from Part II - Physical investigation of minerals
Introduction
Mineralogy relies on a quantitative characterization of minerals and many different techniques are available. We have discussed X-ray diffraction (see Chapter 7) and analyses with the petrographic microscope (see Chapters 9 and 10). In this chapter we will describe briefly some other methods. Reading it you will not become experts, but at least you will have an idea of how to pursue more in-depth studies, particularly if you happen to find employment in a mineralogical laboratory. We have put this chapter into Part II of the book, which deals with physical investigations. You may want to skip it for now and return to it after knowing more about mineral systems (Part IV), in order to better appreciate the discussion of examples.
Every mineralogist must be able to use a petrographic microscope to identify minerals in thin sections, or opaque minerals in polished sections, and to determine their associations and intergrowths. A quick look at a thin section provides a wealth of information about the geological history of an assemblage of minerals. For example Figure 12.1a is olivine basalt, recognizable by phenocrysts of olivine and plagioclase of labradorite composition, and by a fine groundmass. Figure 12.1b is a metamorphic amphibolite with hornblende, plagioclase (An50), and biotite. The plagioclase composition in this metamorphic rock indicates that it is of similar bulk chemical composition to the basalt, crystallized at 600–700 ℃ and 400–600 MPa. The alignment of the hornblende crystals suggests that crystallization was associated with tectonic deformation.
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