Ch. 3: TexturesCh. 3: TexturesSome Useful DefinitionsSome Useful Definitions

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

ObservationsObservations Simple structures nucleate more easilySimple structures nucleate more easily Oxides easier > Olivines > Pyrox. > Plag. > K-sparsOxides easier > Olivines > Pyrox. > Plag. > K-spars

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

prevalentprevalent

Igneous TexturesIgneous Textures

Figure 3-3. a.Figure 3-3. a. Volume of liquid Volume of liquid (green) available to a corner or (green) available to a corner or edge of a crystal is greater than edge of a crystal is greater than for a side. for a side. b.b. Volume of liquid Volume of liquid available to the narrow end of a available to the narrow end of a slender crystal is even greater. slender crystal is even greater. After Shelley (1993).After Shelley (1993). Igneous Igneous and Metamorphic Rocks Under and Metamorphic Rocks Under the Microscope. © Chapman the Microscope. © Chapman and Hall. London.and Hall. London.

ba

A mineral will deplete the adjacent melt of its constituentsA mineral will deplete the adjacent melt of its constituents

Expect the narrow ends growth > corner > edge > sideExpect the narrow ends growth > corner > edge > sideAnd erosion the oppositeAnd erosion the opposite

Igneous TexturesIgneous Textures

Figure 3-4. a.Figure 3-4. a. SkeletalSkeletal olivine phenocryst with rapid growth at corner enveloping melt olivine phenocryst with rapid growth at corner enveloping melt at sides. Taupo, N.Z. From Shelley (1993).at sides. Taupo, N.Z. From Shelley (1993). Igneous and Metamorphic Rocks Under Igneous and Metamorphic Rocks Under the Microscope. © Chapman and Hall. London.the Microscope. © Chapman and Hall. London.

EpitaxisEpitaxis

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

1900 1900 ooCC

1553 1553 ooCC

3-D3-D

sheetssheets

CompositionalCompositionalZoningZoning

Figure 3-5. a. Figure 3-5. a. Compositionally Compositionally zonedzoned hornblende phenocryst with hornblende phenocryst with pronounced color variation visible pronounced color variation visible in plane-polarized light. Field in plane-polarized light. Field width 1 mm. width 1 mm. b.b. Zoned plagioclase Zoned plagioclase twinned on the Carlsbad law. twinned on the Carlsbad law. (twins 180 on b) Andesite, Crater (twins 180 on b) Andesite, Crater Lake, OR. Field width 0.3 mm.Lake, OR. Field width 0.3 mm. © © John Winter and Prentice Hall.John Winter and Prentice Hall.

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

Anorthite CaAlAnorthite CaAl22SiSi22OO88

Albite NaAlSiAlbite NaAlSi33OO88

NaSi (AlSiNaSi (AlSi22OO88 ) )

CaAl (AlSiCaAl (AlSi22OO88 ) )

Figure 3-6.Figure 3-6. Examples of plagioclase zoning profiles determined by microprobe point traverses. Examples of plagioclase zoning profiles determined by microprobe point traverses. a. a. Repeated sharp reversals attributed to magma Repeated sharp reversals attributed to magma mixing, followed by normal cooling increments. mixing, followed by normal cooling increments. b.b. Smaller and irregular oscillations caused by local disequilibrium crystallization. Smaller and irregular oscillations caused by local disequilibrium crystallization. c.c. Complex Complex oscillations due to combinations of magma mixing and local disequilibrium. From Shelley (1993).oscillations due to combinations of magma mixing and local disequilibrium. From Shelley (1993). Igneous and Metamorphic Rocks Under the Igneous and Metamorphic Rocks Under the Microscope. © Chapman and Hall. London.Microscope. © Chapman and Hall. London.

Ca++ decreases locally so less Anorthite, mixing restores Ca++ Ca++ decreases locally so less Anorthite, mixing restores Ca++

Oscillatory ZoningOscillatory Zoning

Figure 3-7. Figure 3-7. Euhedral early pyroxene with late Euhedral early pyroxene with late interstitialinterstitial plagioclase (horizontal twins). Stillwater plagioclase (horizontal twins). Stillwater complex, Montana. Field width 5 mm.complex, Montana. Field width 5 mm. © John Winter and Prentice Hall.© John Winter and Prentice Hall.

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

Figure 3-8. Figure 3-8. OphiticOphitic texture. A single Clinopyroxene envelops several well-developed texture. A single Clinopyroxene envelops several well-developed plagioclase laths. Width 1 mm. Skaergård intrusion, E. Greenland.plagioclase laths. Width 1 mm. Skaergård intrusion, E. Greenland. © John Winter and © John Winter and Prentice Hall. Discuss Diabase definition.Prentice Hall. Discuss Diabase definition.

Figure 3-9. a. Figure 3-9. a. GranophyricGranophyric quartz-alkali feldspar intergrowth at the margin of a 1-cm quartz-alkali feldspar intergrowth at the margin of a 1-cm dike. Golden Horn granite, WA. Width 1mm. dike. Golden Horn granite, WA. Width 1mm. b.b. GraphicGraphic texture: a single crystal of texture: a single crystal of cuneiform quartz (darker) intergrown with alkali feldspar (lighter). Laramie Range, WY. cuneiform quartz (darker) intergrown with alkali feldspar (lighter). Laramie Range, WY. © John Winter and Prentice Hall.© John Winter and Prentice Hall.

<= 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

Figure 3-10.Figure 3-10. Olivine Olivine mantledmantled by orthopyroxene in by orthopyroxene in ((aa)) plane-polarized light and plane-polarized light and ((bb) ) crossed nicols, in which olivine is extinct and the pyroxenes stand out clearly. Basaltic crossed nicols, in which olivine is extinct and the pyroxenes stand out clearly. Basaltic andesite, Mt. McLaughlin, Oregon. Width ~ 5 mm. andesite, Mt. McLaughlin, Oregon. Width ~ 5 mm. © John Winter and Prentice Hall.© John Winter and Prentice Hall.

Note the erosion of the Olivine. Note the erosion of the Olivine.

Is Olivine stable at theIs Olivine stable at theconditions suitable for Pyroxeneconditions suitable for Pyroxenecrystallization?crystallization?

Figure 3-11. b.Figure 3-11. b. ResorbedResorbed and and embayedembayed olivine phenocryst. Width 0.3 mm. olivine phenocryst. Width 0.3 mm. © John © John Winter and Prentice Hall.Winter and Prentice Hall.

Figure 3-11. c.Figure 3-11. c. Hornblende phenocryst Hornblende phenocryst dehydratingdehydrating to Fe-oxides plus pyroxene due to to Fe-oxides plus pyroxene due to pressure release upon eruption. Andesite, Crater Lake, OR. Width 1 mm. pressure release upon eruption. Andesite, Crater Lake, OR. Width 1 mm. © John Winter © John Winter and Prentice Hall.and Prentice Hall.

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.

Figure 3-12. a.Figure 3-12. a. TrachyticTrachytic texture in which texture in which micromicrophenocrysts phenocrysts of plagioclase are of plagioclase are alignedaligned due to flow. Note flow around due to flow. Note flow around phenocryst (P). Trachyte, Germany. Width phenocryst (P). Trachyte, Germany. Width 1 mm. From MacKenzie 1 mm. From MacKenzie et alet al. (1982). . (1982). © © John Winter and Prentice Hall.John Winter and Prentice Hall.

Figure 3-12. b.Figure 3-12. b. FeltyFelty or or pilotaxiticpilotaxitic texture texture in which the microin which the microphenocrysts phenocrysts are are randomly orientedrandomly oriented. Basaltic andesite, Mt. . Basaltic andesite, Mt. McLaughlin, OR. Width 7 mm. McLaughlin, OR. Width 7 mm. © John © John Winter and Prentice Hall.Winter and Prentice Hall.

Figure 3-13. Figure 3-13. Flow bandingFlow banding in andesite. Mt. in andesite. Mt. Rainier, WA. Rainier, WA. © John Winter and Prentice © John Winter and Prentice Hall.Hall.

Caused by mingling of two magmatic Caused by mingling of two magmatic fluidsfluids

Figure 3-15. Figure 3-15. IntergranularIntergranular texture in basalt. texture in basalt. Columbia River Basalt Group, Washington. Columbia River Basalt Group, Washington. Width 1 mm. Width 1 mm. © John Winter and Prentice © John Winter and Prentice Hall.Hall.

Intergranular Intergranular Plagioclase and Pyroxene Plagioclase and Pyroxene crystals are similar in size.crystals are similar in size.

Figure 3-14. Figure 3-14. Development of Development of cumulate texturescumulate textures. . a.a. Crystals accumulate by crystal settling or simply form in Crystals accumulate by crystal settling or simply form in place near the margins of the magma chamber. In this case plagioclase crystals (white) accumulate in mutual place near the margins of the magma chamber. In this case plagioclase crystals (white) accumulate in mutual contact, and an intercumulus liquid (pink) fills the interstices.contact, and an intercumulus liquid (pink) fills the interstices. b.b.  OrthocumulateOrthocumulate: intercumulus liquid : intercumulus liquid crystallizes to form additional plagioclase rims plus other phases in the interstitial volume (colored). There is crystallizes to form additional plagioclase rims plus other phases in the interstitial volume (colored). There is little or no exchange between the intercumulus liquid and the main chamber. After Wager and Brown (1967), little or no exchange between the intercumulus liquid and the main chamber. After Wager and Brown (1967), Layered Igneous RocksLayered Igneous Rocks. © Freeman. San Francisco. . © Freeman. San Francisco.

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

Orthocumulate TextureOrthocumulate Texture

Orthocumulate texture - Harzburgite with cumulate olivine Orthocumulate texture - Harzburgite with cumulate olivine and chromite and intercumulus plagioclase. Stillwater Layered and chromite and intercumulus plagioclase. Stillwater Layered Intrusion. Width = 8 mm. Image © T. E. Bunch, 2007.Intrusion. Width = 8 mm. Image © T. E. Bunch, 2007.

Figure 3-14. Figure 3-14. Development of Development of cumulate texturescumulate textures. . c.c. Adcumulates: open-system exchange between the Adcumulates: open-system exchange between the intercumulus liquid and the main chamber (plus compaction of the cumulate pile) allows components that intercumulus liquid and the main chamber (plus compaction of the cumulate pile) allows components that would otherwise create additional intercumulus minerals to escape, and plagioclase fills most of the available would otherwise create additional intercumulus minerals to escape, and plagioclase fills most of the available space. space. d.d. Heteradcumulate: intercumulus liquid crystallizes to additional plagioclase rims, plus other large Heteradcumulate: intercumulus liquid crystallizes to additional plagioclase rims, plus other large minerals (hatched and shaded) that nucleate poorly and poikilitically envelop the plagioclases. minerals (hatched and shaded) that nucleate poorly and poikilitically envelop the plagioclases. . After Wager . After Wager and Brown (1967), and Brown (1967), Layered Igneous RocksLayered Igneous Rocks. © Freeman. San Francisco. . © Freeman. San Francisco.

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

Figure 3-16.Figure 3-16. a.a. The interstitial liquid (red) between bubbles in pumice (left) become 3-pointed-star-shaped The interstitial liquid (red) between bubbles in pumice (left) become 3-pointed-star-shaped glass shards in ash containing pulverized pumice. If they are sufficiently warm (when pulverized or after glass shards in ash containing pulverized pumice. If they are sufficiently warm (when pulverized or after accumulation of the ash) the shards may deform and fold to contorted shapes, as seen on the right and accumulation of the ash) the shards may deform and fold to contorted shapes, as seen on the right and b.b. in the in the photomicrograph of the Rattlesnake ignimbrite, SE Oregon. Width 1 mm.photomicrograph of the Rattlesnake ignimbrite, SE Oregon. Width 1 mm. © John Winter.© John Winter.

Figure 3-18.Figure 3-18. a.a. Carlsbad twinCarlsbad twin in in orthoclase. Wispy perthitic exsolution orthoclase. Wispy perthitic exsolution is also evident. Granite, St. Cloud MN. is also evident. Granite, St. Cloud MN. Field widths ~1 mm. Field widths ~1 mm. © John Winter © John Winter and Prentice Hall.and Prentice Hall.

Figure 3-18.Figure 3-18. b.b. Very straight multiple Very straight multiple albite twinsalbite twins in plagioclase, set in felsitic in plagioclase, set in felsitic groundmass. Rhyolite, Chaffee, CO. Field groundmass. Rhyolite, Chaffee, CO. Field widths ~1 mm. widths ~1 mm. © John Winter and Prentice © John Winter and Prentice Hall.Hall.

Figure 3-18. Figure 3-18. ((c-dc-d)) Tartan twinsTartan twins in in microcline. Field widths ~1 mm. microcline. Field widths ~1 mm. © © John Winter and Prentice Hall.John Winter and Prentice Hall.

Figure 3-19.Figure 3-19. Polysynthetic Polysynthetic deformation twinsdeformation twins in plagioclase. Note how they concentrate in in plagioclase. Note how they concentrate in areas of deformation, such as at the maximum curvature of the bent cleavages, and taper away areas of deformation, such as at the maximum curvature of the bent cleavages, and taper away toward undeformed areas. Gabbro, Wollaston, Ontario. Width 1 mm. toward undeformed areas. Gabbro, Wollaston, Ontario. Width 1 mm. © John Winter and © John Winter and Prentice Hall.Prentice Hall.

Figure 3-20. a.Figure 3-20. a. Pyroxene largely Pyroxene largely replacedreplaced by hornblende. Some by hornblende. Some pyroxene remains as light areas (Pyx) pyroxene remains as light areas (Pyx) in the hornblende core. Width 1 mm. in the hornblende core. Width 1 mm. b.b. Chlorite (green) replaces biotite (dark Chlorite (green) replaces biotite (dark brown) at the rim and along cleavages. brown) at the rim and along cleavages. Tonalite. San Diego, CA. Width 0.3 Tonalite. San Diego, CA. Width 0.3 mm. mm. © John Winter and Prentice Hall.© John Winter and Prentice Hall.

Pyx

Hbl

BtChl

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.

Figure 3-21Figure 3-21.. MyrmekiteMyrmekite formed in plagioclase at the boundary with K-feldspar. Photographs courtesy © L. formed in plagioclase at the boundary with K-feldspar. Photographs courtesy © L. Collins.Collins. http://www.csun.edu/~vcgeo005 http://www.csun.edu/~vcgeo005

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.

MyrmekiticMyrmekitic