Geological Materials Flashcards
Rock
Any formation of natural origin in the earth, composed of a single mineral, or an aggregate of a number of minerals.
Mineral
Naturally occurring, inorganic, solid compound or element, with a definite chemical composition and a regular internal structure composed of repeating bonded atoms or molecules. Crystalline.
Igneous
Crystallised from magma which is produced by partial melting of rocks at depths typically down to 150km.
Sedimentary.
Mostly formed from the products of weathering of rocks exposed at the earths surfaces which have been deposited by (or in) water, wind or ice, buried the lithified.
Metamorphic
Formed by the transformation of pre-existing rocks in the solid state by the influence of changing pressures and temperatures.
Texture of minerals
Way the minerals in a rock exist together:
Interlocking grown together from magma - CRYSTALLINE
Broken fragments/grains of weathered/eroded rocks - CLASTIC
SiO2
Quartz
Al(2)O(3)
Corundum
Fe(2)O(3)
Haematite
CaCO(3)
Calcite
Mg(2)SiO(4)
Olivine
Crystalline
Minerals are arranged in an orderly, repeating, 3D Array
Glass
Absence of mineral formation
Anions
Negative charge (extra electrons)
Cations
Positive charge (loss of electrons)
Habit
The development of an individual crystal, or aggregate of crystals, to produce a particular external shape, with development depending on the conditions obtaining during formation.
Prismatic
Means that mineral has an elongated habit with the bounding faced forming a prismlike shape, as is common in members of the pyroxene and amphibole groups of silicates.
Columnar
Exhibits rounded columns as is common in tourmaline.
Acicular
Means needlelike (zeolite).
Tabular
Describes crystal masses that are flat like a board as commonly seen in barite.
Fibrous
Refers to threadlike masses, as exhibited by chrysotile, the most common mineral included in the commercial term asbestos.
Massive
Describes a mineral specimen that is totally devoid of crystal faces.
Cleavage
Crystals split along planes of weakness inherent in the structure of their atomic lattices.
Set fractures closely spaced.
Planar cleavage
Cleavage along a single planar direction. MICA minerals.
Prismatic cleavage
Two different cleavage directions whose lines of intersection are commonly parallel to a specific crystallographic direction.
Cubic cleavage
Mineral fragments that have cubic outlines on account of three cleavage directions at 90 degrees to one another.
Rhombohedral cleavage
Results in fragments with an external shape with six sides. Carbonates and calcite.
Conchoidal fracture
Curvature of the interior surface of a shell. Same as in glass due to absence of clearly defined planes of weakness in the crystal structure of quartz or glass.
KAlSi(3)O(8)
Orthoclase
Sheet silicates
Silicate minerals characterised by possessing layers of SiO(4)^(4-) to form a flat sheet with the composition Si(4)O(10). Includes micas, chlorite, clays.
How do crystals form? (Temp drop)
As a melt’s temperature drops, it gradually becomes unstable as a melt and reconfigures into ordered minerals that are more stable. Minerals CRYSTALLISE.
MAGMA COOLING
How do crystals form? (Ionic solution)
Ionic solution becomes supersaturated in ions such that crystals must precipitate out of solution.
Salt forming from seawater.
How do crystals form? (Change Physical/chemical conditions)
Change the physical or chemical conditions of an existing system ie temp or pressure. The existing crystals may not be stable at the new conditions so the ions in the solid minerals will rearrange themselves in the solid state to form new minerals which are more stable.
GRAPHITE TO DIAMOND.
Mineral stability depends on
Temperature Pressure pH Water availability, Gas content, Biological activity. Proportions of elements available - depend on environment crystallisation occurs in.
Mineral stability (carbon polymorph)
Form diamond and graphite = mineral polymorphs.
Formed in different environments - graphite is low pressure and temp
- diamond is the high pressure and temp
- ~50/60 kilobits pressure = 150 km down.
8 main minerals that make up 99% of crust.
Oxygen, Silicon, Aluminium, Iron, Magnesium, Calcium, Potassium, Sodium.
Silicate anion
Tetrahedral, most minerals on earth’s building block.
Chains or sheets.
Bond with cations.
Key mineral groups
Silicates Native elements Halides Oxides Sulphides Carbonates Sulphates
Main silicate minerals are
Olivine Pyroxene Amphibole Mica Feldspar Quartz
Example of isolated tetrahedra
Olivine
Garnet
Example of single chain silicate
Pyroxene
Example of double chain silicates
Amphibole
Example of sheets of tetrahedra silicates
Mica
Clay minerals
Examples of 3D frameworks of tetrahedra silicates
Quartz
Feldspar
Isolated tetrahedra
Each silicate tetrahedra exists on its own, it does not share oxygens with any other silicate tetrahedra.
Net charge remains as 4-
Tetrahedra bonded by cations that satisfy the negative charge. 2 divalent cations (Mg2+,Fe2+)
OLIVINE
Silicate tetrahedra chemical symbol
SiO(4)
Olivine mineral group
(Mg.Fe)(2)SiO(4)-isomorphic substitution
Any proportion or Mg and Fe
Solid solution!
No shared oxygens between tetrahedra only weak ionic bonds.
No regular repeating planes of weakness
Fractures in an irregular fashion - no cleavage.
Hardness 6-7
Conchoidal fracture.
Solid solution
Solid crystalline phases representing a mixture of two or more end members which may vary in composition within finite limits without the appearance of a another phase.
Single chain silicate basic formula
[SiO(3)]2-
Single chains ORTHO-PYROXENE formula
(Mg,Fe)SiO(3) Or (Mg,Fe)(2)Si(2)O(6)
Single chain CLINO-PYROXENE formula
Ca(Mg,Fe)Si(2)O(6)
PYROXENE
Black, hard to tell difference between ortho and Clint pyroxene.
2 perfect cleavages at 90 degrees
Strong Si-O and O-O bonds
Double chain silicates AMPHIBOLE
[Si(8)O(22)]12-
With an OH- is hydrated
Charge is balanced by variety of cations.
Hornblende is most frequent and black
Hornblende amphibole
Black or green
Two prominent cleavages at 60/120 degrees
Sheet silicate MICAS
[Si(4)O(10)]4-
Two sheets linked octahedrally
Flakey
Framework silicates QUARTZ
SiO(2) Pale coloured or transparent. Chemically indestructible No cleavage 7 on Mohs scale. Conchoidal fracture.
Framework silicates FELDSPAR
XAl(Si(2)O(8)
Potassium=orthoclase feldspar
Prismatic cleavage. (ALKALI FELDSPAR)
Plagioclase feldspar - Na to Ca
Key mineral properties
Colour Lustre Streak Hardness Crystal habit Cleavage Fracture
Microcline twinning
Really really slow cooling of orthoclase. corsshatched.
Magmas form from
Partial melts of mantle or crust through decompression melting or hydration melting. Pressure and volatiles influence temp.
Magna generation is linked to
Plate tectonics
Partial melt of mantle produces a _____ magma
Mafic
Magmatic differentiation.
How we get intermediate and silicic felsic magmas and rocks
3 main processes of magmatic differentiation
Fractional crystallisation
Assimilation/contamination
Magma mixing/ replenishment
Bowen’s reaction series
The evolution of magma is linked to the specific order of crystallisation of the silicate minerals
Fixed order in which the silicate minerals crystallise.
3 key groups of Bowen’s reaction series
Discontinuous reaction series of mafic minerals
Continuous reaction series of plagioclase feldspars
Final crystallisation of felsic minerals
Discontinuous series of mafic minerals
Mafic minerals rich in Mg and Fe (olivine pyroxene amphibole biotite) crystallise at specific temp intervals only with each new mineral adopting an increasingly more polymerised and complex silicate structure as temp descends -
isolated tetrahedra>single chains>double chains>sheets
Mafic minerals rich in Mg and Fe
Olivine
Pyroxene
Amphibole
Biotite
Continuous reaction series of plagioclase feldspar
Crystallises at high temp in parallel with mafic series.
Ca rich anothites and becomes more Na rich Albite Down temp. Series is continuous as mineral structure doesn’t change only Ca:Na ratio changes
Remaining felsic minerals crystallise together at lowest temps (orthoclase quartz and muscovite)
Bowen’s reaction series is a model :
Doesn’t predict mafic melt with result in a rock with all silicate minerals but some.
Bowen reaction series depends on temp and one key condition:
That the minerals crystallising do not react back with the magma but are effectively removed from the system as crystallisation proceeds.
FRACTIONAL CRYSTALLISATION
Process by which the composition of a magma can change or evolve due to successive batches of crystals being removed from the magma.
How does fractional crystallisation happen?
Different crystals form at different temps
Crystals forming remove specific proportions of elements from the magma.
Crystals forming early tend to be more dense than the magma
The denser minerals sink to the bottom of the magma chamber effectively removing them from the residual magma….
The composition of remaining magma is now different to what it was at start so had differentiated or evolved.
Fractional crystallisation with Bowen’s reaction series
Olivine plagioclase And pyroxene are forming in a mafic magma and left there they will react with magma continuously so final rock will be a mafic igneous rock containing those three minerals.
REMOVE minerals so cannot react the residual magma composition will be changed.
New magma will be of a composition that is only able to crystallise minerals of intermediate composition.
AMPHIBOLE BIOTIT AND NA RICH PLAGIOCLASE
Bowen’s reaction series predicts that fractional crystallisation. If more mafic magmas can lead to more felsic acidic silicic magmas as long as …..
fractions of crystals are removed from the melt!
Other ways of separating crystals and magma
Filter pressing
Convective separation
Sidewall crystallisation
Assimilation/contamination
Mafic magmas are high temp and have enough heat to partially melt some crust.
Crustal rocks are felsic! Melt at lower temps than mafic magmas exist.
Felsic minerals melt first and become assimilated into mafic magma contaminating it - results in magma more enriched in felsic components.
Where does assimilation contamination occur
SUBDUCTION ZONES Hydration melting (subduction zones) - water/volatiles are added to the mantle then the melting temp is reduced giving partial melt. Partially melts more felsic continental crust and operating alongside fractional crystallisation a magma that is more intermediate - acidic in composition results = volcanic mountains such as Andes.
Examples of where assimilation happens
Magma chambers in deep crust
Large sills and dykes in shallow crust
In areas where mantle plumes invade the crust.
Magma mixing / replenishment.
Mafic + felsic = intermediate magma
Can happen anywhere where two magmas meet (feel in crust)
Magma chambers can evolve through fractionation as we saw and become more felsic in composition but are often replenished with new magma as they are objected with plumes of new mafic magma rising from deep mantle.
Intermediate magma facts
Medium temp ~1000-900 55-65% wt% silica Moderately viscous - resist flow Moderate to high volatile content Erupt effusively or explosively Convergent plate boundaries ANDESITIC DIORITIC OR GRANODIORITIC MAGMAS.
Minerals in intermediate rocks
Amphibole and plagioclase some biotite
A lil bit of quartz and orthoclase.
Colour of intermediate rocks
Medium grey
Fine grained intermediate rock
Andesite
Medium grained intermediate rock is
MICRODIORITE
Coarse grained intermediate rock
DIORITE
Andesites form as
Lavas erupted from subduction zone volcanoes
Diorites are formed from
Subduction zone magma chambers solidifying
Microdiorites are formed from
Dykes and sills of intermediate magma
Eruption style of andesites
Lots of volatiles = PYROCLASTIC ERUPTIONS
erupt effusively as lavas
Steep sided than mafic volcanic edifices
Caldera eruptions.
PYROCLASTIC eruptions
Viscous volatile rich magmas produce PYROCLASTIC eruptions.
Depressurises and gases come out of solution - sticky magmas don’t deform for gasses release bubbles - magma froths And shatters producing TEPHRA
Tephra
Tiny glassy shards of magma - Ash is larger crystalline class called lapilli and larger coasts (blocks)
Mafic Rock
Igneous rock rich in magnesium and iron bearing minerals
Basic Rock
Same as mafic
Felsic Rock
Igneous rock rich in feldspathic minerals and silica
Acidic rock
Same as felsic
Intermediate rock igneous
a rock with a composition between felsic and mafic
Ultramafic Rock
igneous rock that has a similar composition to the mantle, peridotite is an unltramafic rock
2 cooling rates occur when
magma cools deep underground for a bit growing large crystals, then the crystals and remaining melt cam be erupted onto the surface where the rest of the magma cools rapidly to form smaller crystals
2 cooling rates lead to
2 crystal sizes in rocks:
- Large slow cooled crystals; PHENOCRYSTS
- Tiny quick cooled crystals whcih crystallise last to form the rest of the rock known as GROUNDMASS (textural term)
PORPHYRITIC TEXTURE
A rock with large phenocrysts and then a small groundmass
Dikes/Dykes
Intrusive igneous rock formation that cuts across layers of county rock
Sills
Intrusive Igneous rock that runs parallel to layers of country rock
Type of rock formed from an Effusive eruption
Basic/Mafic magma
Types of magma from a pyroclastic eruption
Acidic/Silicic
Rocks formed at an ISLAND ARC PLATE SUBDUCTION
Mafic to Intermediate (Volcanism and plutonism)
Rocks formed at a plate divergence
Basaltic extrusives and intrusives
Rocks formed at Hot Spot Volcanism
Basaltic extrusives and intrusives
Rocks formed at Continental Plate subduction
Mafic to felsic intrusives and extrusives
Partial melting -
Why and where do they form?
minerals melt at different temps - most magmas formed from partial melts. They form at mid ocean ridges and in mantle wedges above subduction zones
Partial melting of mantle - does the mantle always melt at same temperature?
doesn’t always melt at the same temperature because of pressure (melts at higher temp) and volatile content (water and gas reduce the melting temp)
2 main processes by which the mantle partially melts
Decompression melting and dehydration melting
Experiments in melting rocks tell us
At a given pressure a rock will melt over a range of temperatures - the melting interval. We can plot these on a graph. Point at which melting begins is the SOLIDUS
point when the rock is completely molten is the LIQUIDUS
Solidus
Point on graph of melting interval where melting begins
Liquidus
Point on graph of melting interval when the rock is completely molten
Liquid produced by partial melting will be completey enriched in
the lowest temp components especiall Silica
The presence of water _____ the melting temperature
reduces
Rocks rich in silica melt at ____ temps than rocks poor in silica
LOWER
Decompression (adiabatic) melting
Solid mantle, partially melts with exactly right pressures and temps at MID OCEAN RIDGES because mantle gets closest to earths surface - solid mantle flows earths surface where the crust is thinnest, it retains its heat while at lower pressures = PARTIAL MELTING
MAFIC or BASIC magma composition.
Partial melts of the mantle
confining pressure drops but temp stays same and amount of partial melt can be tiny (3-5%)
Dehydration Melting (subduction zones)
Water or volatiles added to the mantle, melting temp is reduced=Partial melt.
CONVERGENT PLATE BOUNDARIES-SUBDUCTION ZONES
Subducted slab contains hydrated ocean crust - at depth the water and volatiles are liberated from the slab as it heats.
Reduces melting temp of mantle wedge above the subducting slab creating a partial melt of the mantle which is a mafic basic composition of magma.
BASIC/MAFIC MAGMAS
main magmas formed from partial melts in mantle.
lowest Wt% Silica of the 3 main magma types. High temps, not viscous (=runny), erupt gently - rarely explode violently like pyroclastic eruptions EFFUSIVE
Mineralogy of Mafic Rocks
Pyroxene, Plagioclase feldspar, ~Olivine ~Glass
Fine grained MAFIC ROCK
BASALT - lava flows on ground or at top of ocean crust
Medium grained MAFIC ROCK
DOLERITE - mafic magma chambers solidifying and also form lower ocean crust
Coarse Grained MAFIC ROCK
GABBRO - dykes or sills in shallow levels of crust.
Where do we find Mafic rocks
OCEAN CRUST- continents rifting apart and fissure eruptions, dykes and sills, shield volcanos
