Geoscience 001 Fall 2007 Name: Identifying RocksBring a hand lens to lab (available in bookstore).INTRODUCTIONThe purpose of this week's lab is to learn how to identify rocks in preparation for later labs, including our field trips to investigate the local geology. Rocks can tell us interesting stories about the past, and the construction of these stories begins with basic rock identification. Before coming to lab, you should read Chapter 5 and Interlude A of your text, which is all about minerals and rocks.Identifying rocks is an important skill and the more you practice, the easier it becomes. You start out by determining the general class of rock — igneous, metamorphic, or sedimentary — and then proceed to identify the minerals that make up the rock and then the texture of the rock. The texture is the size, shape, and arrangement of those grains. Fossils are an especially important component of sedimentary rocks because they can tell us about the environment in which the rocks formed (and also the rocks age), so we'll also gain some familiarity with commonly occurring fossils.The first part of lab will be spent learning some of the basics of rock identification; then you will identify 5 unknown samples. You will use this information to reconstruct a geologic history, and then after the lab period, you will identify the rocks used in some of the buildings on campus. Part I: Basic Rock Identification Rocks are identified based on their mineral content and their texture, or the way the mineral particles are arranged in the rock as well as their sizes and shapes. Classification schemes for igneous, metamorphic, andsedimentary rocks are displayed in the lab - these serve as one of your most important resources as you begin to learn how to identify rocks. Additional information on identifying rocks can be found in some of the lab manuals that are in the lab.IGNEOUS ROCKS Study the igneous rock classification and the igneous textures displays on the side counter in the lab. The texture (i.e., grain size) of an igneous rock reflects the cooling history of the magma. The mineral composition of an igneous rock reflects the chemical composition of the magma from which it crystallized. The three most important mineralogical criteria are:1) the presence or absence of quartz, which is identified by its glassy luster, hardness, and conchoidal fracture;2) the composition of feldspar minerals, most of which have good cleavage surfaces at 90° to each to each other and have pearly to glassy luster (K-spar is commonly pink or salmon-colored, plagioclase is identified by the closely spaced parallel striations or twin planes); and3) the proportion and types of dark (generally green or black) ferromagnesian (mafic) minerals - biotite, hornblende, pyroxene, and olivine. The diagram on igneous rocks below should be particularly helpful inassigning a rock name based on the mineral content. This diagram can also be used to test hypotheses - e.g.,"if this rock is an andesite, we expect to find little or no quartz, lots of plagioclase, some amphibole and pyroxene".One important thing to bear in mind is that if individual mineral grains are too small to be seen with a hand lens, then you cannot identify the rock on the basis of mineralogy (you would have to use a more powerful microscope) and you are forced to make a tentative identification based mainly on the color and other textures. Refer to the classification schemes for help here.Also a couple of terms need to be defined: aphanitic = mineral grains too small to be seen w/ naked eye; phaneritic = mineral grains big enough to be seen with naked eye; these two terms could be modified by adding another term - porphyritic = some grains significantly larger than the others. So a rock could be aphanitic porphyritic or, more rarely, phaneritic porphyritic.METAMORPHIC ROCKSMetamorphic rocks (sedimentary or igneous rocks transformed by the effects of high pressures and temperatures) are also classified by their mineralogy and texture. Rocks with abundant platy minerals (usually micas or clays) are typically layered on a fine scale; this layering is called foliation. Three foliated metamorphic rocks are slate, phyllite, and schist. The size of the mica grains increases from slate to phyllite to schist, corresponding with an increase in the temperatures and pressures that the rocks were subjected to during the metamorphism. If micas are clearly visible with a hand lens, the rock is a schist. If the rock has a shiny luster (caused by microscopic-sized micas), the rock is a phyllite. If the rock is foliated,but has a dull luster (not shiny), the rock is a slate. Another foliated rock is gneiss, but it is usually foliated on a coarser scale. Foliation in gneisses is caused by segregation of minerals into different layers. In most gneisses, equi-dimensional mineral grains of quartzand feldspar are more abundant than the platy micas.Non-foliated metamorphic rocks include marble, quartzite, and hornfels (a fine-grained, hard rock). Someof the lab manuals in lab have more complete descriptions of metamorphic rocks. Boundaries between mostof these textural categories are fuzzy.Metamorphic rocks are named by prefacing the textural name (schist, gneiss, etc.) with the names of the minerals present (those that you can identify). Some typical rock names are glaucophane-garnet-muscovite schist, biotite-garnet gneiss, and quartz-biotite-garnet schist.Study the metamorphic rocks on display and familiarize yourself with some of the varieties of foliated and non-foliated metamorphic rocks. SEDIMENTARY ROCKS Sedimentary rocks may be classified in a number of ways, and there is no general agreement or a single bestclassification. Most systems rely on the mineralogy of the particles or clasts. 1.Siliciclastic Sedimentary Rocks - composed of fragments (clasts) of silicate minerals and rocks of various sizes. These rocks may also contain fossils, but mineral grains form the major part of these rocks. Siliciclastic rocks are grouped according to the size of the particles:Particle Size Type of Particle Rock Name64 mm and larger cobbles conglomerate2-64 mm pebbles conglomerate1/16 - 2 mm sand sandstone1/256 - 1/16 mm silt siltstonesmaller than 1/256 mm clay (mud) mudstone or shaleStudy the siliciclastic rocks on classification chart to familiarize yourself with the characteristics these rocks.2.Carbonate Sedimentary Rocks - composed of the shells and