Classification of Igneous Rocks

Magma

Molten rock. Has 3 components. Formed at depths of ~250km. Becomes lava at the surface.

Melting

Will happen when ions vibrate strong enough to break chemical bonds.

Crystallization

Reverse of melting. Forces of chemical bonds will confine the ions to an orderly crystalline arrangement. Happens at temperatures > 200 degrees Celsius.

Liquid component of magma / Melt

Mobile ions of the 8 most abundant elements (O, Si, Al, Fe, Ca, Na, K, and Mg). O and Si most abundant.

Solid component of magma

Crystallized minerals, assimilated rocks.

Gas component of magma / Volatiles

H2O, CO2, and SO2. Separate from melt when magma approaches the surface.

Intrusive / Plutonic rocks

Coarse-grained (visible mineralogy). Forms plutons (magma chambers of various sizes, shapes, and depths). Observed when uplifted.

Extrusive / Volcanic rocks

Fine-grained (minerals too small due to fast cooling). From magma that rises and erupts from within due to dissolved gases acting as a fuel blaster.

Igneous rocks

Composed mainly of silicates. Si and O are abundant.

Ultrabasic

Ultramafic rocks.

Basic

Mafic rocks. 45-52% weight silica. Dark-colored rocks containing pyroxene, amphibole ± olivine ± biotite.

Intermediate

52-66% weight silica. Grayish to salt and pepper-colored rocks rich in plagioclase, amphibole ± biotite ± quartz.

Acidic

Felsic rocks. >66% weight silica. Light-colored or red rocks rich in K-feldspar, quartz ± biotite or muscovite.

Peridotite

A very dark-colored ultramafic rock depleted with silica and enriched with minerals like olivine, amphibole, and plagioclase.

Rare and occur in the Earth's mantle.

Basalt and Gabbro

Dark-colored, silica-poor rocks rich in plagioclase, pyroxene, and olivine.

Basalt is a very common volcanic rock (most common); encompassing the upper few kilometers of the oceanic crust.

Gabbro crystallizes more slowly at depth at the lower portions of the ocean basin.

Andesite and Diorite

Gray-colored to salt and pepper-colored rocks rich in hornblende, pyroxene, and plagioclase; have almost 2/3 silica.

Most common volcanic rock in the Pacific Ring of Fire.

Dioritic plutons underlies most andesitic volcanoes.

Dacite and Granodiorite

Light-colored rocks containing 2/3 silica, rich in plagioclase, K-feldspar, and quartz (also some biotite and hornblende but minor amounts).

Dacite also occurs along the Pacific Ring of Fire.

Granodiorite underlies most andesitic-dacitic volcanoes.

Rhyolite and Granite

Light-colored to red-colored rocks containing more than 2/3 silica, and rich in quartz, K-feldspar with minor amounts of plagioclase and biotite.

Rhyolite commonly erupts on thick continental crusts.

Granite crystallizes also occur at thick continental crusts.

Texture

Overall appearance of rock based on size, shape, and arrangement of minerals. Infers rock origin.

Holocrystalline texture

Wholly crystalline

Hypocrystalline texture

Partially crystalline / partially glass

Holohyaline texture

Wholly glass

Euhedral grains

Contain complete crystal faces that are not impinged by other crystals.

Develop as early mineral phases in the crystallization of magma wherein crystals have abundant free space for growth, enhancing the likelihood to form perfectly formed crystal faces.

Subhedral grains

Contain partially complete crystal forms in which at least one of the crystal faces is impinged by adjacent rock material.

What causes formation of subhedral grains?

(1) Contact against previously formed minerals

(2) Nucleation on pre-existing surfaces such as early formed crystals on the margins of the magma chamber

(3) Resorption in which pre-existing euhedral crystals are partially remelted

(4) Other secondary alteration processes that destroy pre-existing euhedral faces

Anhedral grains

Lack any observable crystal faces.

The space available for the development of euhedral and subhedral crystals diminishes as crystallization progresses.

Shape determined by the available space left (interstitial spaces).

Factors affecting texture

1. Cooling rate

2. Depth

3. Crystal nucleation rate

4. Crystal growth rates

5. Rate of magma undercooling

6. Ion availability

7. Chemical diffusion rates

8. Viscosity

9. Chemical composition and volatile content of magma

Pegmatitic texture

Exceptionally coarse-grained (>30mm). Display large, euhedral crystals surrounded by subhedral grains. Commonly develops on granitic plutons with high volatile components.

Source of many gemstones and valuable ore deposits of metals such as Sn, Ag, and Au.

Phaneritic texture

Diameters of 1-30mm. Fine-grained varieties associated with shallow intrusions while coarse-grained ones associated with deep intrusions.

What are the subdivisions of the phaneritic texture?

Fine (1-3mm)

Medium (3-10mm)

Coarse (10-30mm)

Aphanitic texture

Fine-grained due to fast cooling.

Microcrystalline

Term used if minerals can be identified under the microscope.

Cryptocrystalline

Term used if minerals cannot be identified individually under the microscope.

Porphyritic texture

Consists of two distinct size of crystals: phenocrysts and groundmass. Shows a two-stage cooling process where larger phenocrysts form slowly at depth, while the finer grained groundmass crystals cool rapidly as magma approaches the surface.

How is magma generated?

Increase in temperature

Decompression

Addition of volatiles

Crystal nucleation

Involves the formation of new crystals through nuclei, or seed crystals which are large enough to persist and grow to even larger crystals. Affected by rate of undercooling, ion availability, and ease of migration.

Crystal growth rate

Measure of the increase in crystal radius over time (cm/s).

Governed by the rate of undercooling as well as the availability of elements and magma viscosity.

Low nucleation rate = high growth rate.

Ion availability

Refers to the availability of ions that can fill specific ionic site in a crystal lattice structure.

Enhanced crystal growth = appropriate ions readily available and migrate immediately.

Viscosity

Resistance of a fluid to shear stress. Determined by silica content.

Higher viscosity = lower strain rate.

Chemical diffusion rate

Diffusion rate is the rate at which elements migrate through magma; depends on viscosity.

Low viscosity = high rate of diffusion and growth rate.

High viscosity = low rate of diffusion and growth rate.

Viscosity and magma composition

Increased molecular linkages results in higher magma viscosity

Molecular linkage is determined by the relative abundance of network formers and network modifiers.

Silica rich = more viscous

Silica poor = less viscous

Network formers

Elements that tend to increase the molecular linkage thereby increasing viscosity (Si, Al, and O).

Network modifiers

Elements that decrease molecular linkage thereby reducing viscosity.

Role of gases

Magma contain 7% gas.

Gases reduce molecular bonding; important network modifiers.

More gases = less viscous because dissolved volatiles partially bonds with the corners of the silica tetrahedra.

Temperature

Inversely proportional to viscosity and molecular bonding (that's why hotter magma is more fluid and cooler magmas are more viscous).

Molecular linkage in magma

Dependent on temperature and magma composition.

Glassy texture

Very quick cooling. Ions frozen in place due to very quick cooling. Due to rapid loss of dissolved gases.

Quenching

Occurs when melts of any composition come into contact with liquid water or air causing it to solidify before crystals have the time to nucleate and grow.

Vitrophyric texture

Results from the presence of vitrophyres - porphyritic glassy rocks.

Devitrification

Occurs when glass crystallize in solid state by growing on pre-existing microlitic or cryptocrystalline nuclei.

Spherulites

Radiating crystals that grow outward from an existing crystal nuclei.

Perlites

Glassy, silica-rich volcanic rocks with higher water content than obsidian; exhibit perlitic texture, characterized by cloudy appearance and curved or sub-spherical cooling cracks.

Vesicular texture

Spherical to ellipsoidal void spaces.

Develop due to exsolution and entrapment of gas bubbles in lava as it cools and solidifies. Vesicular rocks contain >30% vesicles by volume (like pumice and scoria). Rocks that have 5-30% vesicles are named using the modifier 'vesicular'.

Amygdules

Form when hot fluids flow through vesicular rocks and later precipitate secondary minerals in the void spaces (can be quartz, calcite, epidote, zeolite, and metals).

Level of exsolution

The depth on where magma cannot hold the gas in solution.

Vesiculation

Process of nucleating gas bubbles.

Fragmentation surface

Observed when bubbles constitute 70-80% of the magma volume.

Exsolution

When gases separate from the magma due to decompression; gas bubble develop, expand, and rise towards the surface as a separate phase.

Pyroclasts

Broken materials/rock particles ejected from a volcanic eruption.

What are the types of pyroclasts?

Lithic (rock fragments)

Vitric (glassy fragments)

Crystal (mineral fragments)

Volcanic breccia

Deposited proximal to the volcanic vent and subjected to minimal transport such that the angular blocks are not abraded.

Agglomerate

Composed of volcanic bombs that are abraded and rounded during transport.

Unwelded tuff

Produced by small or single event eruptions. They display random shard orientations and spherical to ellipsoidal pumice vesicles.

Welding

Results as fragments become progressively fused together as porosity decreases during compaction. Plastic flow deforms hot shard making them flattened and rotated to a more parallel orientation.

Partially welded tuff

Created by more compaction in the pyroclastic deposits.

Densely welded tuff

Formed by intense compaction of still hot deposits have dark colors and glassy lusters that resembles obsidian.

Major elements

Have concentrations of >1% in the crust.

O, Si, Al, Fe, Ca, Na, K, and Mg.

Join with O to form the major oxide compounds present in igneous rocks.

Minor elements

Have concentration of 0.1-1%.

Most common minor elements are Cr, Mn, P, H, and Ti.

Economically important.

Trace elements

Have concentration of

Rare earth elements (REE)

Lanthanoid elements.

REE with odd atomic numbers are more abundant that those with even atomic numbers.

Relative abundance in crust acquired by dividing to the amount present in chondrites.

Divided into LREE and HREE.

Light Rare Earth Elements (LREE)

Incompatible

Left side of lanthanoid series (La, Ce, Pr, Nd, and Sa).

Heavy Rare Earth Elements (HREE)

Compatible

Right end of lanthanoid series (Eu, Gd, Tb, Ho, Er, Tm, Yb, and Lu)

High Field Strength Elements (HSFE)

High ionic charge (+3 or +4).

Immobile, remain in restite.

Ionic radius:valence charge < 0.20

Includes Ti, Ni, Cr, V, Zr, Hf, Nb, Ta, and Y

Large Ion Lithophile Elements (LILE)

Mobile.

Ionic radius:valence charge > 0.20.

Useful in determining the role of hydrous fluid interaction and the parental source of the partial melt.

Have difficulty entering the solid phase.

Includes Cs, Ba, Rb, Sr, U, Pb, K, Zr, Th, and Ta.

Element compatibility

Measure of the ease with which an element fits into the crystal structure, and is analogous to the description of a compatible or incompatible person.

Compatible elements

Form long-lasting bonds and incorporate into crystal structures.

They are immobile which means they do not easily migrate.

Incompatible elements

Do not fit easily into crystal structures and their bonds are easily broken.

They are mobile which means they migrate from crystal structures into the melt when the rock is subjected to partial melting.

Primary minerals

Crystallize directly from the magma at elevated temperatures. Divided into major and accessory minerals.

Major minerals

Formed from the 7 oxide compounds.

Have abundance >5%.

Include quartz, K-feldspar, plagioclase feldspar, feldspathoid, mica, amphibole, pyroxene, and olivine mineral groups.

Not economically important.

Accessory minerals

Concentration

Secondary minerals

Form later in response to chemical changes that affect the existing rock.

Replace primary minerals or infill voids through alterations by hot solutions, chemical reactions with country rock, or other secondary alteration processes.

Color Index / Mafic Index

CI = [%mafic / (%mafic + % felsic)] x 100

Felsic (Hyndman, 1985)

Intermediate (Hyndman, 1985)

40-70% dark-colored minerals

Mafic (Hyndman, 1985)

70-90% dark-colored minerals

Ultramafic (Hyndman, 1985)

>90% dark-colored minerals

Leucocratic

Mesocratic

35-65% dark-colored minerals

Melanocratic

65-90% dark-colored minerals

Hypermelanic

>90% dark-colored minerals

IUGS color index rock modifier terms

Leucocratic

Mesocratic

Melanocratic

Hypermelanic

Modal composition / Mode

Abundance of minerals by percentage by volume.

Through actual mineral identification.

Direct measuring technique.

Works well for crystalline rocks but fails in fine-grained or glassy ones.

Normative compositon

Uses data derived from chemical analysis of a rock as devised by Cross, Iddings, Pirsson, and Washington.

Also called the CIPW Norm.

Commonly used in aphanitic or glassy rocks.

Takes a rock bulk chemical composition and creates a hypothetical set of minerals using the bulk chemical data set.

Normative minerals

Quartz

Orthoclase

Albite

Anorthite

Nepheline

Magnetite

Ilmenite

Apatite

Corundum Diopside

Enstatite

Hypersthene

Olivine

Silica oversaturated

Implies that all available cation oxides have been used to make normative minerals and additional SiO2 remains available to generate normative quartz.

Evidence of presence of free quartz or tridymite alongside plagioclase.

More than 2/3 silica present to convert nepheline to albite.

Quartz ±feldspars and/or magnesium orthopyroxene.

Silica saturated

Exactly enough SiO2 to consume all the other oxides.

Occurs when there is exact (2/3) amount of silica to convert nepheline to albite.

Evidenced by the presence of feldspar and absence of silica and feldspathoids.

Feldspars and/or magnesium orthopyroxene only.

Silica undersaturated

SiO2 is depleted before all the other oxides have been used to form normative minerals

Insufficient SiO2 to make quartz, feldspars, or OPX.

Occurs when there is not enough silica to convert nepheline to albite

Evidenced by presence of feldspathoids (ex: nepheline).

Forsterite olivine, nepheline, leucite, and other feldspathoids ± feldspars and/or OPX minerals. Excludes quartz.

Peraluminous

Characterized by unusually high Al2O3 contents.

Al2O3 > CaO + Na2O + K2O.

Muscovite, corundum, topaz, garnet, tourmaline, cordierite, andalusite, biotite.

Peralkaline

Contain normative or modal minerals with unusually high K2O and or Al2O3 contents.

Al2O3 < Na2O + K2O.

Aegirine, riebeckite, arfvedsonite, aenigmatite, astrophyllite, columbite, pyrochlore.

Metaluminous

Contain mafic minerals with average aluminum contents.

Na2O + K2O < Al2O3 < CaO + Na2O + K2O.

Hornblende, epidote, melilite, biotite, pyroxene.