Lesson 20

Lesson 20 - Lesson 20: Pegmatites and Tourmaline Learning...

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Lesson 20: Pegmatites and Tourmaline Learning Objectives By the end of these three Lessons (Lessons 20-22) on pegmatites and pegmatite gems, learners will: Describe the diagnostic properties of the three main pegmatite gems varieties: tourmaline, topaz, and spodumene Describe the basic crystal chemistry of tourmaline, topaz, and spodumene Explain the mineralogical differences between the different tourmaline and spodumene varieties (e.g., rubellite and kunzite) Describe the external morphology of tourmaline, topaz, and spodumene crystals Define the main variables used to value gem tourmaline, topaz, and spodumene Describe the term "pegmatite" Describe the geological setting and physical morphology of gem pegmatites Describe the global and local distribution of gem pegmatite Introduction If you want to find gemstones in the rough, the best rock type to concentrate on is pegmatite. Pegmatites supply the world with the best tourmaline, topaz, and beryl along with a large selection of other rare stones - some so rare that their faceted varieties are only cherished by the few collectors who can get their hands on them. In addition to the wonderful gems that pegmatites produce, these rocks are also important hosts for rare metal deposits, including lithium (Li), tantalum (Ta), niobium (Nb), and tin (Sn). Pegmatites are known for growing the largest crystals and are the environment for many "largest crystal" records for specific minerals. In fact, the term pegmatite is used as a descriptor for igneous rocks with large crystal sizes. In these Lessons on pegmatite, we'll first look at the different types of minerals hosted in pegmatites and then take a closer look at tourmaline, spodumene, and topaz. Later, we'll describe the environments where pegmatites occur, and look at some specific examples in the USA and Canada.
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Essential readings and topics for this lesson in your textbook Topic Pages Granite 34 - 35 Pegmatite 36 Tourmaline 288 - 289 Pegmatites and their Mineralogy First, what is a pegmatite? Prof. "Skip" Simmons, a "pegmatologist" from the University of New Orleans , defines these wonderful rocks as intrusive igneous rocks that are texturally very coarse to gigantic in size. Your textbook describes them similarly and goes into a bit more detail. In the next lesson, we'll dive deeper into the geology of pegmatites but for now we'll start with the minerals that comprise pegmatites. The mineralogy of pegmatites is directly tied to their geochemistry and most pegmatites can be characterized by a base composition similar to granite but with significant enrichment in rare elements. The enrichment in rare elements typically facilitates the growth of rare minerals (one of the prerequisites for a valuable gemstone) that require these particular elements in their crystal structure. An example of one of these rare minerals that you're already familiar with is beryl. This
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Lesson 20 - Lesson 20: Pegmatites and Tourmaline Learning...

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