Lab 7: Minerals and Rocks

Transcription

1 SJSU Geol 4L Planet Earth Lab Lab 7 p. 1 of 7 Lab 7: Minerals and Rocks In this lab, we ll investigate minerals and rocks the basic building blocks of the Earth. Part I. Minerals Minerals are naturally occurring combinations of elements. You re familiar with some of them, though maybe not by formal name: common table salt is halite, rust is chiefly hematite, beach sand is mostly quartz and feldspar, and most bath and baby powders contain talc. Rocks consist of minerals in various combinations. Some rocks are made up of many minerals; some only have one. Thousands of minerals exist, but most of them are very rare. Use the testing tools supplied by your instructor, the key mineral properties described below, and the flow chart on p. 3 to try to name each unnamed sample in the chart on p. 4. Some samples have a very distinctive property or properties, but others are tougher. Color Easily observed, but many minerals occur in a variety of colors, so don t rely on this property alone for identification. Luster How a mineral reflects light. The simplest distinction is between metallic (shiny gold or silver) and non-metallic (doesn t look like metal; dull, earthy, or glassy). Streak The color of a powdered mineral, determined using a porcelain streak plate. More dependable than color. Hardness A measure of relative resistance to scratching (use Mohs scale, p. 2). Crystal shape How a mineral grows if it has sufficient growing space to form a nice crystal. Most mineral samples don t have well-defined crystal faces. Cleavage or Fracture How a mineral breaks. Cleavage is orderly breaking along welldefined planes. Fracture is random breakage. Reaction w/hcl Calcite (and only calcite, or rocks made of calcite) will vigorously react ( fizz ) with dilute hydrochloric acid (HCl). Magnetism A few minerals, such as magnetite, are naturally magnetic.

2 SJSU Geol 4L Planet Earth Lab Lab 7 p. 2 of 7 Mohs Scale of Relative Hardness ( What scratches what? ) The hardness of a mineral can be tested by scratching. A mineral can only be scratched by a harder substance. A hard mineral can scratch a softer mineral, but a soft mineral can not scratch a harder mineral. Therefore, a relative scale can be established simply by seeing which mineral scratches another. The scale proposed by the French mineralogist Friedrich Mohs (in 1812) is still used throughout the world. The higher the number, the harder the mineral. The list also includes several common items that can be used to help determine the relative hardness of a mystery mineral. 1 Talc 2 Gypsum 3 Calcite 4 Fluorite 5 Apatite ~2.5: fingernail ~3.5: copper penny ~5.5: window glass or typical knife blade 6 Orthoclase 7 Quartz 8 Topaz ~6.5: streak plate or good steel file 9 Corundum 10 Diamond FYI: The Mohs scale scale is relative, not absolute; fluorite (4) is not twice as hard as gypsum (2). Here s the true relation among these ten minerals, setting talc as the baseline (= 1). 1 Talc 3 Gypsum 9 Calcite 21 Fluorite 48 Apatite 72 Orthoclase 100 Quartz 200 Topaz 400 Corundum 1600 Diamond

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5 SJSU Geol 4L Planet Earth Lab Lab 7 p. 5 of 7 Part II. Rocks In this part of the lab, we ll examine examples of the most common igneous, sedimentary, and metamorphic rocks. Complete (in any order) each of the three stations located around the room. A. Igneous rocks Igneous rocks form when molten magma or lava cools (the ign- prefix means fire in Latin). When magma stays underground, it cools very slowly. The resulting rock consists of interlocking crystals visible to the naked eye. These are plutonic rocks (after Pluto, god of the underworld). plutonic: underground; big crystals When magma erupts on Earth s surface, it cools quickly. The resulting rock consists mostly or completely of crystals that are too small to see. These are volcanic rocks (after volcanoes). volcanic: on Earth s surface; few or no crystals The composition of igneous rocks also is important to geologists, but we ll just use the following informal rule of thumb: Lighter-colored rocks have more silica (SiO2); darker-colored rocks have relatively less. lighter: more SiO2 darker: less SiO2 Look carefully at your samples of granite and rhyolite. 1. Which is relatively coarser-grained? 2a. Which cooled more quickly? 2b. Why do you think so? Now look at the samples of gabbro, diorite, andesite, and basalt. 3a. Which are plutonic? 3b. Why do you think so? 4a. Which probably have the least SiO2? 4b. Why do you think so? Some volcanic eruptions produce lava that cools so quickly that no crystals grow at all. Two examples of these glassy-looking rocks are obsidian (be careful of its sharp edges) and pumice. 5. Why do you think pumice has such a low density?

6 SJSU Geol 4L Planet Earth Lab Lab 7 p. 6 of 7 B. Sedimentary rocks History of a typical sedimentary rock: (1) Rocks (of any type) are exposed at Earth s surface and erode (broken down) into bits of sand, gravel, and mud collectively known as sediment. (2) Sediment is transported downhill, usually by running water, and deposited in low-lying areas like valleys, lakes, and oceans. Sediment is deposited in layers that geologists call beds. (3) Accumulated sediment turns into sedimentary rock when it compressed (by the overlying sediments) and cemented together (by chemical precipitates from ocean water or groundwater). Examine the bowls of loose sediment, and the samples of sandstone and conglomerate. 1. Sand is to sandstone as is to conglomerate. 2. How would you describe the difference(s) between the sandstone and conglomerate? 3. In what way(s) are the sandstone and conglomerate similar (i.e., more similar to each other than to an igneous rock)? 4. Some sedimentary rocks, such as limestone, contain fossils the remains of ancient plants and critters. Find the samples with fossils. What types of fossils are present? (just use general, everyday terms). 5. Find sample C6R-84-VIII. Estimate how much of this rock is composed of fossils: % 6. Sedimentary rocks also tell us about the environment in which the sediments accumulated. Find the sample labeled ripples. Based on what you know about where modern ripples can be found, where do you think the sediment in this rock may have been deposited?

7 SJSU Geol 4L Planet Earth Lab Lab 7 p. 7 of 7 C. Metamorphic rocks Metamorphic rocks form when pressure, temperature, or corrosive fluids change the texture or mineral composition of any pre-existing rock (which could have been an igneous, a sedimentary, or even a different metamorphic rock). We will divide metamorphic rocks into two types: Foliated metamorphic rocks look streaked, sheeted, or banded. In a foliated rock, flat minerals are roughly parallel to one another due to tectonic squeezing of the rock. Unfoliated metamorphic rocks either weren t squeezed very hard, or don t have enough flat minerals to show a foliation. Compare the samples of shale and slate. Shale, a sedimentary rock made of compacted mud, is dull, earthy, and comparatively lightweight. Slate, a metamorphic rock, is somewhat shiny, foliated, and denser. Don t continue until you re sure you can see these differences. 1. Which of your metamorphic rock samples is foliated? a. slate? yes b. schist ( shist )? c. gneiss ( nice )? d. marble? e. quartzite? f. serpentinite? When metamorphism happens at high temperature and pressure, rocks may deform like soft plastic. Find the folded rock sample. 2. Draw a picture in the space at the right showing the direction of compressional force necessary to produce the folding you see in this rock (experiment with a sheet of paper). Find the serpentinite, which is California s state rock. Serpentinite forms when corrosive fluids metamorphose the rocks of Earth s mantle. Serpentinite is pretty rare, and most of it is found on the ocean floor. Serpentinite followed a long, arduous trek to get from the ocean floor to the California hills where we found it, so please be nice to it. 3. Why do you think serpentinite has that name? Before you answer this question, look at as many serpentinite samples as possible.