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Chromite can also be presented in a thin section. The grains seen in thin sections are disseminated with crystals that are euhedral to subhedral. [12] Chromite contains Mg, ferrous iron [Fe(II)], Al and trace amounts of Ti. [5] Chromite can change into different minerals based on the amounts of each element in the mineral.
The following tests are some examples of those that are used on hand specimens, or on field samples, or on thin sections with the aid of a polarizing microscope. Color; Color of the mineral. Color alone is not diagnostic. For example quartz can be almost any color, depending on minor impurities and microstructure. Streak
Dark layers of chromite-rich cumulate rock alternating with light layers of plagioclase-rich rock in the Bushveld Igneous Complex, South Africa Oxide mineral cumulates form in layered intrusions when fractional crystallisation has progressed enough to allow the crystallisation of oxide minerals which are invariably a form of spinel .
The opposite is anhedral (also known as xenomorphic or allotriomorphic), which describes rock with a microstructure composed of mineral grains that have no well-formed crystal faces or cross-section shape in thin section. Anhedral crystal growth occurs in a competitive environment with no free space for the formation of crystal faces.
An ordinary 30 μm thin section is prepared as described above but the slice of rock is attached to the glass slide using a soluble cement such as Canada balsam (soluble in ethanol) to allow both sides to be worked on. The section is then polished on both sides using a fine diamond paste until it has a thickness in the range of 2–12 μm.
Altered samples of the Blue Ridge Ophiolite are green, with white and black crystals that are visible. In thin section some samples have no olivine such as the sample form Todd, North Carolina. Altered minerals that can be seen in thin section are chlorite and chromite. [3] Sample of Talc-Actinolite Schist taken from Todd, North Carolina ...
In geology, texture or rock microstructure [1] refers to the relationship between the materials of which a rock is composed. [2] The broadest textural classes are crystalline (in which the components are intergrown and interlocking crystals), fragmental (in which there is an accumulation of fragments by some physical process), aphanitic (in which crystals are not visible to the unaided eye ...
Extinction is a term used in optical mineralogy and petrology, which describes when cross-polarized light dims, as viewed through a thin section of a mineral in a petrographic microscope. Isotropic minerals, opaque (metallic) minerals, and amorphous materials (glass) do not allow light transmission under cross-polarized light (i.e. constant ...