Getting Optic Figures
Looking for grains that yield centered optic figures
Grains that yield centered optic figures are Isotropic as well as opaque minerals that have no optic figures. Then, for optic axis figures on uniaxial and biaxial minerals like find grains with minimum, if possible zero, and birefringence. Then, for flash figures, uniaxial and biaxial minerals are like finding grains with utmost birefringence. And last are BXA and BXO figures, biaxial minerals merely as it is hard to find except for flat lying sheet silicate plates. These grains have middle birefringence.
Three things control the birefringence that you see
The three things to manage the birefringence that you see is first the dissimilarity flanked by the maximum and minimum refractive indices, then the thickness of the grain, then the grain orientation and finally in thin sections, thickness is steady so birefringence is restricted just by 1 and 3.
Setting up to get a figure
Discover the grain you desire to get a figure on. Next is to move back and forth in the high power objective. Focus on the grain. NOTE: It is very important that this objective be centered. Next is to put in the analyzer. Then, if your petrographic microscope has one, swing in the sub stage condenser lens. Then, perform one of the following, first to swing in the Bertrand lens. Finally is to take away the left ocular plus sight down the axis of the ocular tube. Putting in a pinhole into the left ocular tube is able to help. Next is to create your optic figure observation. And finally is to backtrack throughout the steps to get back to normal observation.
What is Petrography?
Petrography is that branch of petrology which focuses on thorough descriptions of rocks. Someone who studies petrography is named a petrographer. The mineral substance and the textural relationships inside the rock are described in detail. Petrographic descriptions begin with the field notes at the outcrop and contain megascopic description of hand specimens. Though, the mainly important tool for the petrographer is the petrographic microscope. The complete analysis of minerals by optical mineralogy in thin section and the micro texture as well as structure are dangerous to considering the origin of the rock. Electron microprobe analysis of individual grains as well as whole rock chemical analyses by atomic absorption or X ray fluorescence is used in a recent petrographic lab. Individual mineral grains from a rock sample might also be analyzed by x ray diffraction whenever optical means are insufficient. Analysis of microscopic fluid additions in mineral grains with a heating stage on a petrographic microscope provides clues to the temperature as well as pressure conditions present throughout the mineral formation.
Macroscopic characters
The macroscopic characters of rocks, those visible in hand specimens with no the aid of the microscope, are extremely diverse and hard to illustrate precisely and completely. The geologist in the field depends mainly on them and on a few rough chemical as well as physical tests and to the practical engineer, architect and quarry-master they are all significant. Though often inadequate in themselves to decide the true nature of a rock, they typically serve for a preliminary classification and frequently give all the information which is actually wanted.
With a small bottle of acid to test for carbonate of lime, a knife to determine the hardness of rocks and minerals, and a pocket lens to enlarge their structure, the field geologist is hardly ever at a loss to what group a rock belongs. The fine grained species are frequently indeterminable in this way, and the minute mineral components of all rocks can typically be determined only by microscopic examination. But it is simple to see that a sandstone or grit consists of more or less rounded, waterworn sand grains plus if it holds dull, weathered particles of felspar, shining scales of mica or small crystals of calcite these as well hardly ever escape observation. Shales as well as clay rocks generally are soft, fine grained, frequently laminated and rarely have minute organisms or fragments of plants. Lime stones are easily marked with a knife blade, fizz readily with weak cold acid and often have complete or broken shells or other fossils. The crystalline nature of a granite or basalt is obvious at a glance, and while the previous have white or pink felspar, clear vitreous quartz and glancing flakes of mica, the other will show yellow green olivine, black augite and grey stratiated plagioclase.
Some easy tools include the blowpipe, the goniometer, the magnet, the magnifying glass and the specific gravity balance.
