Sheet Silicates

• Micas

• Chlorite

serpentine Mg3Si2O5(OH)4

A B

C D

The term serpentine refers to three polymorphs (lizardite, antigorite, crysotile) that may occur together and cannot be easily distinguished in thin section.  Serpentine primarily forms by hydrous alteration of olivine, and in some cases pyroxene, and many crustal peridotites are extensively altered to serpentine.  It is typically colourless to pale yellow or pale green in plane light, and is commonly intergrown with opaque magnetite (A).  The very low maximum interference colours of first order white to yellow are evident in B.  In C, the original outlines and internal fracture pattern of olivine grains are highlighted by opaque magnetite.  (Coarse subhedral opaque minerals are primary Cr-spinels grains.)  In D, more highly birefringent calcite and tremolite accompany the serpentine. Serpentine can be confused with pale-coloured varieties of chlorite (Mg-rich), but in serpentine the slow ray is parallel to the fiber length whereas chlorites with Mg/(Mg+Fe) < 0.50 are length fast.  A and B are from the Hunting Hill, Maryland serpentinite and C and D are from a serpentinized peridotite cumulate from the Abitibi Belt, Ontario.  A and B are 2.2 mm across and C and D are 5.5 mm across.  A and C ppl, B and D x-nicols. 

Perfect micaceous cleavage

Moderate relief

May be dark pleochroic halos around inclusions of zircon or other mildly radioactive minerals (B)

trioctahedralTOT layer + interlayer cationK (Mg,Fe)3(AlSi3O10)(OH)2

MonoclinicSpace group: C2/m

Pleochroism: Orange, brown or dark green(A)

Perfect cleavageDark halosDark colourStrong pleochroism

High refractive indices

Strong pleochroism:From dark brown to light brown

mottled texture

Plane-Polarized Light•Low relief •Clear •Perfect micaceous cleavage (closely spaced cleavage in one (!) direction

Crossed Polarizers•Bright second-and third-order interference colors• Mottled texture

dioctahedral

TOT layer + interlayer cation

KAl2(AlSi3O10)(OH)2

Space group: C2/c

Igneous muscovite is generally colorless with good cleavage.

Distinguishing features:Lighter than biotiteHigher 2Vx than other micasHigher birefringence than clays and chlorite

High birefringence(upper second order) Mottled texture

Sericite: Not really aSeparate species, butmuscovite with lowbirefringence. Probably K+ replacedby H3O+; i.e. it ishydromuscovite

Muscovite foliation

talc Mg3Si4O10(OH)2

A B

C D

Talc is colourless in plane polarized light, and its high birefringence causes its relief to change with rotation of the stage (compare A and B which are rotated 90 degrees relative to one another), a feature that can be mistaken for faint pleochroism.  Maximum interference colours are in the third order (C).  Extinction is parallel to cleavage traces and, like the micas, the grains have a mottled or “crinkly” appearance at and near extinction (D).  Talc and muscovite appear very similar in thin section and usually cannot be reliably distinguished based on optical characteristics alone.  Paragenesis, or mineral assemblage, can be a useful guide.  Although muscovite occurs in a wide variety of igneous and metamorphic rocks, talc is rather restricted in occurrence, typically to high magnesium rocks (mafic or ultramafic) in which is it associated with tremolite, dolomite, serpentine, magnesite and other Mg-rich minerals.  Sample is from a talc-tremolite schist from St. Lawrence County, New York.  All views are 2.2 mm across. A and B ppl, C and D x-nicols.

prehnite Ca2Al(AlSi3O10)(OH)2

A B

CD

D

Prehnite occurs in amygdules and veins in low-grade meta-basic rocks (e.g., basalts) but is more common as a constituent of low-grade meta-greywackes and meta-basic rocks where it is typically too fine-grained for identification by optical techniques.  It is colourless in plane light (A) and has moderately high positive relief relative to quartz and sodic plagioclase, with which it is commonly associated.  In image A prehnite dominates the amygdule and stands out relative to minor interstitial quartz and a quartz core.  In amygdules and veins it commonly forms radiating sheaves, apparent in crossed nicol views B and D.  Maximum interference colours range to upper second order and extinction is parallel, or nearly so, to the length of the plates in the bundles.  All views are 5.5 mm across.  A, C ppl, B, D x-nicols.

Chlorite, trioctahedral - Mg3Si4O10(OH)2.Mg3(OH)6

Plane-Polarized LightLow relief, colorless to green. Light green is most common shade. Micaceous cleavageCrossed PolarizersLow birefringence results in first-order whites. Anomalous interference colors very common (A). Dark blue, brown, purple and green are possible. Fine-grained varieties often show undulose extinction.

A. alteration product of ferromagnesian minerals

B. low-grade metamorphic rocks

General occurrence

low first order, anomalous Berlin blue interference color = Fe-rich chlorite

Chlorite after biotite Small residual brownish patches of biotite still occur

A. alteration product

B. low-grade metamorphic rocks