The physical characteristics of
minerals include traits which are used to identify and
describe mineral species. These traits include color, streak, luster,
density, hardness, cleavage, fracture, tenacity, and crystal habit.
Certain wavelengths of light are reflected by the atoms of a mineral's crystal lattice while others are absorbed. Those wavelengths of light which are reflected are perceived by the viewer to possess the property of color. Some minerals derive their color from the presence of a particular element within the crystal lattice. The presence of such an element can determine which wavelengths of light are reflected and which are absorbed. This type of coloration in minerals is termed idiochromatism; different samples of an idiochromatic mineral species will all display the same color. Other minerals are colored by the presence of certain elements in mixture. Different samples of such a species may exhibit a range of similar colors. Still other mineral species may usually be colorless, but may display several different and startling colors when trace amounts of impurities, or elements which are not an integral part of the crystalline lattice, are present. Coloration which is caused by the presence of an element foreign to the crystal lattice, whether in mixture or in trace amounts, is termed allochromatism. Certain elements are strong pigmenting agents and may lend vivid colors to specimens when they are present, whether as a part of the crystal lattice, in mixture, or as an impurity. These elements are termed the chromophores.
Streak is the color which a mineral displays when it has been ground to a fine powder. Trace amounts of impurities do not tend to affect the streak of a mineral, so this characteristic is usually more predictable than color. Two different specimens of the same species may be expected to possess the same streak, whereas they may display different colors.
Minerals are either opaque or transparent. A thin section of an opaque mineral such as a metal will not transmit light, whereas a thin section of a transparent mineral will. Typically those minerals which possess metallic bonding are opaque whereas those where ionic bonding is prevalent are transparent. Relative differences in opacity and transparency are described as luster. The characteristic of luster provides a qualitative measure of the amount and quality of light which is reflected from a mineral's exterior surfaces. Luster thus describes how much the mineral surface 'sparkles'.
The property of density is defined as mass per unit volume. Certain trends exist with respect to density which may sometimes aid in mineral identification. Native elements are relatively dense. Minerals whose chemical composition contains heavy metals, or atoms possessing an atomic number greater than iron (Fe, atomic number 26), are relatively dense. Species which form at high pressures deep within the earth's crust are in general more dense than minerals which form at lower pressures and shallower depths. Dark-colored minerals are typically fairly dense whereas light-colored ones tend to be less dense.
Hardness is defined as the level of difficulty with which a smooth surface of a mineral specimen may be scratched. Hardness has historically been measured according to the Mohs scale. Mohs' method relies upon a scratch test to relate the hardness of a mineral specimen to the hardness of one of a set of reference minerals. Hardness may also be measured according to the more quantitative but less accessible diamond indentation method.
Cleavage refers to the splitting of a crystal along a smooth plane. A cleavage plane is a plane of structural weakness along which a mineral is likely to split. The quality of a mineral's cleavage refers both to the ease with which the mineral cleaves and to the character of the exposed surface. Not every mineral exhibits cleavage.
Fracture takes place when a mineral sample is split in a direction which does not serve as a plane of perfect or distinct cleavage. A mineral fractures when it is broken or crushed. Fracture does not result in the emergence of clearly demarcated planar surfaces; minerals may fracture in any possible direction.
The characteristic of tenacity describes the physical behavior of a mineral under stress or deformation. Most minerals are brittle; metals, in contrast, are malleable, ductile, and sectile.
The term crystal habit describes the favored growth pattern of the crystals of a mineral species. The crystals of particular mineral species sometimes form very distinctive, characteristic shapes. Crystal habit is also greatly determined by the environmental conditions under which a crystal develops.
When different wavelengths of visible
light are incident upon the eye they are perceived as being of different
colors. Three different varieties of color receptors in the eye
correspond to light possessing wavelengths of approximately 660 nm (red),
500 nm (green), and 420 nm (blue-violet). The eye then interprets the
color of incident light according to which color receptors have been
stimulated. For example, if monochromatic light which stimulated the red
and green color receptors equally and did not affect the blue-violet
receptors was detected, then the eye would interpret this light as
possessing a wavelength halfway between those of red and green light. The
eye would therefore register an incident light wave with a wavelength of
approximately 580 nm and the viewer would percieve the incoming light
as yellow. Incident polychromatic light which stimulated the red and
green color receptors equally and did not affect the blue-violet ones
would also be interpreted as yellow light, regardless whether or not the
incoming light actually contained a component with a wavelength close to
580 nm. The incident polychromatic light might possess only a red and a
green component of equal intensity; it would nevertheless be interpreted
by the eye as yellow light. The phenomenon called color is thus a
description of the differentiation by the eye between various wavelengths
and combinations of wavelengths of visible light.
When light is incident upon a mineral specimen, some wavelengths are absorbed by the atoms of the crystal lattice while others are reflected. Those wavelengths which were not absorbed are reflected off of the mineral's surfaces and enter the eye of the viewer. The color which is perceived by the viewer depends on the wavelengths of light which are reflected rather than absorbed by the mineral. The property of color in minerals is thus due to the absorption of particular wavelengths of light and the reflection of others by the atoms of the crystal lattice.
The color exhibited by certain mineral species may depend upon which crystallographic axis is transmitting the light. Such species may demonstrate several different colors as light is transmitted along various different axes. This phenomena of directionally selective absorption is termed pleochroism.
The color of many mineral species is
derived directly from the presence of one or more of the elements which
constitute the crystal lattice. The color of such minerals is a
fundamental property directly related to the chemical composition of the
species. Minerals which exhibit this type of coloration are called
idiochromatic minerals. Idiochromatic coloration is a property
possessed by a mineral species as a whole. In such species color can
successfully be utilized as a means of identification.
Ions of certain elements are highly absorptive of selected wavelengths of light. Such elements are called chromophores; they possess strong pigmenting capabilities. The elements vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu) are chromophores. A mineral whose chemical formula stipulates the presence of one or more of these elements may possess a vivid and distinctive color.
Examples of idiochromatic minerals abound. For instance, the copper carbonate malachite is consistently green; the copper carbonate azurite and the copper silicate chrysocolla are each a distinctive and predictable blue. Rhodochrosite is always red or pink; samples of sulphur are a bright, recognizable yellow. Each of these distinctive colors is due to the fact that the chemical composition which defines the mineral species specifies inclusion of one of the chromophores within the lattice structure.
Most minerals which are composed
entirely of elements other than the chromophores are nearly colorless.
However, certain specimens are sometimes observed to possess vivid
coloration. Color in such instances is due to the presence of an
impurity. If one of the chromophores is present within a mineral whose
chemical formula does not include it, then the foreign element constitutes
an impurity or a defect in the lattice structure. Coloration in minerals
which is due to the presence of a foreign element is termed
allochromatism. In such cases the color of the mineral may differ
radically from the nearly colorless shade expected of the species.
Some minerals demonstrate a range of colors due to the presence in mixture of one of the chromophores. For example, the substitution of a quantity of iron for zinc atoms within the crystal lattice of sphalerite (ZnS) implements a change from white to yellow in the color of the mineral. Proportionally larger inclusions of iron will progressively result in a brown and eventually a black mineral specimen. In such cases the color of the sample is directly proportional to the amount of the pigmenting element which is present in the crystal lattice.
Not all allochromatism in minerals is due to presence of substantial amounts of a chromophore in mixture, however. The property of color may sometimes be highly dependent on the inclusion of trace amounts of impurities. The presence of even a minute quantity of a chromophore within the crystal lattice can cause a mineral specimen to exhibit vivid color. For example, trace inclusions of chromium (Cr) in beryl are responsible for the deep green of emerald, while the purple of amethyst is due to trace amounts of iron (Fe) in quartz and the pink of rose quartz is due to trace inclusions of titanium (Ti). Samples of the mineral corundum which include tiny amounts of chromium are deep red, and the gem is then called a ruby, while samples containing iron or titanium impurities produce blue gems termed sapphire.
Trace amounts of an impurity do not
affect the basic chemical composition or the chemical formula of a
mineral, and thus do not affect its classification as a species. Trace
amounts of the various chromophores, however, can cause several samples of
a single species to differ radically in color. (Beryl, corundum, and
quartz provide examples of this possibility.) Because it varies so
widely, color is a property which is sometimes of little use in
identification. However, the idiochromatic minerals are consistently of
distinctive color. The green of malachite, the blue of azurite, the pink
of rhodocrosite, and the yellow of sulphur are easily recognized and are
therefore quite useful in the identification of these species.
Streak is the color of a mineral
substance when it has been ground to a fine powder. Typically an edge of
the sample will be rubbed across a porcelain plate, leaving behind a
'streak' of finely ground material. The material in a streak sample thus
consists of a powder composed of randomly oriented microscopic crystals
rather than a lattice structure containing the uniformly oriented unit
cells which compose a macroscopic crystal.
Although color is a property which may vary widely between two different specimens of the same mineral, streak generally varies little from sample to sample. The presence of trace amounts of an impurity may radically affect the property of color in a macroscopic crystal because each unit cell is aligned within the crystal structure, thereby forming a diffraction grating. Minute amounts of a strongly absorptive impurity within the structure may highly affect which wavelengths of light are reflected from this diffraction grating. This change may greatly modify the absorption of certain wavelengths of incoming light, altering the percieved color of the specimen. In a streak sample, however, each of the microscopic crystal grains of the sample is randomly oriented and the presence of an impurity does not greatly affect the absorption of incoming light. Because it is not typically affected by the presence of an impurity, streak is a more reliable identification property than is color.
Minerals may be categorized according
to whether they are opaque or transparent. A thin section
of an opaque mineral such as a metal will not transmit light, whereas a
thin section of a transparent mineral will. The absorption index of an
opaque mineral is high. Light which is incident upon an opaque
mineral such as a metal is unable to propagate through the mineral due to
this high rate of absorption, and will thus be reflected. Opaque minerals
typically reflect between 20% to 50% or more of the light incident upon
them. In contrast, most of the light which is incident upon a transparent
mineral passes into and through the mineral; transparent minerals may
reflect as little as 5% of the incident light and as much as 20%.
Typically those minerals which possess metallic bonding are opaque whereas
those where ionic bonding is prevalent are transparent.
Relative differences in opacity and transparency are described as luster. The term luster refers to the quantity and quality of the light which is reflected from a mineral's exterior surfaces. Luster provides an assessment of how much the mineral surface 'sparkles'. This quality is determined by the type of atomic bonds present within the substance. It is related to the indices of absorption and refraction of the material and the amount of dispersion from the crystal lattice, as well as the texture of the exposed mineral surface.
Minerals are primarily divided into the two categories of metallic and nonmetallic luster. Minerals possessing metallic luster are opaque and very reflective, possessing a high absorptive index. This type of luster indicates the presence of metallic bonding within the crystal lattice of the material. Examples of minerals which exhibit metallic luster are native copper, gold, and silver, galena, pyrite, and chalcopyrite. The luster of a mineral which does not quite possess a metallic luster is termed submetallic; hematite provides an example of submetallic luster.
The property of streak can aid in distinguishing whether a specimen has a metallic or a nonmetallic luster. Metals tend to be soft, implying that more powdered material may be obtained from the streak sample of a metal than a nonmetal. Metals are also opaque, transmitting no light. Minerals which possess a metallic luster therefore tend to exhibit a thick, dense, dark streak whereas those which possess a nonmetallic luster tend to produce a thinner, less dense streak which is also lighter in color.
Adjectives such as "vitreous', 'dull', 'pearly', 'greasy', 'silky' or 'adamantine' are frequently used to describe various types of nonmetallic luster.
The property of density is defined as mass per unit volume:
The geometric structure of the unit cell of a mineral determines the
volume which it occupies. The masses of the atoms which compose the unit
cell decree the mass of each cell. The identity of the atoms which
compose the unit cell is specified by the chemical formula of the mineral.
Density is therefore directly related to both the physical structure of
the unit cell and the chemical composition of each species of mineral.
One method of measuring the density of a sample entails the use of one dense liquid and another miscible liquid of lower density. A solution of the two substances is created in which a crystal of the mineral in question remains suspended and neither sinks nor floats. The weight of a known volume of the solution is then measured, and the density of the solution and thus the density of the crystal are calculated from this information. Bromoform (CHBr3, density 2.9 g/cm3), soluble in acetone; di-iodomethane (CH2I2, density 3.3 g/cm3), soluble in chloroform, CHCl3; and Clerici's solution (a solution of thallium formate and thallium malonate; density 4.4 g/cm3), soluble in water, are some heavy liquids and their solvents which are commonly used in this process.
Density has historically been equated by mineralogists with the concept of specific gravity. Specific gravity is a unitless quantity which is defined as the ratio of the weight of a substance to the weight of an equal volume of water at a temperature of 4° Celsius. This ratio is equal to the ratio of the density of the substance to the density of water at 4° Celsius.
Specific gravity has therefore classically been measured by weighing a mineral specimen on a balance scale while it is submerged first in air and then in water. The difference between the two measurements is the weight of the volume of water which was displaced by the sample. The specific gravity of the mineral specimen is thus:
Because the density of water at 4° Celsius is 1.00 g/cm3, the density of a mineral in units of grams per centimeter cubed (g/cm3) is equal to its (unitless) specific gravity.
The field geologist sometimes uses a
very rough estimation of the density of a hand-held sample as a clue to
identification. Certain rough trends relating mineral density to various
other factors are sometimes useful. Native elements, which contain only
one type of atom and whose molecular structure is that of cubic or
hexagonal closest packing, are relatively dense. Minerals whose chemical
composition contains heavy metals - atoms of greater atomic number then
iron (Fe, atomic number 26) - are more dense than atoms
whose chemical composition does not include such elements. Minerals which
formed at the high pressures deep within the earth's crust are in general
more dense than minerals which formed at lower pressures and shallower
depths. A general trend relating color to density is also prevalent; this
trend states that dark-colored minerals are often fairly heavy whereas
light-colored ones are frequently relatively light. A geologist is thus
given cause to remark upon a sample which seems to reverse this trend. For
example, graphite is dark colored but of low density (C;
2.23 g/cm3) while barite is light in color but
unexpectedly heavy (BaSo4; 4.5
g/cm3). The noted oddity of unexpectedly high or
low density with respect to color provides the field geologist with a clue
as to the identification of such atypical materials.
Hardness has traditionally been
defined as the level of difficulty with which a smooth surface of a
mineral specimen may be scratched. The hardness of a mineral species is
dependent upon the strength of the bonds which compose its crystal
structure. Hardness is a property characteristic to each mineral species
and can be very useful in identification.
Certain trends exist in hardness with respect to mineral class. (For a description of the various classes of minerals, please refer to the discussion on mineral classification contained in Section 4.) Native elements are typically soft, although iron (Fe) and platinum (Pt) are relatively hard and diamond (C) is exceptionally hard. Compounds of heavy metals are soft. Sulphides and sulpho-salts, with the exception of pyrite, are relatively soft; halides are soft; carbonates and sulphates are usually soft. Oxides are typically hard while hydroxides are softer. Anhydrous silicates tend to be hard, while hydrous silicates are softer.
The property of hardness has
historically been measured according to the Mohs scale, which was
created in 1824 by the Austrian mineralogist Friedrich Mohs. Mohs based
his system for measuring and describing the hardness of a sample upon the
definition of hardness as resistance to scratching. Mohs' method thus
relies upon a scratch test in order to relate the hardness of a mineral
specimen to a number from the Mohs scale.
In order to define his scale, Mohs assembled a set of common reference minerals of varying hardnesses and labled these in order of increasing hardness from 1 to 10. The reference minerals of the Mohs scale are as follows:
Each reference mineral will scratch a test specimen with a Mohs
hardness less than or equal to its own. Each reference mineral can be
scratched by a specimen with a hardness equal to or greater than its own.
If a reference mineral both scratches and can be scratched by a certain
test specimen, then the specimen is assumed to possess a hardness equal to
that of the reference mineral in question.
The set of reference minerals of the Mohs' scale can be supplemented by a few common household items. A fingernail has a Mohs hardness of 21/2; a copper penny 3, window glass 51/2, and a knife blade approximately 6.
The hardness of an unknown sample can be determined to within 1/2 increment by using the scratch test. Mineral hardnesses determined by the scratch test should never be given in decimal form, because the Mohs scale does not provide measurements of such precision.
The hardness of a mineral may vary with direction and crystallographic plane. This effect is usually small. However, species exist in which the variance in the hardness along different axes is notable. For example, the mineral kyanite (Al2OSiO4) typically forms elongated crystals. The Mohs hardness parallel to the length of a kyanite crystal is 5, whereas the Mohs hardness perpendicular to the length of such a crystal is 7. A second example is provided by the mineral halite, which is softer parallel to its cleavage planes than it is at a 45° angle to the cleavage planes.
Investigations more recent than those
completed by Mohs have used the diamond indentation method to
quantitatively determine hardness. According to this method, a diamond
point is pushed into a planar mineral surface under the weight of a known
load. The diameter of the indentation thereby produced is then measured
under a microscope. The diamond indentation hardness of a sample is equal
to the mass of the load applied divided by the surface area of the
indentation produced. The units in which diamond indentation hardness is
recorded are therefore kilograms per millimeter squared
Tests utilizing the diamond indentation method have shown that in order for a point fashioned from a certain material to scratch a surface the hardness of its constituent material must be 1.2 times that of the surface. Thus on an ideal hardness scale, each subsequent reference material would have a hardness of approximately 1.2 times that of the material preceeding it. It must be noted that the intervals between reference points on the Mohs scale are not, in fact, equal. The interval between subsequent reference points on the scale increases as the hardness of the reference materials increases. The skill with which Mohs chose his reference materials becomes apparent when one notes that each of his samples is approximately 1.6 times the hardness of the last.
The Mohs scale provides a means of testing hardness which is far more readily available to amateur geologists than the diamond indentation method. It has therefore remained the standard scale by which hardness is measured.
A cleavage plane is a plane of
structural weakness along which a mineral is likely to split smoothly.
Cleavage thus refers to the splitting of a crystal between two
parallel atomic planes. Cleavage is the result of weaker bond strengths
or greater lattice spacing across the plane in question than in other
directions within the crystal. Greater lattice spacing tends to accompany
weaker bond strength across a plane, because such bonds are unable to
maintain a close interatomic spacing.
Both the positioning of crystal faces in a mineral and the property of cleavage are derived from the crystalline structure of the species. However, despite the fact that every mineral belongs to a specified crystal system, not every mineral exhibits cleavage. A mineral such as quartz may demonstrate beautiful, well-developed crystals and yet possess no distinct planes of cleavage.
Cleavage planes, if they exist, are always parallel to a potential crystal face. However, such planes are not necessarily parallel to the faces which the crystal actually displays. Fluorite, for example, has octahedral cleavage yet forms cubic crystals. Nonetheless, the property of cleavage, if it is present, can offer important information about the symmetry and inner structure of a crystal.
The quality of a mineral's cleavage refers to both the ease with which the mineral cleaves and to the character of the exposed cleavage surface. The quality of a sample's cleavage is typically described by terms such as 'eminent,' 'perfect,' 'distinct,' 'difficult,' 'imperfect,' or 'indistinct.'
'Eminent' cleavage describes the case in which cleavage always occurs readily and is in fact difficult to prevent from occurring. The mineral mica, for example, cleaves readily into thin, flat sheets. A mineral which demonstrates 'perfect' cleavage breaks easily, exposing continuous, flat surfaces which reflect light. Fluorite, calcite, and barite are minerals whose cleavage is perfect. 'Distinct' cleavage implies that cleavage surfaces are present although they may be marred by fractures or imperfections. 'Difficult' or 'indistinct' cleavage produces surfaces which are neither smooth nor regular; samples possessing such cleavage tend to fracture rather than split.
Cleavage may be determined by the examination of surfaces which have actually broken. It may also be determined by inspection of the interlacing systems of cracks which permeate the structure of certain specimens. These systems of cracks are beautifully apparent within transparent crystals such as fluorite or calcite.
A mineral fractures when it is
broken or crushed. Fracture takes place when a mineral sample is split in
a direction which does not serve as a plane of perfect or distinct
cleavage. In other words, fracture takes place along a plane possessing
difficult, indistinct, or nonexistant cleavage. The difference between
fracture and indistinct cleavage is not clearly delineated.
Unlike perfect or distinct cleavage, fracture does not result in the emergence of clearly demarcated planar surfaces which run parallel to possible crystal faces. Fracture is nondirectional: minerals which do not possess distinct cleavage may fracture in any possible direction.
Fractured surfaces may in some minerals possess a characteristic appearance which can aid in identification. Examples of distinctive types of fracture are 'conchoidal,' 'irregular,' and 'hackly' fracture.
The property of tenacity describes the behavior of a mineral under deformation. It describes the physical reaction of a mineral to externally applied stresses such as crushing, cutting, bending, and striking forces. Adjectives used to characterize various types of mineral tenacity include 'brittle,' 'flexible,' 'elastic,' 'malleable,' 'ductile,' and 'sectile'.
10. Crystal Habit
The term crystal habit describes
the favored growth pattern of the crystals of a mineral species, whether
individually or in aggregate. It may bear little relation to the form of
a single, perfect crystal of the same mineral, which would be classified
according to crystal system. (Please see the following discussion of
crystal system in Section
3.) Subtle evidence of the crystal system to
which a mineral species belongs is, however, frequently observed in the
habit of the crystals which a specimen displays.
The terminology used to describe crystal habit is not intended to replace the precise nomenclature of crystallography. Instead, it is intended as a supplement to this system. Discussions of crystal habit are more descriptive than precise; for this reason the terminology is suited to the discussion of mineral samples discovered in the field. Naturally formed specimens are rarely quantitatively perfect.
The crystals of particular minerals species sometimes form very distinctive, characteristic shapes. Crystal habit is thus often useful in identification.
Although each mineral species typically forms according to a few preferred shapes, crystal habit is largely determined by the environmental conditions under which a crystal develops. For example, aqueous solutions near or surrounding a crystal contain the elemental substances which it needs to continue growth. The direction from which a growing crystal may obtain such solutions is a factor which will affect its eventual shape. Higher environmental temperatures during formation increase ion mobility and aid in crystal formation; the rate at which the environment cools determines how much time a mineral is allowed to form large crystals. The amount of space available for a crystal to fill affects its final shape and size. Surface energy relations are also quite important to the direction of crystal growth; this process is not yet fully understood.
Adjectives used to describe the habit of individual crystals are 'equant,' 'prismatic,' and 'tabular.' Aggregates of crystals may also be termed equant or prismatic, while aggregates of thin, flat, tabular crystals may be 'bladed.' Thin sheets, flakes or scales are termed 'foliated,' 'micaceous,' and, if feathery or delicate, 'lamellar' or 'plumose.' Crystal aggregates resembling long, slender needles, hair, or thread are termed 'acicular,' filiform,' 'capillary,' or 'fibrous.' An aggregate of crystals forming a network or lattice is 'reticulated;' one composed of branches which radiate starlike from a central point is 'stellated' while a branching and treelike mineral growth is 'dendritic.' 'Colloform' crystal habits termed 'botryoidal,' 'mamillary,' and 'reniform' display spherical, bulbous or globular lumps. Smaller spherical forms are of 'pisolitic' or 'oolitic' habit; ovoid clusters or formations are 'amygdaloidal.' Tapered, column-like formations are 'stalactitic' or 'columnar' while concentrically banded formations are of 'concretionary' habit. Minerals whose flat crystal faces are covered with shallow, parallel grooves are 'striated;' a fine furry layer of crystals growing over a massive lump constitutes a formation of 'drusy' habit. Following is a list of descriptive terms which are applied when discussing crystal habit.