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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

The Spinel Series of minerals includes spinel, pleonaste, hercynite, picotite, gahnite, and galaxite. These minerals are known to crystallize in the isometric system. In optical mineralogy, the isometric system comprises crystals having three axes, all of which are perpendicular to one another and all are found equal in lengths. These members of the Spinel series are Oxide minerals. Spinel series of minerals are all isotropic when evaluated under petrographic polarizing microscopes. In optical mineralogy, isotropic minerals have no power to produce any illumination and they are singly refracting and consequently are quite dark when viewed between crossed nicols of petrographic polarizing microscope. They remain in that condition during a complete rotation of the polarized microscope stage. These minerals are also found exhibiting a very high positive relief when viewed under polarizing microscope.

 

            In terms of their composition and structure, the spinel series of minerals may have extensive solid solution among its members. Mineral Spinel is the most common member of the series. If the composition would contain substantial amount of Fe(+2), then it will be called Pleonaste. Picotite on the other hand is a variety of mineral Hercynite in which substantial amount of Cr has been substituted for Al.

 

            Hand sample color of the minerals is quite variable. Spinel is commonly found colorless, green, blue, or red. Pleonaste is usually found green to blue green in color. Hercynite is commonly dark green in appearance. Picotite is usually colored olive brown to brown. Gahnite is commonly exhibiting blue green, yellow, and brown colors. While Galaxite, on the other hand, is usually found red brown or black. The hardness measure of the minerals in the Spinel series is usually found ranging from 7.5 to 8 when evaluated with the use of the Mohs scale method. The specific gravity measure of the Spinel series minerals ranges from 3.55 grams per cubic centimeters to 4.62 grams per cubic centimeters. The colors of the mineral members of the series if evaluated in thin section correspond to their colors in hand sample. Some samples may appear deeply colored and these are the ones that are practically opaque in appearance. Gahnite, when evaluated under petrographic polarizing microscope, may show anomalous pleochroism.

 

            The crystal forms of the mineral members of the Spinel series are usually octahedrons yielding triangular, square, or diamond-shaped cross sections in thin section. Subequant anhedral grains are also common. There is no cleavage reported when the samples are examined closely under the polarizing microscopes. But sometimes, there is a prominent octahedral parting on one direction parallel to the octahedral faces. Single twins or sometimes multiples twins are commonly found on the contact plane. Since the mineral is isotropic, twinning will not be visible unless betrayed by the crystal shape.

 

            The indices of refraction of the mineral members vary in linear fashion. That is, between the end member composition. Spinel is commonly found having an index of 1.714, Hercynite is 1.835, Gahnite is 1.805, and Galaxite is 1.920. It has been found in optical mineralogy that there are so many variables in the solid solution to allow the index of refraction alone be used to determine the composition. Some samples of Gahnite may show weak anomalous birefringence when viewed closely with the aid of the petrographic polarizing microscopes used in optical mineralogy.

 

            Spinel minerals are relatively resistant to weathering or other alteration although there are alterations reported such as those to various phyllosilicates and other minerals. Zinc-bearing spinels may alter to Sphalerite and various phyllosilicates. Spinel minerals are generally distinguished by their high relief in thin section, strong color, and isotropic character. The different crystal shapes and lighter color distinguishes garnet from spinel. In grain mounts, Spinel minerals may be difficult to differentiate between the spinel and garnet groups, and thus, x-ray diffraction and other techniques are required to use.

 

            Common spinel, including pleonaste, is relatively commonly found in highly aluminous or silica poor metamorphic rocks. These are usually associated with andalusite, corundum, Kyanite, orthopyroxene, sillimanite, cordierite, and in regionally metamorphosed carbonate rocks associated with chondrondite, phlogopite, forsterite, and calcite. Mineral Hercynite is commonly found in metamorphosed iron-rich argillacenous sediments and also in some mafic and ultramafic igneous rocks. It can be also possibly found in some granulites. Gahnite on the other hand is usually found in hydrothermal deposits and in granitic pegmatites. Galaxite is relatively rare and is found in hydrothermal vein deposits that are Mn-rich and this is commonly associated with Mn-bearing minerals like rhodonite and spessartine.



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Wednesday, May 28th, 2008 at 5:01 am
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The Oxides and Hydroxides Mineral Class
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