The optic sign is determined by the orientation of the colors which result when a quartz plate is slid into the microscope. Therefore, in order for the position to be additive, the slow direction of the slide must line up with that of the quartz plate. https://en.wikipedia.org/w/index.php?title=Extinction_(optical_mineralogy)&oldid=961817878, Creative Commons Attribution-ShareAlike License, This page was last edited on 10 June 2020, at 15:34. It is also one of the most common rock-forming minerals. Isotropic minerals, opaque (metallic) minerals, and amorphous materials (glass) do not allow light transmission under cross-polarized light (i.e. 90 degrees would be considered zero degrees, and is known as parallel extinction. When the stage is rotated 45 degrees to the left from it's original position, the color of the quartz plate changes to a straw yellow. The extinction angle is the measure between the cleavage direction or habit of a mineral and the extinction. In order to find the Sign of Elongation of Quartz, we used a thin section and the quartz plate to determine whether quartz is a length-slow or -fast mineral. Minerals with undulose extinction, solid solution/zonation, or other factors (e.g. Given this, Quartz must be length-slow since the slow-direction is parallel to the long axis of the grain. Interference color: Order I white; quartz is very useful in determining the thickness of any slide in which it occurs. It is easy to determine on account of its lack of alteration, absence of cleavage or twinning. Refractive Index Quartz is a uniaxial hexagonal-rhombohedral mineral with extremely low birefringence. Since this is below that of the the mineral the Becke lines should be inside of the mineral. In the first, quartz is fully visible, and the second has it completely extinct. bird's eye extinction in mica) that may inhibit this measure and may be more difficult to use. Extinction Angle Minerals that have no cleavage or elongation can not have an extinction angle. The Refractive Index of quartz is 1.552, so in order to observe the birefringence of the mineral, we made a slide of the mineral in a 1.538 oil. This indicates that the slide is in an additive position. these both happen at exactly ninety degrees from eachother, and along the axis. Extinction angle: – Twins: Absent Uniaxial/Biaxial: Uniaxial (+) Optic axial angle (2V… When the quartz plate is inserted, yellow appears in the upper left, and lower right corners, and blue appears in the upper right and lower left, The Extinction Angle of Quartz is zero, which can be seen in the following graphics. Minerals with two cleavages can have two extinction angles, with symmetrical extinction occurring when minerals have multiple angles that are the same. 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. We then picked an oil with a higher refractive index than quartz, and the Becke lines are outside the mineral: Bringing the Becke lines into focus, the white halo goes along the mineral boundary: In order to match the epsilon ray of the quartz, we then made a new slide with 1.552 oil, which should match the refractive index: Optic Sign The Extinction Angle of Quartz is zero, which can be seen in the following graphics. To find this, simply line up the cleavage lines/long direction with one of the crosshairs in the microscope, and turn the mineral until the extinction occurs. Without the quartz plate, the quartz appears as such: Although the color in this picture of the slide in the additive position is not true, the slide should be the bright blue. constant extinction). Sign of Elongation Inclined extinction is a measured angle between 1-89 degrees. If the quartz plate were added without rotating the stage at all, the color of the quartz slide would become a first-order red. To find this, simply line up the cleavage lines/long direction with one of the crosshairs in the microscope, and turn the mineral until the extinction occurs. In the first, quartz is fully visible, and the second has it completely extinct. The number of degrees the stage was rotated is the extinction angle, between 0-89 degrees. Anisotropic minerals specifically will show one extinction for each 90 degrees of stage rotation. these both happen at exactly ninety degrees from eachother, and along the axis. when it is the stage is rotated 45 degrees to the right, the color of the quartz will change to a second order blue. 90 degrees would be considered zero degrees, and is known as parallel extinction. When the quartz plate is pushed in, the slow axis is perpendicular to the direction it comes in at. The number of degrees the stage was rotated is the extinction angle, between 0-89 degrees. Before the quarz plate is inserted, the quartz has a cross without color and no shading: This indicates that this is a subtractive position for the quartz slide. This signifies that quartz is positive.
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