Read an Excerpt
Rocks and Minerals
By Charles A. Sorrell, George F. Sandström
St. Martin's PressCopyright © 1973 St. Martin's Press
All rights reserved.
MINERALOGY AS A SCIENCE
Minerals are the natural crystalline materials that form the Earth and make up most of its rocks. Though minerals have been used and metals extracted from them for all of recorded history, mineralogy as a science is relatively young. Serious study of minerals began in the 1800's, after the development of the petrographic microscope (for studying rocks) and the reflecting goniometer (for accurately measuring angles between faces of a mineral's crystals). During that century most of the minerals known today were described, optically studied, and chemically analyzed. Largely from these studies grew the schemes used today to classify minerals. The internal crystal structure of minerals, however, could only be guessed from their external symmetry and optical properties.
Wilhelm Roentgen's discovery of X-rays in 1895 provided mineralogists with the tool they needed to study crystal structures, but it was not until 1912 that Max von Laue and his assistants proved that when X-rays are scattered by a crystal their behavior gives clues to the internal arrangement. Since then, the structures of all important mineral groups have been analyzed. By correlating this structural knowledge with physical, chemical, electrical, thermal, and mechanical properties, mineralogists have gained an understanding of the forces that hold crystalline matter together. This understanding has in turn enabled scientists to synthesize crystalline compounds with properties to fill special needs. These compounds have been used in the manufacture of high-temperature ceramics, electrical insulators, transistors, and many other items.
Knowledge of the behavior of crystals at high temperatures and pressures has been applied to research on the formation of mountains, the eruption of volcanoes, and other geologic processes. The forces that cause these activities become more understandable when analyzed in terms of structural changes in the mineral components of rock.
Rocks, metals, concrete, bricks, plaster, paint pigments, paper, rubber, and ceramics all contain mineral or synthetic crystals. In fact, almost all solids except glass and organic materials are crystalline. This is why knowledge of the structure and behavior of crystals is important in nearly all industrial and technical endeavors. Even organic materials form crystals when isolated in a pure state. By studying these crystals, biologists and medical researchers have learned much about life processes and heredity.
Unquestionably mineralogy is a subject of widespread importance in all of science. Consequently, persons trained in mineralogy and crystallography may be found doing work in the parent science, geology, or may be engaged in research in metallurgical, ceramic, or polymer materials, in solid state physics or chemistry, or in the biological sciences. Interdisciplinary cooperation among scientists has led to many important discoveries.CHAPTER 2
MINERALS IN ROCKS
Minerals are the constituents of rocks, which make up the entire inorganic, solid portion of the earth. Mineral formation and rock formation are, in fact, one process. To know minerals, therefore, it is important to know rocks. A single mineral may form a rock, but usually rocks are cohesive aggregates of two or more minerals. Depending on how they were formed, rocks are divided into three types: igneous, metamorphic, and sedimentary.
IGNEOUS ROCKS are formed by the cooling and hardening of magma, a complex molten material that originates within the earth. Some important types of igneous rocks are shown in the illustration on the facing page. The major mineral constituents of acid, intermediate, and basic rocks shown provide the basis for the classification given here.
IGNEOUS MINERALS important in the formation of igneous rocks are relatively few in number. This is because the magma from which the minerals crystallize is rich only in certain elements: silicon, oxygen, aluminum, sodium, potassium, calcium, iron, and magnesium. These are the elements that combine and form the silicate minerals. A limited number of the silicates — the olivines, pyroxenes, amphiboles, micas, feldspars, and quartz — account for over 90 percent of all igneous rocks.
As magma cools, minerals crystallize at different temperatures. Olivine and calcium feldspar form at high temperatures and may separate early from the melt. Other minerals solidify as the temperature falls (see Bowen's Reaction Series). The last to crystallize are potassium feldspar, muscovite mica, and quartz, the major constituents of granite. Finally, water in the magma, carrying valuable metals and sulfur in solution, moves outward through fractures in the surrounding rock and deposits sulfides in veins. The water is also important because it affects the temperature at which crystallization occurs and the types of minerals formed during cooling.
INTRUSIVE IGNEOUS ROCKS, also called plutonic rocks, crystallize from magma that cools and hardens within the earth. Surrounded by pre-existing rock, called country rock, the magma cools slowly. As a result, these rocks are coarse-grained.
Central cores of major mountain ranges consist of large masses of plutonic rock, generally granite, intruded as a part of the mountain-building process. When exposed by erosion, these cores, called batholiths, may occupy millions of square miles of surface area. Offshoots of batholiths bear different names, such as laccoliths and sills, depending on their size and their relationship to the country rock. The term abyssal is commonly used to describe coarse-grained rocks formed at depth; hypabyssal is used to describe intrusive rocks formed near the surface.
EXTRUSIVE IGNEOUS ROCKS, also called volcanic rocks, are formed at the earth's surface as a result of volcanic activity. Likebatholith formation, this activity is associated with mountain-building forces within the earth. Temperatures only a few miles beneath the earth's surface are higher than the temperatures at which most rocks would melt at the surface. The below-surface rocks remain solid, however, because of the pressure exerted by overlying rocks. If the rocks fracture — as the result of mountain-building forces, for example — the pressure may be released, and a sizable volume of rock will melt. The resulting magma will be forced through the fractures to the surface, forming a volcano.
Molten rock, or lava, will flow from the volcano and spread onto the ground. Because the lava cools and crystallizes rapidly, it is fine-grained. Material may be blown violently from the volcanic pipe as blocks, pellets, and dust, or as a liquid that hardens before it falls to the surface. These pyroclastics may fall nearby, forming part of the volcano, or may be spread great distances by winds.
CLASSIFICATION of the many and greatly different kinds of igneous rocks can provide important information as to the conditions of formation. Two obvious variables that may be used as criteria for classification are particle size, which depends largely on cooling history, and composition, both chemical and mineralogical. Because feldspars, quartz, olivines, pyroxenes, amphiboles, and micas are the important minerals in the formation of igneous rocks, they are basic to the classification of those rocks. All other minerals are nonessential (accessory).
In the simplified classification on the opposite page, rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz, and, in rocks with no feldspar or quartz, the type of iron and magnesium minerals present. Rocks with crystals large enough to be seen by the eye are called phaneritic; those with crystals too small to be seen are called aphanitic. In general, phaneritic implies an intrusive origin; aphanitic, an extrusive origin. Porphyritic refers to crystals embedded in a fine-grained rock. More detailed classifications using these terms are given in geology and petrology texts.
GRANITES show evidence of being the result of either igneous or metamorphic processes. Some granites obviously have crystallized from a melt; blocks of partially assimilated country rock may be found in granite, clearly indicating that the country rock fell into a liquid magma that hardened around it. Other granites, however, bear evidence of having been formed by metamorphism; variations in composition of pre-existing sedimentary rocks are reflected in banding preserved in the granite. The conversion of sedimentary rock to granite by metamorphism is called granitization.CHAPTER 3
Rocks formed under one set of temperature, pressure, and chemical conditions and then exposed to a different set of these conditions may undergo structural and chemical changes, without melting, that produce rocks with different textures and new minerals. This process is known as metamorphism (change in form). Metamorphic rocks are formed deep beneath the earth's surface by the great stresses and high pressures and temperatures associated with mountain building. They are also formed by the intrusion of magma into rock, particularly at the place of contact where the temperatures are high. The study of metamorphic rocks provides valuable information about temperatures and pressures at great depths. Laboratory studies of the stabilities of minerals at temperatures and pressure similar to those within the earth are essential.
METAMORPHIC MINERALS form only at the high temperatures and pressures associated with metamorphism. Among these are kyanite, staurolite, sillimanite, andalusite, and some garnets. Other minerals — the olivines, pyroxenes, amphiboles, micas, feldspars, and quartz — may be found in metamorphic rocks, but are not necessarily the result of metamorphism. These minerals, formed during crystallization of igneous rocks, are stable at high temperatures and pressures and may remain unchanged during metamorphism of the rock. All minerals, however, are stable only within certain limits of pressure and temperature. Thus the presence of some minerals in rocks indicates the approximate temperatures and pressures at which the rocks were formed.
RECRYSTALLIZATION is the change in particle size of minerals during metamorphism. Small gray calcite crystals in limestone, for example, change to large white crystals in marble. Both temperature and pressure contribute to recrystallization. High temperatures allow the atoms and ions in solid crystals to migrate, thus reorganizing the crystals. High pressures cause solution of crystals at their contacts and deposition in the pore spaces between them.
FOLIATION is a layering in metamorphic rock. It occurs when a strong compressive force is applied from one direction to a recrystalling rock. This causes the platy or long crystals of such minerals as mica and tourmaline to grow with their long axes perpendicular to the direction of the force. The result is a banded, or foliated, rock, the bands showing the colors of the minerals that form them. Rocks subjected to uniform pressure from all sides or lacking minerals with distinctive growth habits will not be foliated. Slate is a very fine-grained foliate. Phyllite is a coarse foliate, schist coarser, and gneiss very coarse. Marble is commonly a nonfoliate.
SOLID-STATE REACTION is one of the important mechanisms of metamorphism. It is a chemical reaction between two minerals without either of them melting. In the process atoms are exchanged between the minerals, and new minerals are formed. Consider the minerals quartz and calcite. Each is stable alone at high temperatures. Together in a siliceous limestone, they do not change at low temperatures, but at high temperatures they react with one another and form the metamorphic mineral wollastonite. The chemical equation of the reaction is: SiO(quartz, solid) + CaCO (calcite, solid) CaSiO (wollastonite, solid) + CO (carbon dioxide, gas). Many complex high-temperature reactions take place among minerals, and each mineral assemblage produced is a clue to the temperature and pressure at the time of metamorphism.
METASOMATISM is a drastic change in the bulk chemical composition of a rock that often occurs during metamorphism. It is due to the introduction of chemicals from other rocks. Water can transport these chemicals rapidly over great distances. Because of the role played by water, metamorphic rocks generally contain many elements that were absent from the original rock and lack some that were originally present. The introduction of new chemicals is not necessary for recrystallization and solid-state reaction to take place, but it does speed up metamorphic processes.
CONTACT METAMORPHISM describes the chemical changes that take place when magma is injected into cold rock (country rock). These changes in the rock are greatest wherever the magma comes in contact with it, for temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock formed by the cooling of the magma is a metamorphosed zone called a contact metamorphic aureole (halo). Aureoles are important in the study of metamorphism because a single rock type may show all degrees of metamorphism from the contact area to the unmetamorphosed country rock some distance away. Formation of important ore minerals may occur by metasomatism at or near the contact; limestone is particularly susceptible to this type of mineralization.
REGIONAL METAMORPHISM, in contrast to contact metamorphism, involves changes in great masses of rock over wide areas. The high temperatures and pressures in the depths of the earth are the cause. If the resulting metamorphosed rocks are uplifted and exposed by erosion, they may cover many thousands of square miles. Their mineralogy and texture provide important information about mountain building and earth processes. The metamorphism, however, destroys features that would have revealed the rock's previous history. Recrystallization destroys fossils and sedimentary textures; solid-state reaction and metasomatism change the original compositions.CHAPTER 4
All rocks disintegrate slowly as a result of mechanical and chemical weathering. Rock particles — in the form of clay, silt, sand, and gravel — and dissolved materials are transported by the agents of erosion (water, ice, and wind) to new locations, generally at lower elevations, and deposited in layers. The deposited particles eventually become cemented together, forming clastic sedimentary rocks. The dissolved materials may precipitate as crystals that accumulate in layers in oceans and lakes and are cemented to form chemical sedimentary rocks.
Sedimentary rocks provide abundant information about the most recent half-billion years of the earth's history. They contain in fossil form the preserved remains of evidences of ancient animals and plants. The manner in which particles of sediment are worn and deposited, the relationships of the different layers, the color and composition, the presence of ripple marks or raindrop impressions — these are among the features that enable geologists to reconstruct ancient landscapes and climates as well as the general sequence of geologic events.
MECHANICAL WEATHERING is the breakdown of rock into particles without changing the identities of the minerals in the rock. Ice is the most important agent of mechanical weathering. Water percolates into cracks and pore spaces, freezes, and expands. The force exerted is sufficient to widen cracks and break off pieces, in time disintegrating the rock. Heating and cooling of the rock, with resulting expansion and contraction, also helps. Mechanical weathering contributes further to the breakdown of rock by increasing the surface area exposed to chemical agents. The breakdown of rocks and erosion of the fragments has been greatly accelerated over the past several centuries by the activities of man through farming and construction.
Excerpted from Rocks and Minerals by Charles A. Sorrell, George F. Sandström. Copyright © 1973 St. Martin's Press. Excerpted by permission of St. Martin's Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.