1. Define the term rock.
A rock is a more or less hardened (lithified) aggregate of minerals and/or amorphous solids such as natural glass and organic matter (Fig. 2.2).
2. List the three main particles of an atom and explain how they differ from one another?
The particles are electrons, protons, and neutrons. The latter two are heavy, nuclear particles; electrons are tiny, very light-weight particles that form a "cloud" surrounding the nucleus. The mass and charge data are as follows:
proton - one atomic mass unit, 1+ electrical charge
neutron - one atomic mass unit, electrically neutral
electron - tiny fraction of one atomic mass unit, 1- electrical charge
3. If the number of electrons in a neutral atom is 35 and its mass number is 80, calculate the following:
(a) The number of protons - A neutral atom with 35 electrons has 35 protons (element bromine, Br; Fig. 2.3).
(b) The atomic number - The atomic number is 35, equal to the number of protons in the nucleus.
(c) The number of neutrons - The mass number (80) is the sum of protons (35) and neutrons. Thus the nucleus contains 45 (80 - 35) neutrons.
4. What is the significance of valence electrons?
Valence electrons are those outermost, few electrons in an atom or molecule that participate in chemical reactions and bond formation. Valence electrons are the bonding electrons.
5. Briefly distinguish between ionic and covalent bonding.
Ionic bonds are strong attractive forces between closely-spaced ions of opposite (+ and -) electrical charges. The ions are formed by chemical reactions in which valence electrons are removed from a donor atom or molecule, producing a positively charged ion (+ ion) and acquired by another atom or molecule, producing a negatively charged ion (- ion). These reactions (ionizations) enable both ions to achieve much higher chemical stability (more stable valence electron configurations) than the respective neutral atoms.
In covalent bonding, the more stable, outer, electron configurations are achieved by sharing of valence electrons among two or more neighboring atoms in a molecule or crystalline compound. Charged atoms (ions) do not form.
6. What occurs in a atom to produce an ion?
One or more valence electrons are simultaneously gained and lost by atoms participating in a chemical reaction. The atoms that gain electrons are negative ions; those that lose electrons are positive ions.
7. What is an isotope?
Isotopes are atoms of the same element (same atomic number) that differ in mass number (numbers of neutrons are different). Thus natural uranium includes a small fraction of atoms with mass 235 (143 neutrons and 92 protons) together with the more abundant atoms with mass 238 (146 neutrons). In general, isotopes of the same element have very nearly identical chemical characteristics.
8. Although all minerals have an orderly internal arrangement of atoms (crystalline structure), most mineral samples do not demonstrate their crystal form. Why?
Crystal form refers to the geometrically regular, external growth shape that mineral grains can exhibit if grain growth is free and unobstructed by other mineral grains (the grain grows into a fluid-filled cavity, for example). Most mineral grain growth in nature is obstructed (not free), so grains showing their characteristic, geometric, growth forms are not that common. Mineral samples broken from larger masses have their shapes determined by fractures and cleavage, not by grain growth.
9. Why might it be difficult to identify a mineral by its color?
A particular mineral may exhibit many, different colors. Thus by itself, color is seldom definitive in mineral identification, but it may be helpful. Mineral color is highly sensitive to relatively small contents of impurity atoms and to bulk chemical compositions in members of a solid solution series. Thus in corundum, an aluminum oxide mineral, small quantities of chromium account for the red variety (ruby) and small quantities of iron and titanium account for the blue variety (sapphire). Colors of rock-forming silicate minerals with variable compositions such as biotite, pyroxenes, and amphiboles are very sensitive to the contents of iron and other first-row transition elements such as titanium, chromium, and manganese. Iron-poor biotite, pyroxenes, and amphiboles are colorless or only faintly colored; iron-bearing varieties are generally deeply colored, ranging from green to black. Garnets for example, show virtually every color in the rainbow, depending on the bulk composition.
10. If you found a glassy-appearing mineral while rock hunting and had hopes that it was a diamond, what simple test might help you make a determination?
A hardness comparison with quartz would establish that the grain was above 7 in the Mohs scale. So are many other minerals. A jeweler could quickly determine the refractive index, thus verifying or dashing your hopes. Diamond has the highest refractive index of any mineral.
11. Explain the use of corundum as given in Table 2.4 in terms of the Mohs hardness scale.
Any mineral listed in Mohs scale (Table 2.2), corundum for example, will scratch softer minerals (those with lower hardness values) and will not scratch harder minerals. Corundum would scratch virtually all other minerals, diamond being the lone exception. Thus corundum is widely used in abrasives and polishing compounds.
12. Gold has a specific gravity of almost 20. If a 25-liter pail of water weighs 25 kilograms, how much would a 25-liter pail of gold weigh?
The specific gravity of water is one by definition. Thus equal volumes of water and gold would have their weights in the ratio 1:20. Since the 25 liters of water weigh 25 kilograms, the 25 liters of gold will weigh almost 500 kilograms (25 liters X 20 kg/l).
13. Explain the difference between the terms silicon and silicate.
Silicon is the name for the element with atomic number 14; the chemical symbol is Si. Elemental silicon is a semiconductor and is widely utilized today in computer chips. Silicate refers to any chemical compound that contains the elements silicon and oxygen; additional elements may be present but the term silicate is still applicable. Most rock-forming minerals are silicates. The native element does not occur naturally; it is manufactured from quartz, silicon dioxide, at high temperatures under strongly-reducing conditions.
14. What do ferromagnesian minerals have in common? List examples of ferromagnesian minerals.
Ferromagnesian is a word derived from the chemical elements magnesium and iron (ferro, ferrous, ferric, etc.). The term refers to rock-forming, silicate minerals that contain some iron (Fe) and/or magnesium (Mg) in addition to silicon and oxygen. Additional elements such as aluminum, sodium, and calcium may be present without changing the designation. Ferromagnesian minerals comprise most of the dark-colored (dark green and black) mineral grains in igneous rocks.
15. What do muscovite and biotite have in common? How do they differ?
They are both micas with layered (sheet-silicate), internal, crystalline structures and one direction of perfect cleavage. Muscovite is the light-colored, potassium aluminum (KAl) mica; and biotite is the darker-colored, ferromagnesian mica (contains Mg and Fe).
16. Should color be used to distinguish between orthoclase and plagioclase feldspar? What is the best means of distinguishing between these two types of feldspar?
Twinning striations are definitive for plagioclase; unfortunately, they are not always visible without a microscope. Orthoclase doesn't have striations. Both feldspars can be white or colorless, but pale-pink or pale-orange colors usually indicate orthoclase. Ca-rich plagioclase may be fairly dark gray. Thus color alone is not definitive; however, in rocks with a pinkish orthoclase and a white plagioclase, color is very helpful in telling the two feldspars apart.
17. Each of the following statements describes a silicate mineral or mineral group. In each case, provide the appropriate name.
(a) The most common member of the amphibole group - hornblende
(b) The most common nonferromagnesian member of the mica family
- muscovite
(c) The only silicate mineral made entirely of silicon and oxygen - quartz (SiO2); other polymorphs such as tridymite, cristobalite, and high-pressure forms are known, but by far, quartz is the most abundant silicon dioxide phase in rocks
(d) A high-temperature silicate with a name that is based on its color - olivine, green
(e) Characterized by striations - plagioclase with twinning striations
(f) Originates as a product of chemical weathering - the clay minerals
18. What simple test can be used to distinguish calcite from dolomite?
Both minerals are carbonates. Calcite reacts vigorously with dilute, strong acids such as hydrochloric (HCl), with the evolution of carbon dioxide (CO2) gas bubbles. In contrast, dolomite must first be finely powdered before reacting vigorously enough with the same dilute acid to produce visible bubbling.
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