Mineral types

Quartz

Plagioclase

Potassium feldspars

Myrmeckite

Micas

Amphiboles

Pyroxenes

Other rock-forming primary minerals

Accessory minerals

Secondary (subsolidus) minerals


Many of the images have two views, most showing paired plane- and cross-polarized light images. Move the cursor over the visible image to see the other view.


Quartz


Quartz crystals in alkali granite. Quartz is typically the most transparent mineral in rocks, because it is not very succeptible to alteration to fine-grained minerals, and it has no cleavages. Birefringence in the low to middle first order.


Plane/cross-polarized light, field width is 6 mm.


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, but not feldspars.


Cross-polarized light, field width is 6 mm.


DIG-D


Fluid inclusions in quartz in alkali granite. The center inclusion 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 the fluid was trapped, the fluid was a homogeneous H2O-CO2 fluid.


Plane-polarized light, field width is 0.12 mm.


NEIGC86-B2-7


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


Plane-polarized light, field width is 0.12 mm.


NEIGC86-B2-7


Plagioclase


Plagioclase, unzoned, in a hornblende diorite. Ateration to fine-grained material typically occurs along fractures, twin planes, or cleavages. In the solid solution series, the plagioclase refractive index varies from slightly lower than quartz to somewhat above it. Albite twins are prominent, and some small pericline twins can also be seen.


Plane/cross-polarized light, field width is 3 mm.


NEIGC84-A5-5C


Plagioclase, zoned, in a dacite porphyry. This grain appears quite homogeneous in plane light, without concentric zones of inclusions that are commonly seen elsewhere. In cross-polarized light fine oscillatory zoning is visible, in which the composition varies back and forth between more and less anorthite-rich compositions. Internal unconformities followed by euhedral overgrowths are also visible.


Plane/cross-polarized light, field width is 3 mm.


Rhyolite-2


Plagioclase, zoned, in a dacite porphyry. Notice the concentric layers (zones) of inclusionsl. These probably formed during faster crystal growth than the clear zones. In cross-polarized light you can see that the zones also have different birefringence, indicating they 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 zones.


Plane/cross-polarized light, field width is 1.2 mm.


Rhyolite-2


Potassium feldspars


Orthoclase in a dacite hypobyssal intrusive. In plane-polarized light K-feldspars may be difficult to tell from plagioclase, though plagioclase has higher refractive indexes. Hint: to highlight K-feldspar, switch to a medium or low magnification objective (usually 4 or 10X works well), 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/cross-polarized light, field width is 3 mm.


Py-28


Microcline from a peraluminous granite. Characteristally featureless in plane light, but the inverse Becke line highlighting technique described for orthoclase works just as well for microcline. In cross-polarized light you can see the striking "grid" or "tartan plaid" twinning pattern that results from crossing albite and pericline twin domains. These form during cooling, as the grain changes from monoclinic orthoclase to triclinic microcline. This transformation is associated with progressive ordering of aluminum and silicon. 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 in both the albite and pericline twin directions. However, once they start, that is how the twin continues to grow. Because many twin domains nucleate throughout the crystal, there are large numbers of thin, spindly and discontinuous domains in the final microcline product.


Plane/cross-polarized light, field width is 6 mm.


Kinsman


Microcline from a peraluminous granite. Close-up of the grid twinning. Notice how the twin domains are spindly and somewhat wispy. This is in contrast to the straight, and generally continuous twins in plagioclase. Note the variable spacing of the twin domains, indicating that different volumes of the crystal nucleated different numbers of twin domains during inversion.


Cross-polarized light, field width is 1.2 mm.


Kinsman


Perthite from a metaluminous biotite granite. Perthite is an unmixing texture of an originally homogeneous feldspar grain. Microcline and albite both exsolved (unmixed) from the homogeneous solid solution during cooling. 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 adjacent microcline, which is grayish because of lots of minute alteration minerals. In cross-polarized light you can see the lighter, irregular stripes and patches of bright yellowish-white birefringence albite between gray microcline. The birefringent color difference is mostly caused by the different optical orientations of the two different minerals. In this photo the thin section was rotated to obscure twinning.


Plane//cross-polarized light, field width is 6 mm.


4.7.84H


Perthite from a metaluminous biotite granite. The somewhat less-altered and narrower albite exsolution lamellae are in sharp contact with larger microcline domains. A Becke line test tells you which phase is which: the Becke line goes into the higher index phase, albite.


Plane/cross-polarized light, field width is 1.2 mm.


4.7.84H


Perthite from a metaluminous biotite granite. Close-up 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, field width is 0.6 mm.


4.7.84H


Myrmeckite


Myrmekite patch that appears to be replacing microcline. Though its presence is obscure in plane-polarized light, it is obvious in cross-polarized light as quartz worms in plagioclase. The plagioclase is twinned. 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.


Plane/cross-polarized light, field width is 3 mm.


Kinsman


Micas


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). Birefringence is in the high first or second order.


Plane/cross-polarized light, field width is 3 mm.


NEIGC84-A5-6


Biotite, metaluminous granite, showing several grains in different orientations. The grain on the far right is oriented with cleavages N-S, and 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). In cross-polarized light the birefringent colors of the biotite are muted by the color of the biotite itself.


Plane/cross-polarized light, field width is 3 mm.


4.7.84G


Biotite, metaluminous granite, showing a close-up of one crystal. Damage produced during thin section grinding causes speckles of light in 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, field width is 6 mm.


4.7.84G


Biotite, peraluminous granite. The red-brown biotite in this graphite- and garnet-bearing granite has high titanium and low Fe3+ content. This patch of biotite shows a range of pleochroic colors caused by different crystal orientations.


Plane-polarized light, field width is 0.6 mm


Kinsman


Amphiboles


Green hornblende in a diorite. Pleochroism in this sample ranges from light-yellow-green to bluish-green to brownish-green. Birefringent colors are typically up to middle second order.


Plane/cross-polarized light, field width is 3 mm.


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, field width is 1.2 mm.


NEIGC84-A5-5C


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


Plane/cross-polarized light, field width is 6 mm.


4.8.84Q


Pyroxenes


Enstatite (orthopyroxene, OPX) in norite. The large OPX in the center is oriented with its c crystallographic axis N-S. In cross-polarized light that crystal is at extinction, in keeping with the orthorhombic symmetry of this mineral. Birefringence ranges to middle first order.


Plane/cross-polarized light, field width is 1.2 mm.


NEIGC83-C1-14


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


Cross-polarized light, field width is 1.2 mm.


NEIGC83-C1-14


Augite (clinopyroxene, CPX), in 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. Augite has birefringence up to second order blue. Birefringencetends to be somewhat irregular in single grains because of compositional variations.


Plane/cross-polarized light, field width is 6 mm.


4.8.84A


Aegirine (Na–Fe3+ monoclinic pyroxene), alkaline granite. These crystals have a lot of inclusions of quartz and feldspar. The pleochroic colors are similar to hornblende, but it has approximately right-angle cleavage intersections like the all pyroxenes. In cross-polarized light aegerine can be seen to have a smaller extinction angle than most hornblende, and negative elongation. Aegirine birefringence tends to be up to upper second to third order, higher than amphiboles and most other clinopyroxenes.


Plane/cross-polarized light, field width is 3 mm.


4.8.84F


Other rock-forming primary minerals


Olivine, phenocryst in an Iceland basalt. Notice the fractures concentric with the crystal margin. In cross-polarized light smaller "microphenocrysts" of brightly birefringent olivine and gray to white plagioclase can be seen. Augite occurs in the matrix as small, brownish crystals with brirefringence up to second order blue.


Plane/cross-polarized light, field width is 3 mm.


I-1


Garnet in a peraluminous granite. The high refractive index of garnet cause fractures and the grain margin to stand out as dark lines because of total internal reflection. The lack of birefringence distinguishes garnet from all other common high-index minerals. The bright cracks seen in cross-polarized light have thin films of calcite.


Plane/cross-polarized light, field width is 3 mm.


NHM-9


Cordierite in a peraluminous granite. In plane- and cross-polarized light cordierite looks much like quartz and feldspar, and it can be twinned or untwinned. It has three distinguishing characteristics. 1) It tends to look dustier than quartz and feldspar, little black specks all over the surface, such as can be seen here. 2) 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). 3) Radiation halos are yellow and pleochroic, unless the it is very Mg-rich. Interference colors are first order gray to white, like quartz and feldspar. It is more commonly euhedral than quartz in plutonic rocks. The muscovite alteration products are easily visible here, but the Mg-rich chlorite is not so visible because of its low birefringence.


Plane/cross-polarized light, field width is 3 mm.


NHM-9


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


Plane-polarized light, field width is 1.2 mm.


NHM-9


Epidote in a calc-alkaline granodiorite. In general, epidote in igneous rocks has rather high Fe3+ content, which may color it pale yellow-green. Birefringence and color may define zoning, and both increase with Fe3+ content. The birefringence is usually 2nd and 3rd order and so epidote, with its high relief and common lack of color, can resemble olivine. It is commonly associated with other Ca-rich minerals like hornblende, plagioclase, and titanite. Epidote may have simple twinning, and can occur with quartz and various hydrous minerals like chlorite and sericite. Olivine is usually not associated with quartz (except in highly Fe-rich Mg-poor rocks), and is likely to be partly altered.


Plane/cross-polarized light, field width is 1.2 mm.


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. Its birefringence is also somewhat lower than feldspars and quartz.


Plane/cross-polarized light, field width is 3 mm.


RH-1


Sodalite in a nepheline syenite. This isotropic mineral has much lower relief than both garnet and fluorite, and its refractive index is closer to alkali feldspars and epoxy than either. In this view, healed fractures are highlighted by minute birefringent grains.


Plane/cross-polarized light, field width is 3 mm.


RH-1


Accessory minerals


Opaques inside olivine in an olivine gabbro. The large crystals are titanomagnetite, which crystallized from the relatively evolved gabbroic magma. There are also peculiar paralellogram- and trapezoid-shaped patches of wormy opaque material in the olivine. These are magnetite that formed during subsolidus oxidation of iron in the olivine. Oxidation turned some Fe2+ to Fe3+, precipitating magnetite by the reaction: olivine + O2 = magnetite + enstatite. Though it may seem obvious, opaque minerals are opaque in plane- and cross-polarized light, which differentiates them from isotropic, transparent minerals.


Common opaque minerals include the oxides magnetite and ilmenite, sulfides pyrite, pyrrhotite, and chalcopyrite, and graphite. There are many others, but they are much less common. Yellow sulfides can be distinguished from gray oxides by using oblique illumination from above the section, with the substage light off. The sulfide and oxide colors become visible from light reflecting off the rough mineral surfaces.


Plane/cross-polarized light, field width is 1.2 mm.


4.8.84D


Chromite crystals in olivine in a primitive basalt. Chromite is dark-brown to opaque at full thin section thickness. Chromite is a spinel-type mineral, like magnetite, and has an extensive solid solution in the magnetite composition direction. Cr-Mg-Al-rich chromites are realtively transparent, and can be dark-brown at full thin section thickness. As the magma evolves toward more Fe-rich compositions, the chromite also becomes more Fe-rich and less transparent. Fe-rich chromite is typically opaque at full thin section thickness, but is transparent dark-brown along thin edges. The chromite shown here is relatively Fe-poor. Chromite is isotropic, like all spinels, with a very high refractive index.


Plane/cross-polarized light, field width is 0.6 mm.


ICE-24B


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. The very low 1st order birefringence is obvious, and it 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.


Plane/cross-polarized light, field width is 1.2 mm.


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. It has high negative relief, its refractive index being considerably lower than adjacent quartz and feldspar, and epoxy. Sodalite has much less pronounced relief and poorer cleavage. Fluorite, of course, is isotropic.


Plane/cross-polarized light, field width is 1.2 mm.


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


Plane/cross-polarized light, field width is 1.2 mm.


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, field width is 0.6 mm.


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. Zircon birefringence is typically in the 3rd order. This example has birefringence zoning that is probably caused by igneous compositional zoning. High uranium layers accumulate more radiation damage and become less birefringent.


Plane/cross-polarized light, field width is 0.6 mm.


DIG-D


Zircon compared to monazite


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


Right: Monazite oriented N-S. Monazite can be very pale yellow-green in thin section. This grain has developed a light-brown radiation halo in the adjacent garnet.


Plane-polarized light, field widths are 0.3 mm.


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


right: 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.


Cross-polarized light, field widths are 0.3 mm.


Allanite in a metaluminous granite. This allanite has concentric zoning, probably 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 radial cracks that extend out into the surrounding minerals. This allanite grain mostly has low birefringence, a result of radiation damage that has essentially turned the crystal lattice into a glass.


Plane/cross-polarized light, field width is 1.2 mm.


4.7.84G


Secondary minerals


Chlorite replacing biotite in a muscovite granite. Small residual brownish patches of biotite still occur, as do dark bits of titanite. Biotite can hold several weight percent TiO2 in solid solution, but secondary chlorite can accommodate ≪1%. The low first order, anomalous Berlin blue interference color indicates that this is an Fe-rich chlorite. Residual biotite patches have higher birefringence.


Plane/cross-polarized light, field width is 3 mm.


NEIGC84-A5-6


Chlorite replacing biotite in a metaluminous granite. Lenses of high-index colorless material (titanite?) occur in the chlorite, and a small amount of brown biotite survives in the lower part of the grain. The chlorite mostly has an anomalous brown low first order interference color, indicative of Mg-rich chlorite. The regions with purple birefringence are more Fe-rich. The adjacent biotite has third order birefringence.


Plane/cross-polarized light, field width is 1.2 mm.


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. 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.


Plane/cross-polarized light, field width is 3 mm.


UN30A


Sericite replacing plagioclase in a metaluminous granite. At higher magnification, some of the grungy-looking material resolves into little, colorless, platy crystals. 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/cross-polarized light, field width is 1.2 mm.


UN30A


Serpentine in an altered harzbergite nodule in a kimberlite dike from Pennsylvania. Much of this sample is made of calcite (colorless) and stained talc (darker browns). The serpentine is the yellowish material in the thin veins. 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, where the fibers are perpendicular to the vein walls.


Plane/cross-polarized light, field width is 3 mm.


Fayette Co. Penn. 33


Calcite in an alkaline granite. This shows two images with the same patch of calcite in two different orientations. In the first view, 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 and the grain has rather low relief. In the second image, the calcite grain first has been rotated so that a N-S line bisects the acute angle between the cleavages, so the c axis is approximately E-W the high calcite refractive index is seen and so it has high relief. Calcite and dolomite are the only two common minerals that can noticeibly change from low to high relief on rotation in thin section.


Both plane-polarized light, field width is 1.2 mm.


NEIGC86-B2-7


Calcite in an alkaline granite. Calcite has very high birefringence, so 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 thin edge to the left of center of the first image has ~8th order birefringence. The second image shows this same pair of grains, with one rotated to extinction. 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.


Both cross-polarized light, field width is 0.6 mm.


NEIGC86-B2-7


Zoisite developed in pagioclase in an anorthosite. Zoisite is essentially an orthorhombic, very low-Fe epidote. As an alteration product it commonly forms as masses or veins of irregular, ragged crystals in Ca-rich plagioclase. Zoisite has low first-order birefringence that is characteristically anamalous Berlin-blue and anomalous brown, caused by very high dispersion of the 2V.


Plane/cross-polarized light, field width is 3.0 mm.


STWR-8