Ch4 Rocks and Minerals | Earth Through Time

If you were a Stone Age (Paleolithic) human and had to choose between limestone and chert as the material for an ax head, which would you select? Why?

Chert is a finely crystalline rock that has no cleavage and has hardness near 7. The chert lends itself to flaking and shaping into a spear point and will keep a sharp edge. Limestone can also be finely crystalline rock that has no cleavage, but limestone tends to break along stratification (layering), which can be spaced at small intervals. Limestone is made of calcite, which is a rather soft mineral (H=3). Therefore chert, which is harder, is preferable for an ax head

What mineral groups discussed in this chapter are particularly common in sedimentary rocks?

Clastic rock grains are most commonly composed of minerals quartz, feldspar, micas, and clay. Clastic rock cements are most commonly calcite, hematite, and quartz. Carbonate rocks are composed mainly of calcite and (or) dolomite. Chert is a finely crystalline form of quartz. Evaporites are composed mainly of halite and (or) gypsum.

What inferences can you draw from the texture of an igneous rock? What is the relation between the viscosity of molten rock and its composition?

Grain sizes (crystal sizes) of igneous rocks indicate cooling history: the larger the crystal, the slower the cooling rate. Mixed crystal sizes indicate two stages of cooling. Magmas that have high viscosity generally are relatively low in silica content.

What igneous rock best approximates the composition of continental crust? Oceanic crust?

Granite, made of quartz and feldspar, is the rock that best approximates continental crustal composition. Basalt, made of ferromagnesian silicates and plagioclase feldspar, best approximates oceanic crustal composition.

List the foliated metamorphic rocks in order of increasingly coarse foliation.

In order of increasingly common foliation, metamorphic rocks are slate, phyllite, schist, and gneiss.

List the clastic sedimentary rock types in order of increasingly finer grain size.

In order of increasingly finer grain size, clastic sedimentary rocks are called: conglomerate (or breccia); sandstone; siltstone; and shale (or claystone)

If you were a sculptor and had to select a metamorphic rock from which to carve a statue, why would you choose marble over schist? What might be your objection to quartzite?

Marble is rather soft and tends to be finely crystalline and without natural planes of weakness like foliation. Gneiss is much harder due to its silicate mineral content and is strongly foliated, which introduces natural planes of weakness into the rock. Quartzite tends to be more like marble in terms of texture, but is made of a rather hard mineral (quartz), which could be difficult to work with.

What is a mineral? What characteristics of a true mineral (like quartz, feldspar, calcite, or pyrite) would not be present in a piece of volcanic glass?

Minerals are naturally occurring solid, inorganic substances that have a definite chemical composition or range of compositions as well as distinctive properties that reflect the composition and regular internal atomic structure. Minerals have regular internal structures of atoms, unlike amorphous non-crystalline materials like volcanic glass.

What are the eight most abundant elements that make up rocks and minerals?

Oxygen (O), Silicon (Si), Aluminum (Al), Calcium (Ca), Iron (Fe), Magnesium (Mg), Sodium (Na), and Potassium (K)

What has happened to an igneous rock that forms through partial melting? Explain how an igneous rock can form by fractional crystallization.

Partial melting occurs where pressure is slightly relieved and minerals with select melting points will liquefy and mobilize at the expense of others left behind. In fractional crystallization, some crystals form in the liquid magma as it cools. These crystals fall out or are removed thus making the liquid a different composition. The former generally works to enrich liquids in mafic minerals; the latter to enrich liquids in silicic (lighter) minerals.

Why are silicate minerals so important in geology? Which silicates would you expect to find in granite? Which silicates would you expect to find in sedimentary rocks?

The silicate minerals are the most common minerals forming about 75% by weight of the Earth's crust. Silicate minerals are important because we live on a silicate world. Silicates are based on silica, a bonding of one silicon atom and four oxygen atoms (the most common elements of the crust). Granite is made of quartz and feldspar, the two most common silicates. Silicate minerals common in sedimentary rocks are quartz, feldspar, mica, and clay.

Which one of the following statements is not valid? ___ a. Ferromagnesian minerals lack cleavage. ___ b. Ferromagnesian minerals are dark-colored. ___ c. Ferromagnesian minerals are common in rocks like basalt and gabbro. ___ d. Ferromagnesian minerals include hornblende, augite, and the green mineral olivine.

a. Ferromagnesian minerals lack cleavage.

Which of the following rocks has the highest percentage of quartz minerals? ___ a. Granite ___ b. Diorite ___ c. Gabbro ___ d. Peridotite

a. Granite

The majority of minerals comprising the Earth's crust are: ___ a. Silicates ___ b. Carbonates ___ c. Oxides ___ d. Sulfates ___ e. Chloride

a. Silicates

Sedimentary rocks may consist of: ___ a. Lithified particles of sand, silt, and clay ___ b. Chemical precipitates like carbonates and sulfates ___ c. Accumulations of organic remains, like whole or broken shells of invertebrates ___ d. All of the above

d. All of the above