Lab 7: Minerals and Rocks

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.
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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.

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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
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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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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?

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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?

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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.