Lenses of high-index colorless material (titanite?) Typical for quartz, but not feldspars. Aegirine (Na–Fe3+ monoclinic pyroxene), alkaline granite. 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 3.0 mm. 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. Serpentine in an altered harzbergite nodule in a kimberlite dike from Pennsylvania. Sericite is grungy-looking fine-grained stuff that commonly replaces feldspars. The serpentine is the yellowish material in the thin veins. Birefringencetends to be somewhat irregular in single grains because of compositional variations. These crystals have a lot of inclusions of quartz and feldspar. Perthite from a metaluminous biotite granite. Many of the images have two views, most showing paired plane- and cross-polarized light images. In a given igneous rock, the major light colored minerals will be K-feldspar, plagioclase, quartz, and/or muscovite.. Muscovite is easiest because of its silvery clear color and perfect flaky cleavage. Birefringence and color may define zoning, and both increase with Fe3+ content. Sodalite has much less pronounced relief and poorer cleavage. Grains with the cleavages E-W have the least absorption (not shown in this image). Indeed, it is rare to find a terrestrial plutonic rock without at least some apatite. Plagioclase, unzoned, in a hornblende diorite. 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. Myrmekite is a subsolidus reaction texture that generally results from fluid flow. Myrmekite patch that appears to be replacing microcline. The uranium and thorium content of zircon causes development of pleochroic radiation halos around it. Because many twin domains nucleate throughout the crystal, there are large numbers of thin, spindly and discontinuous domains in the final microcline product. Notice how the twin domains are spindly and somewhat wispy. Cordierite in a peraluminous granite. Birefringent colors are typically up to middle second order. This patch of biotite shows a range of pleochroic colors caused by different crystal orientations. ... the mineral is said to have relief. Ateration to fine-grained material typically occurs along fractures, twin planes, or cleavages. Chlorite replacing biotite in a metaluminous granite. As the magma evolves toward more Fe-rich compositions, the chromite also becomes more Fe-rich and less transparent. Plane-polarized light, field widths are 0.3 mm. Calcite and dolomite are the only two common minerals that can noticeibly change from low to high relief on rotation in thin section. Here the thin section shown above has been rotated clockwise ~45° to show the birefringence of the large grain. The chromite shown here is relatively Fe-poor. Epidote in a calc-alkaline granodiorite. Albite and pericline twins are impossible in monoclinic orthoclase and sanidine, and develop like this only during inversion to triclinic 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. Sodalite in a nepheline syenite. Move the cursor over the visible image to see the other view. It is more commonly euhedral than quartz in plutonic rocks. Fluid inclusions in quartz in alkali granite. Plane/cross-polarized light, field width is 3 mm. Zircon has relief considerably higher than garnet, pyroxenes, or titanite. This is in contrast to the straight, and generally continuous twins in plagioclase. Left: Zircon oriented N-S. Zircon is usually colorless in thin section. Biotite, metaluminous granite, showing a close-up of one crystal. Calcite in an alkaline granite. Brown hornblende in hornblende gabbro. In this case, the exsolved albite is less altered (clearer) than adjacent microcline, which is grayish because of lots of minute alteration minerals. Some useful links in this series . Close-up of the grid twinning. It has three distinguishing characteristics. Wondering how to tell the difference between feldspar and quartz in a hand sample I'm a first year geology student and was wondering how to tell the difference. This means that biotite in standard thin sections rarely goes completely extinct. Yellow sulfides can be distinguished from gray oxides by using oblique illumination from above the section, with the substage light off. 3) Radiation halos are yellow and pleochroic, unless the it is very Mg-rich. Its relief is less than the pyroxenes but higher than any feldspar. Chromite is a spinel-type mineral, like magnetite, and has an extensive solid solution in the magnetite composition direction. When quartz reads 7 on the Moh’s hardness scale, feldspar reads 6. Sericite replacing plagioclase in a metaluminous granite. Calcite has very high birefringence, and the talc has irregular second and third order interference colors that are modified by the brown staining. Perthite is an unmixing texture of an originally homogeneous feldspar grain. Cross-polarized light, field width is 0.6 mm. Zoisite is essentially an orthorhombic, very low-Fe epidote. These probably formed during faster crystal growth than the clear zones. Because it is generally made up of very small crystals, its birefringence is irregular and generally low. 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). Allanite in a metaluminous granite. Augite (clinopyroxene, CPX), in gabbro. The birefringent color difference is mostly caused by the different optical orientations of the two different minerals. Orthoclase in a dacite hypobyssal intrusive. They have a positive sign of elongation and are probably small white micas that grew during subsolidus hydrothermal alteration. renesem5 renesem5 Hey your ans-----hope it is helpful plz Mark as brainliest☺☺☺☺ New questions in Science. In cross-polarized light you can see that the zones also have different birefringence, indicating they have different anorthite content. There are many others, but they are much less common. Besides hardness, the biggest difference is how the two minerals break. Birefringence ranges to middle first order. The birefringent color difference is mostly caused by the different optical orientations of the two different minerals. Perthite from a metaluminous biotite granite. Residual biotite patches have higher birefringence. The serpentine is most easily seen in the veins, where the fibers are perpendicular to the vein walls. Fluorite in a metaluminous granite. The extensive dark and light brown areas are different hornblende crystals in different orientations. Opaques inside olivine in an olivine gabbro. The regions with purple birefringence are more Fe-rich. Albite twinning in the plagioclase is clearly visible. Fe-rich chromite is typically opaque at full thin section thickness, but is transparent dark-brown along thin edges. occur in the chlorite, and a small amount of brown biotite survives in the lower part of the grain. Apatite crystals in norite. The lack of birefringence distinguishes garnet from all other common high-index minerals. Strain has caused the quartz crystal to deform into domains with slightly different extinction angles. Home Rocks and Minerals Plagioclase Feldspar (thin section) Reference URL Share . In general, epidote in igneous rocks has rather high Fe3+ content, which may color it pale yellow-green. Apatite is colorless, commonly elongate, and typically has hexagonal end sections. Its birefringence is also somewhat lower than feldspars and quartz. As you turn a piece of rock in the light, quartz glitters and feldspar flashes. The mineralogy of sandstones: Feldspar grains Cross-polarized light, field width is 1.2 mm. Microcline and albite both exsolved (unmixed) from the homogeneous solid solution during cooling. In this view, healed fractures are highlighted by minute birefringent grains. Aegirine birefringence tends to be up to upper second to third order, higher than amphiboles and most other clinopyroxenes. The mineralogy of sandstones: Quartz grains . Microcline from a peraluminous granite. Cordierite in a peraluminous granite. In cross-polarized light smaller "microphenocrysts" of brightly birefringent olivine and gray to white plagioclase can be seen. Plane//cross-polarized light, field width is 6 mm. Quartz and Feldspar are minerals that we can find prominently in the earth’s crust. Chromite is isotropic, like all spinels, with a very high refractive index.