Since most rocks contain minerals, some knowledge of minerals is necessary to identify rocks. Because minerals are chemicals, they have special properties which aid in their recognition. Minerals are easily identified by chemical analysis.
One of the properties of minerals which depends on their chemical composition is the specific gravity or relative weight of the minerals. When molecules are packed together with a minimum of waste space, as in the metals, the mineral weighs more. The specific gravity of minerals is compared to water, which has a specific gravity of 1. Common minerals range from 1.7 specific gravity, for borax, to 19.3, for gold.
Most minerals also have a distinct crystal form. This, in turn, depends on the arrangement of the molecules in each mineral. Mineral crystals fall into six systems, and these can be identified by the angles of the crystal. Even a small fragment of a crystal is enough to give a clue to its structure and its crystal form.
The way a mineral breaks in flat planes is called its cleavage. This, too, can be used in identification. Mica is an example of perfect cleavage. Minerals also break in an irregular way. This kind of breakage is called fracture and it, too, helps to identify a mineral.
All minerals have a definite hardness, which is the minerals ability to scratch or be scratched. Hardness is generally measured on an arbitrary scale of 10.
The color of minerals is not important in identification because the color may be due to impurities or surface changes. Streak is the color of a powdered mineral, and luster isthe way the structure of a mineral reflects or breaks up light. Besides these properties, certain minerals respond to ultra-violet light and give off brilliant colors. This fluorescence is also used in identification. Other minerals are magnetic. Some have electrical radioactive properties. These and many other properties of minerals help identify them in the field and in the laboratory.
The rock-forming minerals are a group of little importance as gems or as sources of metal. But they have great importance in the overall history of the earth. The rock-forming minerals are the ones which make our land on which we live.
Of all the rock-forming minerals, the simplest and most widespread is the mineral quartz-silicon dioxide. Quartz occurs in many forms, some of them are beautifully colored. These are used as gems. Ordinary quartz is a colorless, glassy mineral which may form a sixsided crystal. It breaks in the same kind of shell-like surface you find in broken glass. Large crystals of quartz are rare and are valued for their use in radio and electronics. Crystalline quartz is found in rocks which were once melted, though this kind seldom forms good crystals.
Under certain conditions quartz will dissolve in alkali water and will reform as a noncrystalline quartz. These forms of quartz are called agate, onyx or chalcedony. Crystalline quartz is the usual rock-forming mineral. Noncrystalline quartz is not.
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Gypsum, calcite, dolomite, and halite (rock salt) are occasionally rock-forming minerals too, but, by and large, the rest of the rock-forming minerals are silicate minerals. Probably the most important of the rock-forming minerals are the feldspars. This is a difficult family of minerals to understand because they grade off one into the other, and are hard to tell apart. All feldspars contain aluminium, silicon and oxygen. They also contain one or two metals such as sodium, calcium and potassium. In a general way, potash or potassium feldspars are put into one group, and soda or sodium feldspars are put with the calcium feldspars into another group.
Not only the amount of feldspar but the kind of feldspar is important in rock identification, especially of those which were once melted. Most are light colored white, pinkish, orange or pale blue. Feldspar is used to manufacture glaze and enamel for pottery. When feldspars break down they form clay, another important rock.
Micas are better known than feldspars because the "books" of mica can be peeled into flat, thin sheets. This has made mica useful in electrical insulation. Micas nearly always occur in rocks which have been heated, squeezed or folded. They too are silicate minerals. Some varieties contain iron. In a general way they are made of the same elements as feldspars-silicon and oxygen plus metals such as potassium, sodium, magnesium and lithium.
Many rocks consist of mica or some other dark mineral combined with feldspar and quartz, two light minerals. In addition to mica, the two best known dark minerals are the amphiboles and the pyroxenes. These are also silicates. Hornblende is a common, dark green amphibole. Augite is a similar-looking pyroxene. They are easily confused but the cleavage angles are a very good way to tell them apart. Another family of the rock-forming minerals are the zeolites, a group comprising two dozen minerals which are chemically similar to feldspars. Most zeolites are soft, light minerals. Some have attractive crystal forms.
Garnets, which people often think of as gems, are common enough to be a rock-forming mineral. They too are silicate minerals, usually containing two metals. Garnet crystals often form with 12, 24, 36, or 48 faces. Because garnets are hard they are used in making sandpaper.
Other less important rock-forming minerals should be mentioned. There is olivine, a green silicate containing magnesium and iron; chlorite, a darker green mineral, and serpentine, which is mainly magnesium silicate. Talc, from which talcum powder is made, is one of the serpentine forms.
The way minerals form rocks is a complicated process. It involves chemical reactions at high temperatures and pressures. These different conditions, which may occur within or beneath the crust of the earth, produce a variety of rocks. While these rocks are quite alike chemically, they differ greatly in their physical and mineral characteristics. All minerals are found in rocks. Diamonds are found only in a volcanic rock called kimberlite. Other minerals, like quartz and calcite, may be found in many different rocks. The chance of finding gold in limestone is practically zero, but the chance of finding it in rocks which were once melted is much greater.
(5280)
NOTES:
* the specific gravity ;
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* borax - ;
* alkali - ;
* potassium - ;
* amphiboles and pyroxenes - .
Text 6. MINERALOGY
Mineralogy is the identification of minerals and the study of their properties, origin, and classification. The properties of minerals are studied under the convenient subdivisions of chemical mineralogy, physical mineralogy, and crystallography. The properties and classification of individual minerals, their localities and modes of occurrence, and their uses are studied under descriptive mineralogy. Identification according to chemical, physical, and crystallographic properties is called determinative mineralogy.
Chemical mineralogy. Chemical composition is the most important property for identifying minerals and distinguishing them from one another. Mineral analysis is carried out according to standard qualitative and quantitative methods of chemical analysis. Minerals are classified on the basis of chemical composition and crystal symmetry. The chemical constituents of minerals may also be determined by electron-beam microprobe analysis.
The various classes of chemical compounds that include a majority of minerals are as follows: (1) elements, such as gold, graphite, diamond, and sulfur, that occur in the native state, that is, in an uncombined form; (2) sulfides, which are minerals composed of various metals combined with sulfur. Many important ore minerals, such as galena and sphalerite, are in this class; (3) sulfo salts, minerals composed of lead, copper, or silver in combination with sulfur and one or more of the following: antimony, arsenic, and bismuth; (4) oxides, minerals composed of a metal in combination with oxygen, such as hematite. Mineral oxides that contain water, such as diaspore, or the hydroxyl (OH) group, such as bog iron ore, FeO(OH), also belong to this group; (5) halides, composed of metals in combination with chlorine, fluorine, bromine, or iodine; halite, NaCl, is the most common mineral of this class; (6) carbonates, minerals such as calcite, containing a carbonate group; (7) phosphates, minerals such as apatite, Ca5(F,Cl)(PO4)3, that contain a phosphate group; (8) sulfates, minerals such as barite, BaSO4, containing a sulfate group; and (9) silicates, the largest class of minerals, containing various elements in combination with silicon and oxygen, often with complex chemical structure, and minerals composed solely of silicon and oxygen (silica). The silicates include the minerals comprising the feldspar, mica, pyroxene, quartz, and zeolite and amphibole families.
Physical mineralogy. The physical properties of minerals are important aids in identifying and characterizing them. Most of the physical properties can be recognized at sight or determined by simple tests. The most important properties include powder (streak), color, cleavage, fracture, hardness, luster, specific gravity, and fluorescence or phosphorescence.
Crystallography. The majority of minerals occur in crystal form when conditions of formation are favorable. Crystallography is the study of the growth, shape, and geometric character of crystals. The arrangement of atoms within a crystal is determined by X-ray diffraction analysis. Crystal chemistry is the study of the relationship of chemical composition, arrangement of atoms, and the binding forces among atoms. This relationship determines minerals chemical and physical properties. Crystals are grouped into six main classes of symmetry: isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic.
(2950)
NOTES:
* antimony - ;
* streak , , , .
Text 7. MINERAL DEPOSITS
Mineral deposit is the concentrated, natural occurrence of one or more minerals. Mineral deposits can form within any kind of rock and consist of any type of mineral. They are valuable economically because they contain high concentrations of metallic and nonmetallic elements or other valuable materials that are essential to an industrial society.
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The concentration of a mineral in a mineral deposit is important in determining whether it can be mined profitably. For the mining of metals, concentration in a mineral deposit is measured two ways. The grade depends on the percentage by weight of a metal in a mineral deposit. This percentage is measured by dividing the weight of the metal by the weight of the rock. The concentration factor (also called enrichment factor) is the number of times more abundant a metal is in a mineral deposit than it is in average crustal rock. The concentration factor is measured by dividing a mineral deposits grade by the average grade of crustal rocks for that metal. A concentration factor of ten, for example, means that a metal is ten times more abundant in a particular deposit than in the earths crust.
If a metal is to be mined profitably, it must have attained a minimum concentration factor - otherwise, the amount of that metal acquired will be too small to pay for the mining process. Minimum concentration factors vary from one metal to the next. Iron, which is relatively abundant in the earths crust, typically requires a concentration factor of between 5 and 10. Gold and silver, however, require concentration factors in excess of 2,000.
The accessibility of a mineral deposit also plays an important role in determining the cost-effectiveness of mining. In general, deposits that reside deeper in the crust are more difficult and more expensive to mine. Consequently, the minimum required concentration factor increases with the difficulty of extraction.
(1620)
Text 8. GEOLOGY
Geology is the study of the planet earth, its rocky exterior, its history, and the processes that act upon it. Geology is also referred to as earth science and geoscience. The word geology comes from the Greek geo, "earth," and logia, "the study of." Geologists seek to understand how the earth formed and evolved into what it is today, as well as what made the earth capable of supporting life. Geologists study the changes that the earth has undergone as its physical, chemical, and biological systems have interacted during its 4.5 billion year history.
Scientists use geology to understand how geological events and earths geological history affect people, for example, in terms of living with natural disasters and using the earths natural resources. As the human population grows, more and more people live in areas exposed to natural geologic hazards, such as floods, earthquakes, tsunamis, volcanoes, and landslides. Some geologists use their knowledge to try to understand these natural hazards and forecast potential geologic events, such as volcanic eruptions or earthquakes. They also study the geologic record of climate change in order to help predict future changes. As human population grows, geologists ability to locate fossil and mineral resources, such as oil, coal, iron, and aluminum, becomes more important. Finding and maintaining a clean water supply, and disposing safely of waste products, requires understanding the earths systems through which they cycle.
The field of geology includes subfields that examine all of the earths systems, from the deep interior core to the outer atmosphere, including the hydrosphere (the waters of the earth) and the biosphere (the living component of earth). Generally, these subfields are divided into the two major categories of physical and historical geology.
Many other scientific fields overlap extensively with geology, including oceanography, atmospheric sciences, physics, chemistry, botany, zoology, and microbiology. Geology is also used to study other planets and moons in our solar system. Specialized fields of extraterrestrial geology include lunar geology, the study of earths moon, and astrogeology, the study of other rocky bodies in the solar system and beyond. Scientific teams currently studying Mars and the moons of Jupiter include geologists.
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(1990)
NOTES:
* core , , ;
* extraterrestrial geology - ;
* lunar geology - .
Text 9. GEOPHYSICS
Geophysics is a branch of science that applies physical principles to the study of the earth. Geophysicists examine physical phenomena and their relationships within the earth; such phenomena include the earths magnetic field, heat flow, the propagation of seismic (earthquake) waves, and the force of gravity. The scope of geophysics also broadly includes outer-space phenomena that influence the earth; the effects of the sun on the earths magnetic field; and manifestations of cosmic radiation and the solar wind.
This discipline embraces all fields devoted to researching the earths interior, atmosphere, hydrosphere (waters), and ionosphere (ionized upper atmosphere). Related fields are included in the following descriptions.
Solid Earth Physics. Embracing all fields devoted to the earths interior, solid earth physics involves studying the behavior of earth materials from the crust down to the core, particularly as they relate to the earths size and shape, gravity, magnetism, and seismicity. The specialized field of geodesy is concerned with determining the earths size and shape and locating precise points on its surface.
Terrestrial Magnetism. Geomagnetism refers to the study of magnetic phenomena exhibited by the earth and its atmosphere. Generation of the magnetic field seems to be related to the motion of fluid, electrically conducting material within the earth, so that the planet acts as a self-exciting dynamo. The conducting material and the geomagnetic field may mutually control each other. Study of this problem is known as magnetohydrodynamics or hydromagnetics. The study of how the magnetic field has changed throughout the earths history, called paleomagnetism, provided the first strong evidence for the theory of plate tectonics.
Gravity and Tides. Gravity is the attractive force exerted by the mass of the earth. The gradient of the gravitational potential - that is, the force of gravityis perpendicular to the surface of the earth, which means that the force acts in the vertical direction. Gravimeters are highly sensitive balances used to make relative gravity measurements.
The rotation of the earth in the gravity fields of the moon and sun imposes periodicities in the gravitational potential at any point on the earths surface. Tides are the most obvious effect; in addition to marine tides, solid earth tides occur as slight crustal deformations.
Seismology. Understanding of global seismic activity became possible with the recognition that major earthquakes are triggered by movement of the earths tectonic plates. In addition, much of what we are able to surmise about the earths mantle and core has been gained by studying the passage of earthquake waves through the center of the earth.
Hydrology. This is the principal science dealing with continental water on and under the earths surface and in the atmosphere.
Volcanology. Volcanologic studies are concerned with the surface eruption of gas-charged magmas from within the earth and with the structures, deposits, and landforms associated with such activity.
Terrestrial Electricity. Static or alternating electric currents that flow through the ground are induced by natural or artificial electric or magnetic fields. Geophysicists have determined from effects of induced currents or geomagnetic variations that, in general, conductivity increases with depth in the mantle.
Atmospheric Phenomena. Physics of the lower atmosphere, where air is dense enough to be subject to the laws of fluid, is the province of meteorology. Phenomena of the upper atmosphere are the subject of aeronomy and magnetospheric physics.
(3100)
NOTES:
core , , ;
alternating electric current - (.) ;
conductivity - (.) .
Text 10. GEOCHEMISTRY
Geochemistry is the application of chemical principles and techniques to geologic studies, to understand how chemical elements are distributed in the crust, mantle, and core of the earth. Over a period of several billion years, chemical differentiation of the earths crust has created vast rafts of silica-rich rocks, the continents, which float on iron- and magnesium-rich rocks of the ocean basins.
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In its emphasis on the chemical composition of earth materials, geochemistry overlaps with several other branches of earth science, notably mineralogy, petrology, and the study of ore deposits. Pioneering work in the field was done early in the 20th century by Scandinavian petrologists such as V. M. Goldschmidt and P. Eskola, who established the principles governing chemical changes that rocks undergo during metamorphism.
Environmental Geochemistry. Among the various branches of earth science, environmental geochemistry is unique in focusing directly on public health issues related to the environment. Trace elements, normally present in minute amounts in rocks, soil, and water, are a major influence on health. Calcium, magnesium, iron, manganese, cobalt, copper, zinc, and molybdenum are all essential to good health. Other trace elements, such as mercury, are toxic; some, such as selenium and fluorine, are beneficial in minute quantities but toxic if concentrated.
The type of bedrock beneath the soil in an area helps determine the kinds of trace elements in the water and vegetation of the area. Geochemical analyses of soil, water, and plants indicate how trace elements are distributed. These findings may have serious health implications, revealing, for example, correlations between trace-element distribution and incidence of cardiovascular disease.
Exploration Geochemistry. Modern methods of exploration geochemistry begin with systematic collection of samples of soil, rock, vegetation, and water. Data obtained by analysis of the samples is now interpreted using computer programs written specifically for this purpose. In current world markets, with the price of most nonferrous metals at an all-time low, exploration for metallic mineral deposits is confined largely to precious metals, and the chief targets of geochemical prospecting are gold and platinum-group metals.
(1980)
NOTES:
* mantle and core of the earth - ;
* manganese - ;
* bedrock - , ;
* cardiovascular disease - .
Text 11. IGNEOUS ROCKS
The large group of rocks - the igneous rocks - are those which were formed from melted or molten materials. Igneous rocks were once magma, a thick, hot liquid deep inside the earth. Since all igneous rocks come from inside the earth, let us take a quick look at what is inside.
The deeper we go into the earth the less is known about its structure. There is some knowledge based on earthquake waves, the behaviour of the earth as a spinning planet, and laboratory experiments with rocks under high pressure. These suggest that the very core of the earth is probably iron or iron alloyed with nickel and cobalt. Pressure on the rock near the earths center equals to about 25,000 tons per square inch. The rock of this rigid inner core - which extends 790 miles out from the center of the earth - is somewhere between 10 and 15 times as dense as water. Surrounding this inner core is another zone some 1,360 miles in thickness. This outer part of the core of the earth also seems to be of dense material, but certain types of earthquake waves do not go through it. Since these earthquake waves travel through solids and not through liquids, it is possible that this outer part of the earths core acts like a dense liquid. The heavy core of the earth is about 4,300 miles in diameter.
Surrounding the core of the earth is a zone or mantle layer close to 1,800 miles thick. This is a solid, rocky layer which may grade into the iron core. The last 20 or 30 miles from the center forms what is called the crust of the earth. This is a term left over from the old days when people imagined that the interior of the earth was a great mass of molten rock and searing flames. A thin crust was thought to surround this fiery interior. Every now and then the crust would crack and puncture to let flames and volcanic rock pour out. Even though this idea about the interior of the earth is false, the term "crust" is still used for the outermost layers of rock.
The crust of the earth contains two distinct types of rocks - forgetting for a moment the soil, sediments, debris, water and ice that coat the surface. The continents are supported by the crystalline sial. Sial is a word made from the abbreviations for silicon and aluminium and it is used because the rocks underlying the continents are rich in these elements combined with oxygen. Sial rocks are light in color and light in weight. They are the rocks that form our great mountain ranges.
Lying underneath the sial and lying directly under the great Pacific Ocean Basin is the sima. This word is made from the abbreviations for silica and magnesium again because these two elements are abundant in the rock. Volcanic lavas are of a silica-magnesium type. They are dark rocks, and are generally heavier than those of the sial. The islands that jut up from the deep Pacific Basin are volcanic ones of the sima type.
A zone of glassy rock is believed to be just beneath the sima at the upper edge of the mantle. This glassy rock melts easily under the great heat and pressure 30 to 40 miles down inside the earth. The presence of this rock zone may occur due to movements below the crust of the earth and to the shifting of rock as mountains are formed and as ocean basins settle.
The melted magma which forms igneous rock seems to have its beginning at least 20 or 30 miles down. Somewhere in this zone the temperature is high enough to melt rock, while at the same time, the pressure is so high that the rock transmits earthquake waves and acts like a solid.
Earth movements relieving strains and pressures in the crust create zones of weakness or actual breaks. These permit some of the magma to find its way up into the crust either through cracks or by dissolving the weakened rock around it. Sometimes magma moves to the surface, spewing out of volcanoes or spreading over the countryside in huge lava flows. Lava is only one type of igneous rock, but it is probably the best known. Most magma cools well below the surface of the earth. Under these conditions it cools very slowly.
Inside the crust of the earth, magma may flow into branching cracks forming veins. It may cut across layers of rock forming great sheet-like dikes. When magma flows between layers it forces the rock apart. Such an intrusion is known as a sill. Sills may be anywhere from a few inches to hundreds of feet in thickness.
Sometimes intrusive rock, forced between layers, will raise the upper layers like a blister. Such blisters a few miles or so across are called laccoliths. Larger intrusive blisters may cover thousands of square miles.
Magma that intrudes or pushes into other rock cools beneath the surface of the earth and hence cools more slowly. Minerals separate out and crystals develop. Shrinkage may split the cooling rock into huge regular columns. Millions of years later the rocks above may be worn down and the igneous rocks are exposed at the surface. Then these hidden structures can be studied and the valuable minerals in or near them can be mined.
When magma does reach the earths surface it cools much more rapidly. The rock it forms is then called an extrusive rock because it is pushed out into the surface. The cooling of extrusive rock may be so fast that magma does not form mineral at all, but a kind of natural glass or obsidian. This natural glass, usually dark brown or black, is almost exactly the same as the glass used for window-glasses or bottles. Indians prized it for arrows and spearheads. It is sometimes used for simple jewelry.
Magma may contain a great deal of gas. As it reaches the surface this gas escapes, causing the magma to bubble and froth as the rock cools. When there are so many gas bubbles that the natural glass is whipped into a froth, the rock is called pumice - a rock usually light in color and so light in weight that it will float on water. When the gas bubbles are larger, the volcanic rock looks like coarse cinders. Dark, heavy basalt is one of the most abundant lavas, but there are also light colored lavas rich in silica. Some lavas, thrown high in the air, cool as they fall, forming rounded or twisted volcanic bombs.
Igneous rocks are important to us because of the rich mineral deposits in them or in veins which are found in them.
From such veins we get most of our gold, lead, zinc, mercury, arsenic, antimony, nickel, cobalt and titanium.
Igneous rocks were the first kind of rocks to form. Some are known to be over two billion years old. At the same time, some other igneous rocks are the youngest rocks, for there are active volcanoes still spewing lava from their craters this very day. Igneous rocks, more than any other kind, offer proof that the earth is still growing, changing and constantly rebuilding its mountains and hillsides.
(5550)
NOTES:
fiery , ;
debris , , ;
sial , ;
sima , , , ;
blister , ;
shrinkage , , , ;
pumice - ;
cinder , ;
silica , ;
antimony - .
Text 12. INTRUSION
Intrusion, in geology, is a body of igneous rock that has cooled and crystallized beneath the earths surface. Intrusions form when rock in the crust or mantle melts to become magma. This magma is less dense than the surrounding rock and therefore rises, or intrudes, seeping into overlying nonmelted rock. It then slowly cools. As a result of this slow cooling, the magma solidifies into crystals that are often large enough to see with the unaided eye. Magma that reaches the earths surface and solidifies there, on the other hand, forms volcanic rock, or extrusions. Extrusions cool rapidly and therefore tend to contain crystals too small to see with the unaided eye, or they are amorphous with no crystal structure. Intrusions also differ from veins, which form when minerals precipitate from hydrothermal solutions.
An intrusions size, shape, and relationship to the surrounding rock determine what type of intrusion it is. Dikes and sills are slablike, generally tabular, intrusions. Dikes cut across and sills form parallel to the surrounding rock layering.
Volcanic necks are another type of intrusion. They are the preserved remains of magma pipelines from volcanic eruptions. They form nearly vertical, cylindrical intrusions.
Batholiths are especially large intrusions, with volumes on the order of 100 cu km or greater. Most batholiths actually consist of numerous smaller intrusions that formed at deep levels in the crust.
The rock surrounding an intrusion is called country rock. Changes in country rock, called contact metamorphism, occur because the country rock becomes heated when it comes into contact with the hot, intruding magma. The most visible changes occur in sedimentary country rock, closest to the intrusion. The heating there causes sandstone, limestone, and shale to change into the metamorphic rocks - quartzite, marble, and hornfels, respectively. New minerals, such as pyroxene or garnet, may also form in the country rock. Mineral deposits, called skarns, may develop in limestone country rock where hot mineral-rich water or gases given off by the magma invade and react with the limestone. In general, contact metamorphic effects increase as the temperature, size, and water content of the intruding magma body increases and as the original temperature of the country rock decreases.
(1970)
NOTES:
intrusion - (), ;
extrusion - , ;
to precipitate - , ;
volcanic neck - (), ;
country rock - , .
