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Unformatted text preview: 1 Lab 2 Description of Igneous Rock Samples in Hand Specimen and Thin Section A- Field description: 1- Location 2- Structures 3- Color and color index (C.I.; see below) 4- Average grain size (overall): Phaneritic: very coarse (> 1cm), coarse (5mm – 1cm), medim (1 – 5mm); fine (0.1 – 1mm). Aphanitic: microcrsytalline (very fine grained; 0.1 – 1mm); cryptocrystalline (unable to identify minerals, but some birefringence tells you that crystals are there); vitreous (glassy). 6- General texture (see below) and degree of crystallinity (% crystals relative to glass in sample). 5- Mineralogy (see below): Pay special attention to the modal content of Quartz, Feldspars (type is important!), Feldspathoids, and Mafic minerals (especially olivine, pyroxenes, and amphiboles). 6- Name: (see below) B- Thin section description: 1- Colour: Indicate whether the rock is leucocratic (light coloured), mesocratic (intermediate in colour) or melanocratic (dark in colour). Note that colour is a function of the relative percentage of mafic minerals in the rock, and can be quantified by indicating a "colour index" (C.I.) for the rock. This colour index is defined as the volume % of dark minerals quoted to the nearest integer. As such, a rock is leucocratic if it has a C.I. < 30, mesocratic if C.I. = 30 - 60, or melanocratic if C.I. > 60. 2- Mineralogical composition: a) Essential minerals: Are those minerals of igneous origin that are sufficiently abundant in the rock to affect its nomenclature. Following the identification of the essential minerals, you must determine the following: (i) The quartz content of the rock (ii) The type of feldspars and their relative abundance (iii) If the rock contains feldspathoids, then determine their type and modal abundance. (iv) The type and abundance of mafic minerals. For each one of your essential minerals, you must describe the following: (i) Grain size (ii) Crystal form: euhedral, subhedral, or anhedral (iii) Shape: prismatic, granular, lath-like, .... etc....
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- Fall '11