SEM-1 MINERALOGY
UNIT -1 INTRODUCTION OF MINERALOGY
Mineralogy - Minerals and history, Branches of mineralogy
chemical crystal properties physical
Mineralogy is the branch of geology concerned with the study of minerals. A mineral is a naturally occurring, homogeneous solid with a definite chemical composition and a highly ordered atomic structure. A homogeneous substance is one that can be divided into repeating units that are exactly the same. A mineral, by definition, cannot be a liquid or a gas. The chemical composition of a mineral is definite, meaning a particular mineral is always composed of the same ratio of elements, and this composition can be shown using a chemical formula. The atoms in a mineral are arranged in a highly ordered fashion, called a crystal lattice structure.
Minerals have been an important part of our society since the time of prehistoric man. Early humans carved tools out of minerals such as quartz. Pottery has been made of various clays since ancient times. Sodium chloride, also known as the mineral halite, has been used in food preservation techniques for millions of years. Mining of useful minerals out of ores became widespread hundreds of years ago, a practice still in use today.
Mineralogy is the branch of geology concerned with the study of minerals. A mineral is a naturally occurring, homogeneous solid with a definite chemical composition and a highly ordered atomic structure. A homogeneous substance is one that can be divided into repeating units that are exactly the same. A mineral, by definition, cannot be a liquid or a gas. The chemical composition of a mineral is definite, meaning a particular mineral is always composed of the same ratio of elements, and this composition can be shown using a chemical formula. The atoms in a mineral are arranged in a highly ordered fashion, called a crystal lattice structure.
Minerals have been an important part of our society since the time of prehistoric man. Early humans carved tools out of minerals such as quartz. Pottery has been made of various clays since ancient times. Sodium chloride, also known as the mineral halite, has been used in food preservation techniques for millions of years. Mining of useful minerals out of ores became widespread hundreds of years ago, a practice still in use today.
Crystallography
There are several different branches of mineralogy. Mineralogists can focus on very specific studies, from crystal structure to classification or chemical composition. Crystallography, for example, is the study of the crystal lattice structure of minerals. As mentioned above, the atoms in a mineral are arranged in a highly ordered fashion. This ordered arrangement produces crystals of definite size and shape. A particular mineral sample is made up of repeating crystal units. Each crystal that makes up the mineral has the same shape. There are six basic shapes a mineral crystal can have. The shape of the crystal, as well as how tightly packed the atoms are in the crystal, help determine the physical properties of the mineral. Crystals that are allowed to grow with plenty of open space will form nearly perfect structures, and those that form in more cramped conditions will display imperfections in the crystal shape.
There are several different branches of mineralogy. Mineralogists can focus on very specific studies, from crystal structure to classification or chemical composition. Crystallography, for example, is the study of the crystal lattice structure of minerals. As mentioned above, the atoms in a mineral are arranged in a highly ordered fashion. This ordered arrangement produces crystals of definite size and shape. A particular mineral sample is made up of repeating crystal units. Each crystal that makes up the mineral has the same shape. There are six basic shapes a mineral crystal can have. The shape of the crystal, as well as how tightly packed the atoms are in the crystal, help determine the physical properties of the mineral. Crystals that are allowed to grow with plenty of open space will form nearly perfect structures, and those that form in more cramped conditions will display imperfections in the crystal shape.
Crystal and conformational chemistry
Crystal chemistry is the branch of mineralogy that deals with how the chemical composition of a mineral relates to its crystal structure. The chemical bonds formed between atoms determine the crystal shape as well as the chemical and physical properties of the mineral. There are three different types of chemical bonds present in minerals—ionic, covalent, and metallic. In ionic bonding, an atom with a positive charge binds to an atom with a negative charge through electrostatic attraction. Minerals with ionic bonds tend to be poor conductors of heat and electricity, have low melting points, and are brittle. Halite and fluorite are both minerals formed by ionic bonds. In covalent bonding, electrons are shared between two atoms. This type of bonding is stronger than ionic bonding, which means minerals with covalent bonds have higher melting points and are harder than those with ionic bonds. These minerals are also poor conductors of heat and electricity and are brittle. Examples of covalently bonded minerals include quartz and diamond. Metallic bonding occurs between atoms of metals. In this type of bond, the outer electrons of the atom are free to move, and are shared between all of the other atoms in the substance. This special structure is the reason metals are good conductors of heat and electricity, are malleable, soft, and have lower melting points. Copper, silver, and gold are all minerals formed by metallic bonding.
Crystal chemistry is the branch of mineralogy that deals with how the chemical composition of a mineral relates to its crystal structure. The chemical bonds formed between atoms determine the crystal shape as well as the chemical and physical properties of the mineral. There are three different types of chemical bonds present in minerals—ionic, covalent, and metallic. In ionic bonding, an atom with a positive charge binds to an atom with a negative charge through electrostatic attraction. Minerals with ionic bonds tend to be poor conductors of heat and electricity, have low melting points, and are brittle. Halite and fluorite are both minerals formed by ionic bonds. In covalent bonding, electrons are shared between two atoms. This type of bonding is stronger than ionic bonding, which means minerals with covalent bonds have higher melting points and are harder than those with ionic bonds. These minerals are also poor conductors of heat and electricity and are brittle. Examples of covalently bonded minerals include quartz and diamond. Metallic bonding occurs between atoms of metals. In this type of bond, the outer electrons of the atom are free to move, and are shared between all of the other atoms in the substance. This special structure is the reason metals are good conductors of heat and electricity, are malleable, soft, and have lower melting points. Copper, silver, and gold are all minerals formed by metallic bonding.
Physical mineralogy
Physical mineralogy is concerned with the physical properties and descriptions of minerals. Minerals can be described using several physical attributes, including hardness, specific gravity, luster, color, streak, and cleavage.
The hardness of a mineral can be determined by a scratch test. The scratch test establishes how easily a mark can be made on a mineral sample using different materials. If a mark is made easily, the mineral is not very hard. If no mark can be made, then the mineral is quite hard. The hardness is then measured on a scale of 1-10, called Mohs' hardness scale, named after the Austrian scientist F. Mohs, who developed this procedure. If a fingernail can scratch a particular mineral, it would have a hardness of 2.5. If a penny can scratch it, its hardness is around 3. If a mineral can be scratched by glass, its hardness is 5.5. If it can be scratched by unglazed porcelain, it has a hardness between 6 and 6.5, and if a steel file can leave a mark, it has a hardness of 6-7. Talc is the softest mineral with a hardness rating of 1, while diamond is the hardest, rated 10.
The luster of a mineral is the appearance of its surface when light is reflected off of it. Minerals can have metallic or nonmetallic luster. Minerals with metallic luster look shiny like a metal. Nonmetallic minerals can have various appearances, such as vitreous (glassy), greasy, silky, brilliant (like a diamond), or pearly.
The color of a mineral sample cannot be used to definitively identify the mineral because of impurities that may be present, however, the color can narrow down the identity of a mineral to a few choices. The streak of a mineral is the color of its powdered form. Rubbing the mineral across an unglazed porcelain square, called a streak plate, can best show streak color. A mineral will have a characteristic streak color, although more than one mineral may have the same color. Therefore, streak is not a definitive identification tool, although it may be used to verify the identity of a mineral of suspected composition.
A mineral exhibits cleavage when it breaks along a certain direction or plane, producing a flat surface along the break. When a mineral shatters, rather than breaks along planes, it exhibits fracture. Cleavage is characteristic of particular minerals such as feldspar, while minerals such as quartz show fracture. Each of these physical properties can be used to determine the chemical identity of an unknown mineral, and together are the focus of the branch of mineralogy called physical mineralogy.
Physical mineralogy is concerned with the physical properties and descriptions of minerals. Minerals can be described using several physical attributes, including hardness, specific gravity, luster, color, streak, and cleavage.
The hardness of a mineral can be determined by a scratch test. The scratch test establishes how easily a mark can be made on a mineral sample using different materials. If a mark is made easily, the mineral is not very hard. If no mark can be made, then the mineral is quite hard. The hardness is then measured on a scale of 1-10, called Mohs' hardness scale, named after the Austrian scientist F. Mohs, who developed this procedure. If a fingernail can scratch a particular mineral, it would have a hardness of 2.5. If a penny can scratch it, its hardness is around 3. If a mineral can be scratched by glass, its hardness is 5.5. If it can be scratched by unglazed porcelain, it has a hardness between 6 and 6.5, and if a steel file can leave a mark, it has a hardness of 6-7. Talc is the softest mineral with a hardness rating of 1, while diamond is the hardest, rated 10.
The luster of a mineral is the appearance of its surface when light is reflected off of it. Minerals can have metallic or nonmetallic luster. Minerals with metallic luster look shiny like a metal. Nonmetallic minerals can have various appearances, such as vitreous (glassy), greasy, silky, brilliant (like a diamond), or pearly.
The color of a mineral sample cannot be used to definitively identify the mineral because of impurities that may be present, however, the color can narrow down the identity of a mineral to a few choices. The streak of a mineral is the color of its powdered form. Rubbing the mineral across an unglazed porcelain square, called a streak plate, can best show streak color. A mineral will have a characteristic streak color, although more than one mineral may have the same color. Therefore, streak is not a definitive identification tool, although it may be used to verify the identity of a mineral of suspected composition.
A mineral exhibits cleavage when it breaks along a certain direction or plane, producing a flat surface along the break. When a mineral shatters, rather than breaks along planes, it exhibits fracture. Cleavage is characteristic of particular minerals such as feldspar, while minerals such as quartz show fracture. Each of these physical properties can be used to determine the chemical identity of an unknown mineral, and together are the focus of the branch of mineralogy called physical mineralogy.
Other branches
Descriptive mineralogists use the properties discussed in physical mineralogy to name and classify new minerals. Determinative mineralogy is the branch of mineralogy that deals with identifying unknown minerals, also using the physical properties of minerals. Other branches of mineralogy include chemical mineralogy (identifying minerals to determine the chemical composition of the earth's crust), optical mineralogy (using light to determine the crystal structure of minerals), xray mineralogy (using x-ray diffraction techniques to determine the crystal structure of minerals), and economic mineralogy (the study of new, economically important uses for minerals). All of the branches of mineralogy together describe the physical and chemical properties of minerals and their uses.
Mineralogy is an important discipline for several reasons. For one, the study of the composition of the earth's crust gives scientists an idea of how Earth was formed. The discovery of new minerals could provide useful materials for industry. The study of the chemical properties of minerals could lead to the discovery of new uses for Earth's mineral resources. Mining ores for their mineral components provides the materials for lasers, buildings, and jewelry. Each of the branches of mineralogy contributes to the indispensable knowledge base of minerals and their uses.
Descriptive mineralogists use the properties discussed in physical mineralogy to name and classify new minerals. Determinative mineralogy is the branch of mineralogy that deals with identifying unknown minerals, also using the physical properties of minerals. Other branches of mineralogy include chemical mineralogy (identifying minerals to determine the chemical composition of the earth's crust), optical mineralogy (using light to determine the crystal structure of minerals), xray mineralogy (using x-ray diffraction techniques to determine the crystal structure of minerals), and economic mineralogy (the study of new, economically important uses for minerals). All of the branches of mineralogy together describe the physical and chemical properties of minerals and their uses.
Mineralogy is an important discipline for several reasons. For one, the study of the composition of the earth's crust gives scientists an idea of how Earth was formed. The discovery of new minerals could provide useful materials for industry. The study of the chemical properties of minerals could lead to the discovery of new uses for Earth's mineral resources. Mining ores for their mineral components provides the materials for lasers, buildings, and jewelry. Each of the branches of mineralogy contributes to the indispensable knowledge base of minerals and their uses.
Occurrence and formation
Minerals form in all geologic environments and thus under a wide range of chemical and physical conditions, such as varying temperature and pressure. The four main categories of mineral formation are: (1) igneous, or magmatic, in which minerals crystallize from a melt, (2) sedimentary, in which minerals are the result of sedimentation, a process whose raw materials are particles from other rocks that have undergone weathering or erosion, (3) metamorphic, in which new minerals form at the expense of earlier ones owing to the effects of changing—usually increasing—temperature or pressure or both on some existing rock type (metamorphic minerals are the result of new mineral growth in the solid state without the intervention of a melt, as in igneous processes), and (4) hydrothermal, in which minerals are chemically precipitated from hot solutions within the Earth. The first three processes generally lead to varieties of rocks in which different mineral grains are closely intergrown in an interlocking fabric. Hydrothermal solutions, and even solutions at very low temperatures (e.g., groundwater), tend to follow fracture zones in rocks that may provide open spaces for the chemical precipitation of minerals from solution. It is from such open spaces, partially filled by minerals deposited from solutions, that most of the spectacular mineral specimens have been collected. If a mineral that is in the process of growth (as a result of precipitation) is allowed to develop in a free space, it will generally exhibit a well-developed crystal form (see ), which adds to a specimen’s aesthetic beauty. Similarly, geodes, which are rounded, hollow, or partially hollow bodies commonly found in limestones, may contain well-formed crystals lining the central cavity. Geodes form as a result of mineral deposition from solutions such as groundwater.
Minerals form in all geologic environments and thus under a wide range of chemical and physical conditions, such as varying temperature and pressure. The four main categories of mineral formation are: (1) igneous, or magmatic, in which minerals crystallize from a melt, (2) sedimentary, in which minerals are the result of sedimentation, a process whose raw materials are particles from other rocks that have undergone weathering or erosion, (3) metamorphic, in which new minerals form at the expense of earlier ones owing to the effects of changing—usually increasing—temperature or pressure or both on some existing rock type (metamorphic minerals are the result of new mineral growth in the solid state without the intervention of a melt, as in igneous processes), and (4) hydrothermal, in which minerals are chemically precipitated from hot solutions within the Earth. The first three processes generally lead to varieties of rocks in which different mineral grains are closely intergrown in an interlocking fabric. Hydrothermal solutions, and even solutions at very low temperatures (e.g., groundwater), tend to follow fracture zones in rocks that may provide open spaces for the chemical precipitation of minerals from solution. It is from such open spaces, partially filled by minerals deposited from solutions, that most of the spectacular mineral specimens have been collected. If a mineral that is in the process of growth (as a result of precipitation) is allowed to develop in a free space, it will generally exhibit a well-developed crystal form (see ), which adds to a specimen’s aesthetic beauty. Similarly, geodes, which are rounded, hollow, or partially hollow bodies commonly found in limestones, may contain well-formed crystals lining the central cavity. Geodes form as a result of mineral deposition from solutions such as groundwater.
The Nature Of Minerals
Morphology
Nearly all minerals have the internal ordered arrangement of atoms and ions that is the defining characteristic of crystalline solids (see ). Under favourable conditions, minerals may grow as well-formed crystals, characterized by their smooth plane surfaces and regular geometric forms. Development of this good external shape is largely a fortuitous outcome of growth and does not affect the basic properties of a crystal. Therefore, the term crystal is most often used by material scientists to refer to any solid with an ordered internal arrangement, without regard to the presence or absence of external faces.
Nearly all minerals have the internal ordered arrangement of atoms and ions that is the defining characteristic of crystalline solids (see ). Under favourable conditions, minerals may grow as well-formed crystals, characterized by their smooth plane surfaces and regular geometric forms. Development of this good external shape is largely a fortuitous outcome of growth and does not affect the basic properties of a crystal. Therefore, the term crystal is most often used by material scientists to refer to any solid with an ordered internal arrangement, without regard to the presence or absence of external faces.
Symmetry elements
The external shape, or morphology, of a crystal is perceived as its aesthetic beauty, and its geometry reflects the internal atomic arrangement (see ). The external shape of well-formed crystals expresses the presence or absence of a number of symmetry elements. Such symmetry elements include rotation axes, rotoinversion axes, a centre of symmetry, and mirror planes.
A rotation axis is an imaginary line through a crystal around which it may be rotated and repeat itself in appearance one, two, three, four, or six times during a complete rotation. A sixfold rotation axis is illustrated in . When rotated about this axis, the crystal repeats itself each 60° (six times in a 360° rotation).
A rotoinversion axis combines rotation about an axis of rotation with inversion. Rotoinversion axes are symbolized as , , , , and : is equivalent to a centre of symmetry (or inversion, i), is equivalent to a mirror plane, is equivalent to a threefold rotation axis plus a centre of symmetry, is not composed of other operations and is unique, and is equivalent to a threefold rotation axis with a mirror plane perpendicular to the axis. The morphological expression of a fourfold rotoinversion axis is illustrated in .
A centre of symmetry exists in a crystal if an imaginary line can be extended from any point on its surface through its centre and a similar point is present along the line equidistant from the centre (see ). This is equivalent to , or inversion. There is a relatively simple procedure for recognizing a centre of symmetry in a well-formed crystal. With the crystal (or a wooden or plaster model thereof) laid down on any face on a tabletop, the presence of a face of equal size and shape, but inverted, in a horizontal position at the top of the crystal proves the existence of a centre of symmetry.
A mirror plane is an imaginary plane that separates a crystal into halves such that, in a perfectly developed crystal, the halves are mirror images of one another. A single mirror in a crystal, also called a symmetry plane, is illustrated in .
Morphologically, crystals can be grouped into 32 crystal classes that represent the 32 possible symmetry elements and their combinations. These crystal classes, in turn, are grouped into six crystal systems. In decreasing order of overall symmetry content, beginning with the system with the highest and most complex crystal symmetry, they are isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. The systems may be described in terms of crystallographic axes used for reference. The c axis is normally the vertical axis. The isometric system exhibits three mutually perpendicular axes of equal length (a1, a2, and a3). The orthorhombic and tetragonal systems also contain three mutually perpendicular axes; in the former system all the axes are of different lengths (a, b, and c), and in the latter system two axes are of equal length (a1 and a2) while the third (vertical) axis is either longer or shorter (c). The hexagonal system contains four axes: three equal-length axes (a1, a2, and a3) intersect one another at 120° and lie in a plane that is perpendicular to the fourth (vertical) axis of a different length. Three axes of different lengths (a, b, and c) are present in both the monoclinic and triclinic systems. In the monoclinic system, two axes intersect one another at an oblique angle and lie in a plane perpendicular to the third axis; in the triclinic system, all axes intersect at oblique angles.
There are 32 possible crystal classes, which are divided into six crystal systems, as shown in the table. Column 1 of the table lists the six crystal systems; column 2 gives the total symmetry content of each of the 32 crystal classes; and column 3 gives a symbolic representation for each of the 32 combinations of symmetry elements known as the Hermann-Mauguin, or international, notation.
Three different crystals with distinctively dissimilar symmetry contents, as expressed by their external morphology, are given in . shows a well-formed monoclinic crystal with symmetry content i, 1A2, and 1m (2/m); features a crystal in the tetragonal system with symmetry content i, 1A4, and 1m (4/m); and shows a crystal in the isometric system having the highest possible symmetry content of 3A4, 43, 6A2, and 9m (4/m2/m).
The external shape, or morphology, of a crystal is perceived as its aesthetic beauty, and its geometry reflects the internal atomic arrangement (see ). The external shape of well-formed crystals expresses the presence or absence of a number of symmetry elements. Such symmetry elements include rotation axes, rotoinversion axes, a centre of symmetry, and mirror planes.
A rotation axis is an imaginary line through a crystal around which it may be rotated and repeat itself in appearance one, two, three, four, or six times during a complete rotation. A sixfold rotation axis is illustrated in . When rotated about this axis, the crystal repeats itself each 60° (six times in a 360° rotation).
A rotoinversion axis combines rotation about an axis of rotation with inversion. Rotoinversion axes are symbolized as , , , , and : is equivalent to a centre of symmetry (or inversion, i), is equivalent to a mirror plane, is equivalent to a threefold rotation axis plus a centre of symmetry, is not composed of other operations and is unique, and is equivalent to a threefold rotation axis with a mirror plane perpendicular to the axis. The morphological expression of a fourfold rotoinversion axis is illustrated in .
A centre of symmetry exists in a crystal if an imaginary line can be extended from any point on its surface through its centre and a similar point is present along the line equidistant from the centre (see ). This is equivalent to , or inversion. There is a relatively simple procedure for recognizing a centre of symmetry in a well-formed crystal. With the crystal (or a wooden or plaster model thereof) laid down on any face on a tabletop, the presence of a face of equal size and shape, but inverted, in a horizontal position at the top of the crystal proves the existence of a centre of symmetry.
A mirror plane is an imaginary plane that separates a crystal into halves such that, in a perfectly developed crystal, the halves are mirror images of one another. A single mirror in a crystal, also called a symmetry plane, is illustrated in .
Morphologically, crystals can be grouped into 32 crystal classes that represent the 32 possible symmetry elements and their combinations. These crystal classes, in turn, are grouped into six crystal systems. In decreasing order of overall symmetry content, beginning with the system with the highest and most complex crystal symmetry, they are isometric, hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic. The systems may be described in terms of crystallographic axes used for reference. The c axis is normally the vertical axis. The isometric system exhibits three mutually perpendicular axes of equal length (a1, a2, and a3). The orthorhombic and tetragonal systems also contain three mutually perpendicular axes; in the former system all the axes are of different lengths (a, b, and c), and in the latter system two axes are of equal length (a1 and a2) while the third (vertical) axis is either longer or shorter (c). The hexagonal system contains four axes: three equal-length axes (a1, a2, and a3) intersect one another at 120° and lie in a plane that is perpendicular to the fourth (vertical) axis of a different length. Three axes of different lengths (a, b, and c) are present in both the monoclinic and triclinic systems. In the monoclinic system, two axes intersect one another at an oblique angle and lie in a plane perpendicular to the third axis; in the triclinic system, all axes intersect at oblique angles.
There are 32 possible crystal classes, which are divided into six crystal systems, as shown in the table. Column 1 of the table lists the six crystal systems; column 2 gives the total symmetry content of each of the 32 crystal classes; and column 3 gives a symbolic representation for each of the 32 combinations of symmetry elements known as the Hermann-Mauguin, or international, notation.
Three different crystals with distinctively dissimilar symmetry contents, as expressed by their external morphology, are given in . shows a well-formed monoclinic crystal with symmetry content i, 1A2, and 1m (2/m); features a crystal in the tetragonal system with symmetry content i, 1A4, and 1m (4/m); and shows a crystal in the isometric system having the highest possible symmetry content of 3A4, 43, 6A2, and 9m (4/m2/m).
Twinning
If two or more crystals form a symmetrical intergrowth, they are referred to as twinned crystals. A new symmetry operation (called a twin element), which is lacking in a single untwinned crystal, relates the individual crystals in a twinned position. There are three twin elements that may relate the crystals of a twin: (1) reflection by a mirror plane (twin plane), (2) rotation about a crystal direction common to both (twin axis) with the angular rotation typically 180°, and (3) inversion about a point (twin centre). An instance of twinning is defined by a twin law that specifies the presence of a plane, an axis, or a centre of twinning. If a twin has three or more parts, it is referred to as a multiple, or repeated, twin.
UNIT-2 ROCK FORMING MINERALS
If two or more crystals form a symmetrical intergrowth, they are referred to as twinned crystals. A new symmetry operation (called a twin element), which is lacking in a single untwinned crystal, relates the individual crystals in a twinned position. There are three twin elements that may relate the crystals of a twin: (1) reflection by a mirror plane (twin plane), (2) rotation about a crystal direction common to both (twin axis) with the angular rotation typically 180°, and (3) inversion about a point (twin centre). An instance of twinning is defined by a twin law that specifies the presence of a plane, an axis, or a centre of twinning. If a twin has three or more parts, it is referred to as a multiple, or repeated, twin.
UNIT-2 ROCK FORMING MINERALS
Rock-forming mineral, any mineral that forms igneous, sedimentary, or metamorphic rocks and that typically, or solely, forms as an intimate part of rock-making processes. In contrast are those minerals that have a limited mode of occurrence or are formed by more unusual processes, such as the ores of metals, vein minerals, and cavity fillings. Also, some precipitates and secondary minerals are not properly classified as rock-forming minerals; these form at a later time than the original rock and tend to destroy its original character. Some mineralogists limit the rock-forming minerals to those that are abundant in a rock and that are usually called essential minerals, a definition implying that they are those most significant in studying the rock-making processes.
Rock-forming mineral, any mineral that forms igneous, sedimentary, or metamorphic rocks and that typically, or solely, forms as an intimate part of rock-making processes. In contrast are those minerals that have a limited mode of occurrence or are formed by more unusual processes, such as the ores of metals, vein minerals, and cavity fillings. Also, some precipitates and secondary minerals are not properly classified as rock-forming minerals; these form at a later time than the original rock and tend to destroy its original character. Some mineralogists limit the rock-forming minerals to those that are abundant in a rock and that are usually called essential minerals, a definition implying that they are those most significant in studying the rock-making processes.
Metamorphism
Mineral Identification: Although nearly 2,500 minerals are known to occur within Earth's crust, most are rare with only about 100 occurring in abundance. Of those 100 minerals, fifteen make up the common rock-forming minerals and only some of the remaining minerals have any economic value.
Mineral Properties
A mineral is a naturally-occurring, inorganic solid which possesses a characteristic internal atomic structure and a definite chemical composition.
If we take the definition of a mineral term by term, it becomes easier to understand:
Minerals must occur naturally. This means man-made substances such as steel aren't minerals.
Inorganic substances are those substances that are not living and are not formed by living processes.
Crystalline solids are those solids in which the atoms composing the solid have an orderly, repeated pattern.
Minerals will have definite chemical compositions, but these compositions may vary within given limits. 1)Physical Properties: a)Hardness: What is hardness? The hardness of a mineral is its ability to resist scratching.
A mineral is a naturally-occurring, inorganic solid which possesses a characteristic internal atomic structure and a definite chemical composition.
If we take the definition of a mineral term by term, it becomes easier to understand:
- Minerals must occur naturally. This means man-made substances such as steel aren't minerals.
- Inorganic substances are those substances that are not living and are not formed by living processes.
- Crystalline solids are those solids in which the atoms composing the solid have an orderly, repeated pattern.
- Minerals will have definite chemical compositions, but these compositions may vary within given limits. 1)Physical Properties: a)Hardness: What is hardness? The hardness of a mineral is its ability to resist scratching.
Where did the hardness scale originate?
Friedrich Mohs, a German mineralogist, developed a hardness scale over 100 years ago. The hardest mineral known, diamond, was assigned the number 10.
How does the hardness scale work?
The Mohs Hardness Scale ranks the order of hardness of minerals and some common objects. For example, your fingernail can scratch the minerals talc and gypsum, with a hardness of 2 or lower. A copper penny can scratch calcite, gypsum, and talc.
A common misunderstanding of how to identify a diamond is that it will scratch glass. While this is true, other minerals can scratch glass too as long as they have a hardness > 6 .
Where did the hardness scale originate?
Friedrich Mohs, a German mineralogist, developed a hardness scale over 100 years ago. The hardest mineral known, diamond, was assigned the number 10.
How does the hardness scale work?
The Mohs Hardness Scale ranks the order of hardness of minerals and some common objects. For example, your fingernail can scratch the minerals talc and gypsum, with a hardness of 2 or lower. A copper penny can scratch calcite, gypsum, and talc.
A common misunderstanding of how to identify a diamond is that it will scratch glass. While this is true, other minerals can scratch glass too as long as they have a hardness > 6 .
b)colour : What causes color in minerals?
Minerals are colored because certain wave lengths of light are absorbed, and the color results from a combination of those wave lengths that reach the eye.
Some minerals show different colors along different crystallographic axes. This is known as pleochroism.
Which minerals is color useful for identification?
For some minerals, color is directly related to one of the major elements and can be characteristic, thus serving as a means of identification. Malichite is always green; azurite is always blue; and rhondonite is always red or pink.
Most metallic minerals’ color is constant, such as the brass-yellow of chalcopyrite and the copper-red of niccolite. These minerals may also tarnish, which is especially true of the mineral bornite. It is called the "peacock ore" because of the blue-velvet tarnish that develops on the surface.
How useful is color in identifying minerals?
The color of a mineral is the first thing most people notice, but it can also be the least useful in identifying a mineral. Most minerals occur in more than one color. Fluorite can be clear, white, yellow, blue, purple, or green. The other properties, such as hardness, cleavage, and luster, must be used instead.
b)colour : What causes color in minerals?
Minerals are colored because certain wave lengths of light are absorbed, and the color results from a combination of those wave lengths that reach the eye.
Some minerals show different colors along different crystallographic axes. This is known as pleochroism.
Which minerals is color useful for identification?
For some minerals, color is directly related to one of the major elements and can be characteristic, thus serving as a means of identification. Malichite is always green; azurite is always blue; and rhondonite is always red or pink.
Most metallic minerals’ color is constant, such as the brass-yellow of chalcopyrite and the copper-red of niccolite. These minerals may also tarnish, which is especially true of the mineral bornite. It is called the "peacock ore" because of the blue-velvet tarnish that develops on the surface.
How useful is color in identifying minerals?
The color of a mineral is the first thing most people notice, but it can also be the least useful in identifying a mineral. Most minerals occur in more than one color. Fluorite can be clear, white, yellow, blue, purple, or green. The other properties, such as hardness, cleavage, and luster, must be used instead.
b)Luster What is Luster? Luster refers to how light is reflected from the surface of a mineral. The two main types of luster are metallic and nonmetallic. What is Metallic Luster? Minerals exhibiting metallic luster look like metal, such as a silvery appearance or that of a flat piece of steel.
How many types of nonmetallic luster are there?
- Vitreous: The luster of glass
- Resinous: The luster of resin.
- Pearly: The luster of pearls.
- Greasy: Looks like it is covered in a thin layer of oil.
- Silky: The luster of silk.
- Adamantine: A hard, brilliant luster.
Another common nonmetallic luster is called translucent luster, where you can see into the mineral, but not completely through it. A mineral that displays a transparent luster transmits light completely through it, resembling glass.
How many types of nonmetallic luster are there?
- Vitreous: The luster of glass
- Resinous: The luster of resin.
- Pearly: The luster of pearls.
- Greasy: Looks like it is covered in a thin layer of oil.
- Silky: The luster of silk.
- Adamantine: A hard, brilliant luster.
Another common nonmetallic luster is called translucent luster, where you can see into the mineral, but not completely through it. A mineral that displays a transparent luster transmits light completely through it, resembling glass.
c)Streak What is streak?
The streak of a mineral is the color of the powder left on a streak plate (piece of unglazed porcelain) when the mineral is scraped across it. The streak plate has a hardness of glass, so minerals with a Mohs Hardness >7 will scratch the streak plate and won't powder the mineral.
Where do you observe streak?
A mineral’s streak is determined by rubbing it on a streak plate, which is a piece of unglazed porcelain. The streak plate is essentially a type of glass, so it isn't used on minerals with hardness greater than 7.
How useful is streak in identifying minerals?
Streak can be useful for identifying metallic and earthy minerals. Nonmetallic minerals usually give a white streak because they are very light-colored. Other minerals may have very distinctive streaks. Hematite, for example, always gives a reddish-brown streak no matter what type of luster it displays.
The streak of a mineral is the color of the powder left on a streak plate (piece of unglazed porcelain) when the mineral is scraped across it. The streak plate has a hardness of glass, so minerals with a Mohs Hardness >7 will scratch the streak plate and won't powder the mineral.
Where do you observe streak?
A mineral’s streak is determined by rubbing it on a streak plate, which is a piece of unglazed porcelain. The streak plate is essentially a type of glass, so it isn't used on minerals with hardness greater than 7.
How useful is streak in identifying minerals?
Streak can be useful for identifying metallic and earthy minerals. Nonmetallic minerals usually give a white streak because they are very light-colored. Other minerals may have very distinctive streaks. Hematite, for example, always gives a reddish-brown streak no matter what type of luster it displays.
Comments
Post a Comment