And everything that follows this box in Chapter 5 is building to that end. The video finishes up by discussing the most important aspect of optical mineralogy, which is viewing rocks and minerals in thin section. And we will do that.īut because mineral optics can seem arcane, before we jump into the underlying fundamentals, we will begin with a video that gives some background and a broad overview without all the details of optical theory and microscopy.
A standard approach into complicated science topics is to start with a discussion of underlying principles and to build to more complicated concepts. The principles of optical mineralogy and mineral microscopy can be confusing. Box 5-2 Prologue: An Introduction to Optical Mineralogy.For an alternative approach, and to see many excellent videos, go to “Introduction to Petrology” by Johnson, E.A., Liu, J. But there are many ways to approach this topic. This chapter contains the standard and fundamental information about optical mineralogy. A combination of optical properties allows us to identify minerals in thin section and to interpret geologic histories.We use cross-polarized light to learn a crystal’s optic class and optic sign, to measure extinction angles and sign of elongation, and to measure 2V.In cross-polarized light, we distinguish anisotropic from isotropic minerals, we see interference colors related to birefringence, and we can see twinning and related features.In plane-polarized light we can distinguish opaque and nonopaque minerals we can see crystal shape, habit, cleavage, color and pleochroism, and relief.We examine thin sections in two modes: using plane-polarized light or using cross-polarized light.Petrographic microscopes have polarized light sources that illuminate a thin section.Today, most optical mineralogy involves examining thin sections with a petrographic microscope.Optical mineralogy involves studying rocks and minerals by studying their optical properties.Light entering a crystal may be absorbed, refracted, or reflected.No responsibility can be assumed by ThermoWorks for the accuracy or otherwise of the following figures.5.1 Thin section on the stage of a petrographic microscope 5 – Optical Mineralogy
We would recommend, in the first instance, comparing measurements, found with an accurate surface probe or wire probe, and then the Infrared thermometer can be adjusted to match the correct emissivity and used for subsequent measurements. The accuracy of the following figures is almost impossible to guarantee as the emissivity of a surface will not only alter with regard to texture and colour but also with its actual temperature at the time of measurement. In the real world, there are no perfect "black bodies" and very few perfect infrared mirrors so most objects have an emissivity between 0 and 1.
A material with an emissivity value of 0 would be considered a perfect thermal mirror.įor example, if an object had the potential to emit 100 units of energy but only emits 90 units in the real world, then that object would have an emissivity value of 0.90. A black body is a material that is a perfect emitter of heat energy and has an emissivity value of 1. It is defined as the fraction of energy being emitted relative to that emitted by a thermally black surface (a black body). Emissivity is a measure of the efficiency in which a surface emits thermal energy.
0 kommentar(er)
