Ch. 3: Textures Ch. 3: Textures Some Useful Some Useful Definitions Definitions The The Surface Energy Surface Energy is the excess is the excess energy at the surface of a material energy at the surface of a material (unbalanced ionic charges, etc.) (unbalanced ionic charges, etc.) compared to the interior. compared to the interior. Surface energy is measured in the Surface energy is measured in the laboratory as the energy (work = laboratory as the energy (work = force x distance) consumed while force x distance) consumed while cutting a new surface in the plane of cutting a new surface in the plane of interest. interest.
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Ch. 3: Textures Some Useful Definitions l The Surface Energy is the excess energy at the surface of a material (unbalanced ionic charges, etc.) compared.
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The The Surface Energy Surface Energy is the excess energy at the is the excess energy at the surface of a material (unbalanced ionic charges, surface of a material (unbalanced ionic charges, etc.) compared to the interior.etc.) compared to the interior.
Surface energy is measured in the laboratory as Surface energy is measured in the laboratory as the energy (work = force x distance) consumed the energy (work = force x distance) consumed while cutting a new surface in the plane of while cutting a new surface in the plane of interest.interest.
Some Useful Definitions (cont.)Some Useful Definitions (cont.) Instability: the spontaneous separation of Instability: the spontaneous separation of smallsmall clusters of clusters of
compatible ions due to high surface charge. This inhibits compatible ions due to high surface charge. This inhibits nucleation.nucleation.
Nucleation: forming a critical-sized embryonic crystal, a Nucleation: forming a critical-sized embryonic crystal, a crystal that is large enough to have sufficient balanced crystal that is large enough to have sufficient balanced interior volume, so that it is stable and won’t fly apart due to interior volume, so that it is stable and won’t fly apart due to like electrostatic charges.like electrostatic charges.
Undercooling: cooling of a melt below the true Undercooling: cooling of a melt below the true crystallization temperature of a mineral.crystallization temperature of a mineral.
Twin: an intergrowth of two or more orientations of the same Twin: an intergrowth of two or more orientations of the same mineral that share common atoms, typically on a plane.mineral that share common atoms, typically on a plane.
NucleationNucleation Requires either undercooling orRequires either undercooling or A seed crystal of same or similar structure mineralA seed crystal of same or similar structure mineral
So oxides small & numerous, K-spars fewSo oxides small & numerous, K-spars few
DiffusionDiffusion
For growth to proceed, constituents must For growth to proceed, constituents must diffuse through the melt, cross the depleted diffuse through the melt, cross the depleted zone, and reach the crystal surface.zone, and reach the crystal surface.
Crystal formation produces heatCrystal formation produces heat ““Latent heat of crystallization”Latent heat of crystallization” This heat must diffuse away, or the This heat must diffuse away, or the
temperature (KE) may become too high for temperature (KE) may become too high for crystallization to proceed.crystallization to proceed.
MeltMelt
T T ~~ <T <Tmeltmelt
Few xtalsFew xtalsbut grow but grow largelarge
T <<TT <<Tmeltmelt
Many xtalsMany xtalsbut don’tbut don’tgrow largegrow large
T T <<<< <T <Tmeltmelt
no xtalsno xtalsglassglass
Glass Aphanitic Phaneritic
ObservationsObservations
If the cooling rate is very slow, Equilibrium If the cooling rate is very slow, Equilibrium is maintained or closely approximated.is maintained or closely approximated.
Initially, undercooling enhances growth and Initially, undercooling enhances growth and nucleationnucleation
However, further cooling decreases Kinetic However, further cooling decreases Kinetic Energy (KE) and increases viscosity, Energy (KE) and increases viscosity, slowing diffusion and stopping growth.slowing diffusion and stopping growth.
PorphyriticPorphyritic
Rock with distinct difference in xtal sizes.Rock with distinct difference in xtal sizes. Igneous: slow cooling in magma chamber, Igneous: slow cooling in magma chamber,
fast cooling near/at surface.fast cooling near/at surface. Phenocrysts Phenocrysts In glassy matrix/groundmass vitrophyricIn glassy matrix/groundmass vitrophyric If phenocrysts contain numerous inclusions If phenocrysts contain numerous inclusions
poikilitic.poikilitic.
Large Crystal GrowthLarge Crystal Growth
Largest xtals haveLargest xtals have The most plentiful components in adjacent meltThe most plentiful components in adjacent melt The fastest diffusing componentsThe fastest diffusing components
Faster at higher temps MAFICFaster at higher temps MAFIC Faster in material of low viscosity MAFICFaster in material of low viscosity MAFIC Slower in highly polymerized viscous melts FELSICSlower in highly polymerized viscous melts FELSIC Diffusion fastest in fluid > glass > solidDiffusion fastest in fluid > glass > solid
Role of Water and PegmatitesRole of Water and Pegmatites
HH22O dramatically reduces the O dramatically reduces the
polymerization of magma. Melt is less polymerization of magma. Melt is less viscous. viscous.
Large xtal size in Pegmatites may be Large xtal size in Pegmatites may be attributed more to high diffusion of attributed more to high diffusion of components through the Hcomponents through the H22O-rich melt, O-rich melt,
rather than just slow cooling.rather than just slow cooling.
Different Melting PtsDifferent Melting Pts
Different minerals have different melting points.Different minerals have different melting points. At some TAt some Too, some forming slightly undercooled, few , some forming slightly undercooled, few
nucleii but fast growth, others greatly undercooled nucleii but fast growth, others greatly undercooled many nucleii but slow growth.many nucleii but slow growth.
“ “ The popular notion that the large xtals in a The popular notion that the large xtals in a porphyritic rockk must have formed “first” is not porphyritic rockk must have formed “first” is not necessarily true. necessarily true.
Melting Point DepressionMelting Point Depression
Adding additional phases lowers the Adding additional phases lowers the melting/freezing temperature.melting/freezing temperature.
Removing a phase raises the Removing a phase raises the melting/freezing temperaturemelting/freezing temperature
DewateringDewatering
Sudden loss of the water-rich fluid phase will Sudden loss of the water-rich fluid phase will quickly raise the melting point, making the quickly raise the melting point, making the forming minerals significantly more forming minerals significantly more undercooled at the current Temperature. This undercooled at the current Temperature. This can move mineral species to form many new can move mineral species to form many new nuclei, with low growth, producing many nuclei, with low growth, producing many aphanitic crystals. This can produce a aphanitic crystals. This can produce a porphyritic texture in plutonic rocks if porphyritic texture in plutonic rocks if phenocrysts were already present.phenocrysts were already present.
Crystal Crystal GrowthGrowth
Addition of more ions onto existing faceAddition of more ions onto existing face Observations:Observations: Bravais: planes with a high density of lattice Bravais: planes with a high density of lattice
points form a more prominent facepoints form a more prominent face Fast growing faces have smaller interplanar Fast growing faces have smaller interplanar
distancesdistances Faces with low surface energy become more Faces with low surface energy become more
Preferred nucleation of one mineral on another Preferred nucleation of one mineral on another pre-existing mineral.pre-existing mineral.
Ex. 1: Sillimanite on mica rather than on its Ex. 1: Sillimanite on mica rather than on its polymorph Kyanite in equilibriumpolymorph Kyanite in equilibrium
Si-Al-O structures more similar in the micas.Si-Al-O structures more similar in the micas. Ex. 2: Plagioclase overgrows K-spar Ex. 2: Plagioclase overgrows K-spar
Orthoclase rather than nucleate on its own.Orthoclase rather than nucleate on its own.
Bowen’s Reaction SeriesBowen’s Reaction SeriesMolten- VERY HotMolten- VERY HotNo solidsNo solids
Molten- Not so hotMolten- Not so hot
100% Solid100% Solid
First mineral to crystallize outFirst mineral to crystallize outIndependent TetrahedraIndependent Tetrahedra
SingleSinglechainschains
DoubleDoublechainschains
sheetssheets
3-D3-D
3-D3-D3-D3-D
““Basaltic”Basaltic”
““Andesitic”Andesitic”
““granitic”granitic”
Fo Mg++ 1900C Fa Fe++ 1500CFo Mg++ 1900C Fa Fe++ 1500C
Take home lesson: if you leave a Take home lesson: if you leave a crystal exposed to the melt, the melt crystal exposed to the melt, the melt will react with it, sometimes will react with it, sometimes scavenging parts. Olivine examples scavenging parts. Olivine examples seen in Basaltsseen in BasaltsIf a new mineral is stable at new If a new mineral is stable at new conditions, the old mineral will be conditions, the old mineral will be covered with the new. This example covered with the new. This example of Plagioclase in a cooling melt.of Plagioclase in a cooling melt.
Compositional Zoning in PlagioclaseCompositional Zoning in Plagioclase
Plagioclase does not re-equilibrate Plagioclase does not re-equilibrate with the melt when the melt with the melt when the melt changes composition, would changes composition, would require substitution of Al for Si in require substitution of Al for Si in one position. This is difficult one position. This is difficult because: because:
Al-O and Si-O bonds are very Al-O and Si-O bonds are very strongstrong
AlAl+3+3 is a very slow diffuser is a very slow diffuser
Crystallization Sequence – experiment: early euhedral Px, Crystallization Sequence – experiment: early euhedral Px, later interstitial Plag. Suggests euhedral Px formed firstlater interstitial Plag. Suggests euhedral Px formed first
<= Granophyric <= Granophyric In a rapid reaction where In a rapid reaction where two minerals must form simultaneously (here two minerals must form simultaneously (here alkali feldspar and quartz in low H2O magma) alkali feldspar and quartz in low H2O magma) an intergrowth occurs, not large euhedral an intergrowth occurs, not large euhedral crystals.crystals.
b. Graphic b. Graphic Can occur when two solid Can occur when two solid phases (mineral species) form (separate, phases (mineral species) form (separate, exsolve, solidify , freeze, open-system exsolve, solidify , freeze, open-system fractional crystallization) at the same time. fractional crystallization) at the same time. Recall Microcline + Albite (Perthitic Texture) Recall Microcline + Albite (Perthitic Texture) from homogeneous (K,Na)AlSifrom homogeneous (K,Na)AlSi33OO88
When a hydrous magma reaches the surface, decompression releases volatiles, When a hydrous magma reaches the surface, decompression releases volatiles, and hydrous minerals such as hornblende and biotite may develop rims of fine and hydrous minerals such as hornblende and biotite may develop rims of fine iron oxides and pyroxenes.iron oxides and pyroxenes.
Minerals accumulate by sinking or floating or by being plastered (by convective Minerals accumulate by sinking or floating or by being plastered (by convective flows)to the magma chamber, or they form in place as the melt flows by with parts.flows)to the magma chamber, or they form in place as the melt flows by with parts.
Orthocumulate, insterstitial liquid solidifies in place, without exchanging ions with the larger Magma chamber.Orthocumulate, insterstitial liquid solidifies in place, without exchanging ions with the larger Magma chamber.
Example, Palisades Olivine layerExample, Palisades Olivine layer
Poikilitic: a mineral contains randomly oriented crystals of another mineral. Poikilitic: a mineral contains randomly oriented crystals of another mineral.
““In this photomicrograph, euhedral to subhedral biotite and In this photomicrograph, euhedral to subhedral biotite and plagioclase crystals are surrounded by optically-continuous, gray-plagioclase crystals are surrounded by optically-continuous, gray-colored K-feldspar.”colored K-feldspar.”
Poikilitic TexturePoikilitic TextureLow int.color, 2 cleavages ~ 90Low int.color, 2 cleavages ~ 90oo, so Fledspar., so Fledspar.Note no Albite twins, so K-spar. No Tartan twins so Note no Albite twins, so K-spar. No Tartan twins so probably Sanadine or Orthoclaseprobably Sanadine or Orthoclase
If water infiltrates at moderate If water infiltrates at moderate temperatures, pyroxenes are temperatures, pyroxenes are altered to amphiboles, and altered to amphiboles, and Biotite to Chlorite Biotite to Chlorite
Sericites and Sericitic TextureSericites and Sericitic Texture
The feldspars in this Alaskite (a low mafic granite) from The feldspars in this Alaskite (a low mafic granite) from the Boulder Batholith have been largely replaced by fine-the Boulder Batholith have been largely replaced by fine-grained Muscovite. This texture is called sericitic.grained Muscovite. This texture is called sericitic.
As Plagioclase replaces a K-spar , silica SiOAs Plagioclase replaces a K-spar , silica SiO22 is is
released . For example, Careleased . For example, Ca++++ Plagioclases Plagioclases contain less silica than the K-spar s, so the contain less silica than the K-spar s, so the reaction is K-spar = Plagioclase reaction is K-spar = Plagioclase ++ Quartz. Quartz.
Often the quartz all goes Often the quartz all goes bright/extinct at the same time, bright/extinct at the same time, as if it is all the same crystal.as if it is all the same crystal.