geoigneous minerals and rocks.pdf

7/30/2019 GeoIgneous Minerals and Rocks.pdf

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Bowens Reaction Series


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Relief

BirefringencePleochroism

CleavageFracturing

Twinning

Zoning

Phaneritic

Aphanitic

Intrusive (Plutonic)

Extrusive (Volcanic)


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Olivine

Generally found in:Gabbro (intrusive rock)Basalt (extrusive rock)

High ReliefHigh Birefringence

Fracturing


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An intrusive, mafic rock composed of

more than 90% Olivine is called a Dunite.

Mineralogy: A dunite has >90% Olivine

Mineral

Diagnostics: Fracturing of crystalsVery bright colorsHigh Relief under plain polarized light


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Pyroxene

Generally found in:

Mafic and intermediate rocks(Gabbros/basalts to andesites)

Medium ReliefHigh Birefringence

90o-90o Cleavage


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Biotite

Amphibole

PPLXPL

PPLXPL

Amphibole Medium ReliefMed-high Birefringence120o-60o Cleavage

Brown/green in plain polarized light

Generally found in:

Intermediate to Felsic rocks:Diorites/andesites/dacites to Granites.


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BiotiteWhat are these?

PPL

XPL

PPL

Generally found in:Intermediate to Felsic rocks(Diorite/dacite to granite)

Brown to green under plain polarized light (PPL)Dominant cleavage plainBladed to wispy shapeBirds eye or Tree barkextinction


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Polysynthetic Twinningin Plagioclase Feldspar

Generally found in:

Mafic to Felsic rocks(Gabbro/basalt to Granite)

Low reliefColorless in PPL

Best identified by twinningand low-order (greys) colors


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Tartan and CarlsbadTwinning in Alkali Feldspar

Generally found in:Felsic rocks (Granites)

Low reliefColorless in PPLBest identified by twinning

and low-order (greys) colors


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Alkali Feldspar does not have to haveTartan and Carlsbad Twinning

Feldspars also often show zoning.The zones shown in this plagioclasefeldspar indicate that the chemistry ofthe magma was changing as thecrystal grew.


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Muscovite

PPL

Biotite

Muscovite

XPL

Generally found in:

Felsic rocks (Granites)Med-Low reliefColorless to mild pleochroic in PPLBladed to wispy shapeBirds eye or Tree barkextinction

--like biotite, but pinky-green colors


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Quartz

Generally found in:

Felsic rocks (Granites)Low reliefColorless in PPLConchoidal fracturingUndulatory extinction


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Intrusive Mafic Rock: Gabbro

Minerals: Pyroxene, Plagioclase Minerals: Olivine, Plagioclase

Gabbros have more than 35% ferromagnesian minerals, and plagioclasecomposition in greater than 50% Ca in crystal lattice site.

Gabbro is the intrusive equivalent of basalt.


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Basalt: Volcanic (Extrusive) Mafic Rock

Pyroxenes and/orOlivines

Plagioclase Feldspars (High in Ca)


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Intrusive Intermediate Rock: Diorite

PPL

XPL

amphibole

amphibole

Plagioclase feldspars

Diorites contain less than 35% ferromagnesianminerals; 30-50% Ca in plagioclase composition.Diorites are common in batholiths associatedwith subduction zones. They are the intrusiveequivalents of andesites.


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Andesites: Intermediate extrusive (volcanic) rocks

Andesites consist of intermediate composition plagioclase (typically,An30-50), and less than 35% of ferromagnesian minerals such aspyroxene, hornblende, or olivine.

Andesites are most abundant in subduction-related arc volcanoes.

The photographs above (crossed polarizers on left, plane polarizedlight on right) show zoned and twinned plagioclase crystals in a matrix

of glass and plagioclase microcrystals.

Granites: High-silica (felsic) intrusive rocks


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Granites: High silica (felsic) intrusive rocks

Quartz

Feldspars

Amphibole

K Feldspar

Biotite

Quartz

Granites are abundant in continental shield areas and in deeply eroded foldedmountain belts. They are the intrusive equivalent of rhyolites.


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Granites!!!!


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Extrusive (volcanic) Felsic Rock: Rhyolite

K Feldspar (Sanidine)

flow banded matrix

PPL

Rhyolites are quartz-rich (silica rich) volcanic rocks with abundant alkali feldspar(typicallysanidine) and lesser plagioclase.Rhyolites are a volumetrically minor component of some volcanic arcsand occur as vast ash flows in some areas affected by tensional tectonics.

geoigneous minerals and rocks.pdf

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