Augite (clinopyroxene, CPX), in gabbro. In this case, the exsolved albite is less altered (clearer) than adjacent microcline, which is grayish because of lots of minute alteration minerals. The high refractive index of garnet cause fractures and the grain margin to stand out as dark lines because of total internal reflection. Home Rocks and Minerals Plagioclase Feldspar (thin section) Reference URL Share . It has high negative relief, its refractive index being considerably lower than adjacent quartz and feldspar, and epoxy. Though its presence is obscure in plane-polarized light, it is obvious in cross-polarized light as quartz worms in plagioclase. The key difference between quartz and feldspar is that the major chemical element present in quartz is silicon whereas, in feldspar, it is aluminum. Sericite replacing plagioclase in a metaluminous granite. Notice the fractures concentric with the crystal margin. Serpentine in an altered harzbergite nodule in a kimberlite dike from Pennsylvania. Chromite crystals in olivine in a primitive basalt. A Becke line test tells you which phase is which: the Becke line goes into the higher index phase, albite. The extensive dark and light brown areas are different hornblende crystals in different orientations. Olivine, phenocryst in an Iceland basalt. renesem5 renesem5 Hey your ans-----hope it is helpful plz Mark as brainliest☺☺☺☺ New questions in Science. The serpentine is most easily seen in the veins, where the fibers are perpendicular to the vein walls. In this view, healed fractures are highlighted by minute birefringent grains. It is commonly associated with other Ca-rich minerals like hornblende, plagioclase, and titanite. Work systematically across the thin section, zooming on features like grain boundaries and crystal contacts. Albite twinning in the plagioclase is clearly visible. Notice how the twin domains are spindly and somewhat wispy. Fluorite, of course, is isotropic. This example is approximately 6th order. If there is nothing between the two polarizers ... that depends on the difference in travel time. This grain appears quite homogeneous in plane light, without concentric zones of inclusions that are commonly seen elsewhere. Zoisite developed in pagioclase in an anorthosite. Notice the concentric layers (zones) of inclusionsl. Plagioclase, zoned, in a dacite porphyry. The lack of birefringence distinguishes garnet from all other common high-index minerals. In this photo the thin section was rotated to obscure twinning. The adjacent biotite has third order birefringence. 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. This patch of biotite shows a range of pleochroic colors caused by different crystal orientations. Interference colors are first order gray to white, like quartz and feldspar. Aegirine birefringence tends to be up to upper second to third order, higher than amphiboles and most other clinopyroxenes. In the case of quartz, the difference in the preferred orientations of PDF poles between H and V sections is less obvious than in plagioclase, but the number of measured features is much higher in V thin sections with respect to H thin sections (Fig. Apatite crystals in norite. Microcline from a peraluminous granite. Plane-polarized light, field widths are 0.3 mm. The uranium and thorium content of zircon causes development of pleochroic radiation halos around it. It is more reliable to look at a thin edge and count the number of magenta bands. In cross-polarized light that crystal is at extinction, in keeping with the orthorhombic symmetry of this mineral. Fluid inclusions in quartz in alkali granite. These form during cooling, as the grain changes from monoclinic orthoclase to triclinic microcline. Birefringence in the low to middle first order. Much of this sample is made of calcite (colorless) and stained talc (darker browns). Pale brown radiation halos can sometimes be visible around radioactive inclusions (but not really visible here). Here the thin section shown above has been rotated clockwise ~45Â° to show the birefringence of the large grain. The mineralogy of sandstones: Feldspar grains Quartz and feldspar have little relief, ferromagnesian minerals have moderate to high relief. Sodalite in a nepheline syenite. Epidote in a calc-alkaline granodiorite. This shows three radioactive inclusions with their yellow radiation halos, caused by alpha particle radiation damage. 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. Calcite in an alkaline granite. Zircon birefringence is typically in the 3rd order. The very low 1st order birefringence is obvious, and it has negative sign of elongation. Calcite has very high birefringence, so interference order is difficult to judge from the high-order pastel colors. Igneous muscovite is generally colorless with good cleavage. right: In the same orientation Monazite is not extinct, and so it must have inclined extinction. This shows two images with the same patch of calcite in two different orientations. Swelling of the crystal as damage accumulates, and absorption of water, causes radial cracks that extend out into the surrounding minerals. Plagioclase, unzoned, in a hornblende diorite. The birefringent color difference is mostly caused by the different optical orientations of the two different minerals. However, once they start, that is how the twin continues to grow.
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