Contents Image field widths

Quartz

Plagioclase

Potassium feldspars

Myrmeckite

Micas

Amphiboles

Pyroxenes

Other rock-forming primary minerals

Accessory minerals

Secondary (subsolidus) minerals

20x = 6 mm

40x = 3 mm

100x = 1 mm

200x = 0.5 mm

400x = 0.25 mm

1000x = 0.1 mm

 


Quartz crystals in alkali granite. Quartz is typically the clearest mineral in rocks, because it is not very succeptible to alteration to fine-grained minerals, and it has no cleavages.

 

Plane polarized light, 20x

 

NEIGC86-B2-7



Quartz crystals in alkali granite. Birefringence in the low first order.

 

Cross polarized light, 20x

 

NEIGC86-B2-7



Strained quartz crystal in a metaluminous granite. Strain has caused the quartz crystal to deform into domains with slightly different extinction angles. Typical for quartz, not fle feldspars.

 

Cross polarized light, 20x

 

DIG-D



Fluid inclusions in quartz in alkali granite. The inclusion in the center has an irregular outer boundary, inside of which is a layer of liquid water, a layer of liquid CO2, and a central bubble of vapor (mostly CO2). At the time of trapping of this secondary inclusion, the fluid was a homogeneous H2O-CO2 fluid.

 

Plane polarized light, 1000x

 

NEIGC86-B2-7



Fluid inclusions in quartz in alkali granite. Several inclusions containing (probably) water and a vapor bubble.

 

Plane polarized light, 1000x

 

NEIGC86-B2-7


 


Plagioclase, unzoned, in a hornblende diorite. Ateration to fine-grained products typically follows fractures, twin planes, or cleavages. In the solid solution series, the plagioclase refractive index varies from slightly lower than quartz to somewhat above it.

 

Plane polarized light, 40x

 

NEIGC84-A5-5C



Plagioclase, unzoned, in a hornblende diorite. Note the strong, parallel sets of albite twins, and the less visible set of pericline twins inclined almost at right angles to the albite twins.

 

Plane polarized light, 40x

 

NEIGC84-A5-5C



Plagioclase, zoned, in a dacite porphyry. This plagioclase appears quite homogeneous in plane light, without concentric zones of inclusions that are commonly seen in other grains.

 

Plane polarized light, 100x

 

Rhyolite-2



Plagioclase, zoned, in a dacite porphyry. This plagioclase has fine oscillatory zoning, in which the composition varies between more and less anorthite-rich compositions. Internal unconformities followed by euhedral overgrowths are also visible.

 

Cross polarized light, 100x

 

Rhyolite-2



Plagioclase, zoned, in a dacite porphyry. Notice the concentric layers (zones) of inclusions in this crystal. These probably formed during faster crystal growth than the clear zones.

 

Plane polarized light, 40x

 

Rhyolite-2



Plagioclase, zoned, in a dacite porphyry. Notice that the concentric inclusion zones also have different birefringence, indicating these zones have different anorthite content. The interior of this crystal has patchy zoning rather than concentric, indicating skeletal early growth. Albite, carlsbad, and pericline twins cut across the composition zones.

 

Cross polarized light, 40x

 

Rhyolite-2


 


Orthoclase in a dacite hypobyssal intrusive. In plane-polarized light K-feldspars may be difficult to tell from plagioclase, though they have refractive indexes lightly lower than even albite. Hint: to highlight K-feldspar, switch of a medium-low magnification objective (4-10X, depending on the optics), mostly close the substage iris, and raise the stage slightly (yes, raise). Though the image will be somewhat out of focus, the low index K-feldspar grains will be surrounded by bright Becke lines just inside the grain boundaries. This is especially handy in tonalites, in which K-feldspar may be hard to find by other methods.

 

Plane polarized light, 40x

 

Py-28



Orthoclase in a dacite hypobyssal intrusive. Notice the two carlsbad twin domains, separated by the carlsbad composition surface. The lack of polysynthetic twinning and lower refractive index than

 

Cross polarized light, 40x

 

Py-28



Microcline from a peraluminous granite. Characteristally featureless, like orthoclase, but the inverse Becke line highlighting technique (see orthoclase, above) works just as well.

 

Plane polarized light, 20x

 

Kinsman



Microcline from a peraluminous granite. Note the "grid" or "tartan plaid" twinning pattern that results from crossing albite and pericline twin domains. These are formed during the change from monoclinic high temperature orthoclase to triclinic low temperature microcline, associated with progressive ordering of aluminum and silicon during cooling. Albite and pericline twins are impossible in monoclinic orthoclase and sanidine, and develop like this only during inversion to triclinic microcline. The triclinic twin domains can nucleate with the triclinic angles leaning one way or another, but once they start that is how the twin grows. Because many twin domains nucleate, there are large numbers of thin, spindly and discontinuous domains in the final microcline product. Because the grid twinning is an inversion texture, it shows that the original feldspar was once monoclinic. Microcline crystals that grow below the inversion temperature do not have grid twinning like this.

 

Cross polarized light, 20x

 

Kinsman



Microcline from a peraluminous granite. Closeup of the plaid twinning. Notice how the twin domains are spindly and somewhat wispy. This is in contrast to the straight and generally continuous twin domains in plagioclase. Note the different spacing of the twin domains. This shows that different volumes of the crystal nucleated different numbers of twin domains during inversion.

 

Cross polarized light, 100x

 

Kinsman



Perthite from a metaluminous biotite granite. Note the faint, irregular stripes that run from the upper right to lower left. In this case, the exsolved albite is less altered (clearer) than the adjacent microcline, which is grayish here because of lots of minute alteration minerals.

 

Plane polarized light, 20x

 

4.7.84H



Perthite from a metaluminous biotite granite. Note the lighter irregular stripes and patches of bright yellowish-white albite between dimmer gray stripes and patches of microcline. The color difference is mostly caused by the different optical orientations of the two different minerals. The microcline and albite both exsolved (unmixed) from an originally homogeneous high temperature of intermediate composition. In this photo the thin section was rotated to obscure the twinning.

 

Cross polarized light, 20x

 

4.7.84H



Perthite from a metaluminous biotite granite. The somewhat less altered and narrower albite exsolution lamellae are in sharp contact with a much larger microcline domains. You can see a lamella closeup here, and then see a Becke line test from focused to a lowered stage position at the lamella-microcline contact. The Becke line goes into the higher index phase, albite.

 

Plane polarized light, 100x

 

4.7.84H



Perthite from a metaluminous biotite granite. Same as the image above in cross polarized light.

 

Cross polarized light, 100x

 

4.7.84H



Perthite from a metaluminous biotite granite. Closeup showing the characteristic grid twinning in the microcline host (upper center and upper left) and albite twinning in the lamellae (lower center to center right).

 

Cross polarized light, 200x

 

4.7.84H


 


Myrmekite patch that appears to be replacing microcline, though its appearance is obscure in plane-polarized light.

 

Plane polarized light, 40x

 

Kinsman



Myrmekite patch that appears to be replacing microcline. Twins in the myrmekite clearly show that the quartz "worms" are in a plagioclase matrix. Myrmekite is a subsolidus reaction texture that generally results from fluid flow. Here, K-feldpar was removed and quartz and plagioclase deposited in its place.

 

Cross polarized light, 40x

 

Kinsman


 


Muscovite, peraluminous granite. Igneous muscovite is generally colorless with good cleavage. Pale brown radiation halos can sometimes be visible around radioactive inclusions (but not really visible here).

 

Plane polarized light, 40x

 

NEIGC84-A5-6



Muscovite, peraluminous granite.

 

Cross polarized light, 40x

 

NEIGC84-A5-6



Biotite, metaluminous granite, showing several grains in different orientations. Grain on the far right is oriented with the cleavages N-S, and so is almost opaque. The largest grain is inclined and is lighter in color. Grains with the cleavages E-W have the least absorption (not shown in this image).

 

Plane polarized light, 40x

 

4.7.84G



Biotite, metaluminous granite. This is the same area as the image above in cross polarized light. The birefringent colors of the biotite are muted because of the color of the biotite itself.

 

Cross polarized light, 40x

 

4.7.84G



Biotite, metaluminous granite, showing a closeup of one of the same biotite crystals above at extinction, occupying the entire center of the image. Damage produced in this soft mineral during thin section grinding causes speckles of light on the biotite, where the crystal lattice has been deformed. This means that biotite in standard thin sections rarely goes completely extinct. This is called "incomplete extinction" or sometimes "birds eye maple extinction".

 

Cross polarized light, 200x

 

4.7.84G



Biotite, peraluminous granite. This graphite- and garnet-bearing granite has high-Ti, low-Fe3+ red-brown biotite. This patch of biotite rimming garnet (mineral occupying the lower part of the image) shows a range of pleochroic colors caused by different crystal orientations.

 

Plane polarized light, 20x

 

Kinsman


 


Green hornblende in a diorite. Pleochroism in this hornblende ranges from light yellow-green to bluish-green to brownish-green.

 

Plane polarized light, 40x

 

NEIGC84-A5-5C



Green hornblende in a diorite. Birefringent colors are typically up to middle second order.

 

Cross polarized light, 40x

 

NEIGC84-A5-5C



Green hornblende in a diorite. The ~120° and ~60° cleavage intersections are clearly visible in this end section of a crystal.

 

Plane polarized light, 100x

 

NEIGC84-A5-5C



Brown hornblende, hornblende gabbro. The extensive dark and light brown areas are several hornblende crystals. Note the numerous inclusions of opaques and plagioclase.

 

Plane polarized light, 20x

 

4.8.84Q



Brown hornblende, hornblende gabbro, in the image above.

 

Cross polarized light, 20x

 

4.8.84Q


 


Enstatite (orthopyroxene, OPX) in norite. The large OPX in the center is oriented with its c crystallographic axis N-S.

 

Plane polarized light, 100x

 

NEIGC83-C1-14



Enstatite (orthopyroxene, OPX) in norite. The large N-S oriented enstatite grain near the center of the image (see image above) is extinct, in keeping with the orthorhombic symmetry of this mineral. Birefringence ranges to upper first order.

 

Cross polarized light, 100x

 

NEIGC83-C1-14



Enstatite (orthopyroxene, OPX) in norite. Here the section shown above has been rotated clockwise ~45° to show the birefringence of the large grain.

 

Cross polarized light, 100x

 

NEIGC83-C1-14



Augite (clinopyroxene, CPX), gabbro. This augite is slightly brownish, and has tiny exsolved rods and plates of Fe-Ti oxide. These form the darkish, cloudy regions, especially in the grain to the lower right.

 

Plane polarized light, 20x

 

4.8.84A



Augite (clinopyroxene, CPX), gabbro. This augite has birefringence up to second order blue. The birefringence in each grain tends to be somewhat irregular, because of compositional variations.

 

Cross polarized light, 20x

 

4.8.84A



Aegirine (Na-Fe3+ monoclinic pyroxene), alkaline granite. This aegirine has a lot of inclusions in this shallowly emplaced, hypersolvus granite. Its colors are similar to hornblende, but it has negative elongation and approximately right-angle cleavage intersections like the all pyroxenes.

 

Plane-polarized light, 40x

 

4.8.84F



Aegirine (clinopyroxene, CPX), alkaline granite. This aegirine has a lot of inclusions in this shallowly emplaced, hypersolvus granite. Its colors are similar to hornblende, but it has a smaller extinction angle and has negative elongation. Aegirine birefringence tends to be up to upper second to third order, higher than most other clinopyroxenes.

 

Cross-polarized light, 40x

 

4.8.84F


 


Olivine, phenocryst in an Iceland basalt. Notice the fractures concentric with the crystal margin. Smaller "microphenocrysts" of olivine and plagioclase also occur, and olivine and brownish augite also occur in the matrix.

 

Plane polarized light, 4x

 

I-1



Olivine, phenocryst in an Iceland basalt. Smaller "microphenocrysts" of olivine have birefringence generally in the second and third order.

 

Cross polarized light, 4x

 

I-1



Garnet in a peraluminous granite. The high refractive index of this mineral cause fractures and the grain margin to stand out as dark lines because of total internal reflection.

 

Plane polarized light, 40x

 

NHM-9



Garnet in a peraluminous granite. The lack of birefringence distinguishes garnet from all other common high refractive index minerals. The bright cracks have thin films of calcite.

 

Cross polarized light, 40x

 

NHM-9



Cordierite in a peraluminous granite. Cordierite looks much like quartz and feldspar, and can be twinned or untwinned. It has three distinguishing characteristics. First, it tends to look dustier than quartz and feldspar; little black specks all over the surface, such as can be seen here. Second, it commonly alters to brownish or orange material (top right), or to an intergrowth of Mg-rich chlorite and muscovite (most of the right margin to lower left). Third, radiation halos are yellow and pleochroic, unless the cordierite is very close to the Mg end member.

 

Plane polarized light, 40x

 

NHM-9



Cordierite in a peraluminous granite. Interference colors are first order gray to white, like quartz and feldspar. It is more commonly in euhedral crystals than is quartz in plutonic rocks. Polysynthetic twinning can be seen in some cases. The muscovite alteration products are easily visible here, but the Mg-rich chlorite is not so visible because of its low birefringence.

 

Cross polarized light, 40x

 

NHM-9



Cordierite in a peraluminous granite. This shows three radioactive inclusions with their yellow pleochroic halos, caused by alpha particle radiation damage.

 

Plane polarized light, 100x

 

NHM-9



Epidote in a calc-alkaline granodiorite. In general, epidote in igneous rocks has rather high Fe3+ content, and is therefore pistacite. It may be pale yellow-green, but more commonly it is essentially colorless. It has high relief and is commonly associated with other Ca-rich minerals such as hornblende, plagioclase, and titanite, and it is more commonly zoned than most coexisting minerals.

 

Plane polarized light, 100x

 

UN30A



Epidote in a calc-alkaline granodiorite. The higher the Fe3+ content, the higher the birefringence and the darker the yellow-green color. Typically, the birefringence is 2nd and 3rd order and so epidote, with its high relief and common lack of color, can resemble olivine. Epidote, however, commonly has simple twinning, can have zoning in birefringence and extinction angle, and can occur with quartz and various hydrous minerals like chlorite and sericite, with which olivine is likely to have altered to serpentine.

 

Cross polarized light, 100x

 

UN30A



Nepheline in a nepheline syenite. It is distinguished from alkali feldspar by lack of perthitic intergrowths, from plagioclase by lack of polysynthetic twinning, from both by parallel extinction on cleavage and uniaxial negative interference figure, from apatite by its much lower relief, and from quartz by its cleavage, common alteration, and uniaxial negative interference figure.

 

Plane-polarized light, 40x

 

RH-1



Nepheline in a nepheline syenite. Its birefringence is also somewhat lower than feldspars and quartz.

 

Cross-polarized light, 40x

 

RH-1



Sodalite in a nepheline syenite. This isotropic mineral has much lower relief than both garnet and fluorite. Its refractive index is closer to alkali feldspars and to the mounting medium than either.

 

Plane-polarized light, 40x

 

RH-1



Sodalite in a nepheline syenite. Healed fractures are highlighted by minute birefringent grains.

 

Cross-polarized light, 40x

 

RH-1


 


Apatite crystals in norite. This view has several apatite crystals mostly arranged in plagioclase with surrounding OPX. Apatite is colorless, commonly elongate, and typically has hexagonal end sections. Its relief is less than the pyroxenes but higher than any feldspar.

 

Plane polarized light, 100x

 

NEIGC83-C1-14



Apatite crystal in a norite. The very low 1st order birefringence is obvious. Apatite has negative sign of elongation. Apatite is extremely common in small amounts in igneous rocks. Indeed, it is rare to find a terrestrial plutonic rock without at least some apatite.

 

Cross polarized light, 100x

 

NEIGC83-C1-14



Fluorite in a metaluminous granite. The fluorite here is primary, as it occurs as euhedral and subhedral single crystals in a relatively fine-grained matrix of a pluton chilled margin. It has high negative relief, its refractive index being considerably lower than adjacent quartz and feldspar, and the mounting medium. Sodalite has much less pronounced relief and poorer cleavage.

 

Plane polarized light, 100x

 

NEIGC86-B2-3B



Fluorite in a metaluminous granite. Fluorite, of course, is isotropic.

 

Cross polarized light, 100x

 

NEIGC86-B2-3B

 



Titanite in a metaluminous granite. Titanite (formerly sphene) has a typical double wedge or diamond shape, is typically light brown, and has very high relief, higher than the pyroxenes or garnet.

 

Plane polarized light, 100x

 

DIG-D



Titanite in a metaluminous granite. Titanite birefringence is very high, making it difficult to determine interference color order from the high-order pastel interference colors normally seen.

 

Cross polarized light, 100x

 

DIG-D



Titanite in a metaluminous granite. At high magnification, the magenta bands can be counted up a thin edge to get a better idea of the interference order. This example is approximately 6th order.

 

Cross polarized light, 200x

 

DIG-D



Zircon in a metaluminous granite. This is a rather blocky zircon fully enclosed within biotite. Small apatite crystals surround it. Zircon has relief considerably higher than garnet, pyroxenes, or titanite. The uranium and thorium content of zircon causes development of pleochroic radiation halos around it.

 

Plane polarized light, 200x

 

DIG-D



Zircon in a metaluminous granite. Birefringence is typically in the 3rd order. This example has birefringence zoning that is probably caused originally by igneous compositional zoning. High uranium layers accumulate more radiation damage and become less birefringent. A metamict state is basically a glass induced by extreme radiation damage, in which the crystalline structure has been destroyed.

 

Plane polarized light, 200x

 

DIG-D


Zircon compared to monazite.

Zircon oriented N-S. Zircon is usually colorless in thin section.

Zircon compared to monazite.

Monazite oriented N-S. Monazite can be very pale yellow-green in thin section.

Zircon compared to monazite.

In the same orientation zircon is extinct, and so it has parallel extinction.

Zircon compared to monazite.

In the same orientation Monazite is not extinct, and so it must have inclined extinction. Xenotime is tetragonal, like zircon, so it can't be distinguished so easily.


Allanite in a metaluminous granite. This allanite has concentric zoning probably resulting from changing mineral composition during successive stages of growth. Allanite contains substantial amounts of Th and U, and sustains considerable radiation damage over time. Swelling of the crystal as damage accumulates, and absorption of water, causes the grain to form many radial cracks extending out into the surrounding minerals.

 

Plane polarized light, 100x

 

4.7.84G



Allanite in a metaluminous granite. The allanite grain mostly has low birefringence, a result of radiation damage that essentially turns the crystal lattice into a glass.

 

Cross polarized light, 100x

 

4.7.84G


 


Chlorite replacing biotite in a muscovite granite. Small residual brownish patches of biotite still occur, as do dark bits of rutile or titanite. Biotite can hold several weight percent TiO2 in solid solution, but secondary chlorite can accommodate <<1%.

 

Plane polarized light, 40x

 

NEIGC84-A5-6



Chlorite replacing biotite in a muscovite granite. The low first order, anomalous Berlin blue interference color indicates that this is an Fe-rich chlorite. Residual biotite patches have higher birefringence.

 

Cross polarized light, 40x

 

NEIGC84-A5-6



Chlorite replacing biotite in a metaluminous granite. The green chlorite has only replaced the biotite on the top left of side of the grain. Lenses of high-index colorless material (titanite?) occur in the chlorite and near the contact region between the two sheet silicates.

 

Plane polarized light, 100x

 

DIG-D



Chlorite replacing biotite in a metaluminous granite. The chlorite mostly has an anomalous brown low first order interference color, indicative of Mg-rich chlorite. The purple regions are more Fe-rich. The adjacent biotite has third order birefringence.

 

Cross polarized light, 100x

 

DIG-D



Sericite replacing plagioclase in a metaluminous granite. Sericite is grungy-looking fine-grained stuff that commonly replaces feldspars. Close examination of sericite reveals that at least some of it is fine-grained white mica.

 

Plane polarized light, 40x

 

UN30A



Sericite replacing plagioclase in a metaluminous granite. Because it is generally made up of very small crystals, its birefringence is irregular and generally low. Some of the larger crystals can be seen here to have first order birefringence. Albite twinning in the plagioclase is clearly visible.

 

Cross polarized light, 40x

 

UN30A



Sericite replacing plagioclase in a metaluminous granite. At higher magnification, some of the grungy-looking clearly resolves into little, colorless, platy crystals.

 

Plane polarized light, 100x

 

UN30A



Sericite replacing plagioclase in a metaluminous granite. The larger crystals have up to middle first order birefringence. They have a positive sign of elongation and are probably small white micas that grew during subsolidus hydrothermal alteration.

 

Plane polarized light, 100x

 

UN30A



Serpentine in an altered harzbergite nodule in a kimberlite dike from Pennsylvania. Much of this sample is made up of calcite (colorless) and stained talc (darker browns). The serpentine is the yellowish material that occupies the thin veins.

 

Plane polarized light, 4x.

 

Fayette Co. Penn. 33



Serpentine in an altered harzbergite nodule in a kimberlite dike from Pennsylvania. The calcite has very high birefringence, and the talc has irregular second and third order interference colors that are modified by the brown staining. The serpentine is most easily seen in the veins, and veins have lower first order birefringence and fibers perpendicular to the vein walls.

 

Cross polarized light, 4x

 

Fayette Co. Penn. 3



Calcite in an alkaline granite. In this image, the calcite grain to the upper left is oriented so that a N-S line bisects the obtuse angle between the cleavages. This means the c axis of the calcite is approximately N-S, so the N-S polarized light is parallel to the low calcite refractive index. The grain has rather low relief.

 

Plane polarized light, 100x

 

NEIGC86-B2-7



Calcite in an alkaline granite. Here, the calcite grain above has been rotated so that a N-S line bisects the acute angle between the cleavages. This means the c axis of the calcite is approximately E-W, and the N-S polarized light is now parallel to the high calcite refractive index. The grain on the left now has high relief. Calcite and dolomite are the only two common minerals that can vary noticeibly from low to high relief in thin section.

 

Plane polarized light, 100x

 

NEIGC86-B2-7



Calcite in an alkaline granite. Calcite has very high birefringence and interference order is difficult to judge from the high-order pastel colors. It is more reliable to look at a thin edge and count the number of magenta bands. The calcite crystal in the lower center of this image has ~8th order birefringence.

 

Cross polarized light, 200x

 

NEIGC86-B2-7



Calcite in an alkaline granite. Calcite is quite soft and undergoes substantial surface deformation during normal thin section grinding. The distorted crystal surface, combined with its high birefringence, results in incomplete, speckled extinction.

 

Cross polarized light, 200x

 

NEIGC86-B2-7