rocks Lighter Color

Volcanic Rocks

Lessons from Bowen’s Reaction Series • • • •

•

Large variety of igneous rocks is produced by large variety of magma compositions Mafic magmas will crystallize into basalt or gabbro if early-formed minerals are not removed from the magma Intermediate magmas will similarly crystallize into diorite or andesite if minerals are not removed Separation of early-formed ferromagnesian minerals from a magma body increases the silica content of the remaining magma Minerals melt in the reverse order of that in which they crystallize from a magma >>>> partial melting!!!

Magma Evolution • •

•

•

A change in the composition of a magma body is known as magma evolution Magma evolution can occur by differentiation, partial melting, assimilation, or magma mixing Differentiation involves the changing of magma composition by the removal of denser early-formed ferromagnesian minerals by crystal settling Partial melting produces magmas less mafic than their source rocks, because lower melting point minerals are more felsic in composition

Differentiation Magma chamber fills

Early formed mafic minerals crystallize  and settle (or are otherwise separated  from the residual melt)

The remaining melt is  enriched in silica

Magma Evolution I •

Assimilation occurs when a hot magma melts and incorporates more felsic surrounding country rock Xenolith

Insert new Fig. 3.22 here

Magma Evolution II •

Magma mixing involves the mixing of more and less mafic magmas to produce one of intermediate composition

Metamorphism • The transformation of rock by temperature and pressure • Metamorphic rocks are produced by transformation of: • Igneous, sedimentary and igneous rxs

Thanks to CU Boulder Geology Dept for use of some of these slides

Metamorphism • Metamorphism progresses from low to high grades • Rocks remain solid during metamorphism

What causes metamorphism? • Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals

• Pressure (stress) • Increases with depth • Pressure can be applied equally in all directions or differentially

Main factor affecting metamorphism • Parent rock • Metamorphic rocks typically have the same chemical composition as the rock they were formed from • Different minerals, but made of the same stuff. • Exception: gases (carbon dioxide, CO2) and water (H2O) may be released

Progressive metamorphism of a shale

Shale

Progressive metamorphism of a shale

Slate

Progressive metamorphism of a shale

Phyllite

Progressive metamorphism of a shale

Schist

Progressive metamorphism of a shale

Gneiss

Metamorphism • Three types of metamorphic settings: • Contact metamorphism – from a rise in temperature within host rock • Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • Regional metamorphism -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock

Common metamorphic rocks • Nonfoliated rocks • Quartzite – Formed from a parent rock of quartz-rich sandstone – Quartz grains are fused together – Forms in intermediate T, P conditions

Sample of quartzite

Thin section of quartzite

Common metamorphic rocks

• Nonfoliated rocks • Marble

– Coarse, crystalline – Parent rock usually limestone – Composed of calcite crystals – Fabric can be random or oriented

Marble (Random fabric = annealing; nonfoliated)

Change in metamorphic grade with depth

Common metamorphic rocks • Foliated rocks • Slate – Very fine-grained – Excellent rock cleavage – Made by low-grade metamorphism of shale

Example of slate

Slate roof

Common metamorphic rocks • Foliated rocks • Phyllite – Grade of metamorphism between slate and schist – Made of small platy minerals – Glossy sheen with rock cleavage – Composed mainly of muscovite and/or chlorite

Phyllite (left) and Slate (right)  lack visible mineral grains

Common metamorphic rocks • Foliated rocks • Schist – Medium- to coarse-grained – Comprised of platy minerals (micas) – The term schist describes the texture – To indicate composition, mineral names are used (such as mica schist)

Mica Schist - note well developed foliation

A mica garnet schist

Common metamorphic rocks • Foliated rocks • Gneiss – Medium- to coarse-grained – Banded appearance – High-grade metamorphism – Composed of light-colored feldspar layers with bands of dark mafic minerals

Gneiss displays bands of light and dark  minerals

What are metamorphic textures? • Texture refers to the size, shape, and arrangement of mineral grains within a rock • Foliation – planar arrangement of mineral grains within a rock

Outcrop of foliated gneiss

Metamorphic textures • Foliation • Foliation can form in various ways: – Rotation of platy or elongated minerals – Recrystallization of minerals in a preferred orientation – Changing the shape of equidimensional grains into elongated and aligned shapes

Flattened Pebble Conglomerate = flattening

Development of foliation due  to directed pressure