Igneous minerals in thin section

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

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. Birefringence tends to be somewhat irregular in single grains because of compositional variations.

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

4.8.84A

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 2^nd and 3^rd 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

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 1^st 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 6^th 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 3^rd 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.

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

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

Zoisite developed in plagioclase 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