|
|
 |
 |
 |
|
|
|
The Science of Minerals
|
|
Both the physical properties of minerals and gemstones and
their beauty are dependent on their chemical composition and
atomic structure. Physical properties can be useful in
distinguishing between both mineral species and individual
minerals within a group or series. An understanding of
basic crystallography is fundamental to an appreciation of the
relationships between physical properties, chemical
composition, and atomic structure.
|
|
|
|
Crystallography
A crystal is
a solid material with a highly-ordered three-dimensional atomic
arrangement or structure. When conditions allow unimpeded growth of
a crystal, its internal atomic arrangement is reflected in its exterior
shape, producing a regular geometric shape bounded by smooth planar
surfaces (referred to as crystal faces).
Crystallography is the study of crystalline solids and the principles that
govern their growth, external shape and internal structure. A few of
the basic concepts of crystallography are explained here. More
detailed explanations of basic
crystallography and crystal
forms and symmetry classes are available on the web, or in Klein and
Hurlbut's Manual
of Mineralogy.
Early crystallographers were able to deduced many things about the
internal structure of crystals from their studies of crystal
symmetry. For example, in 1869, after studying numerous quartz
crystals, Nicolaus Steno observed that although they varied in size and
shape the angle between any two adjacent prism faces was always 120o.
This observation is known as the Law of Constancy of Interfacial
Angles.
Further
study eventually determined that all crystals can be organized into 32
crystal classes (also called point groups)
based on the presence or absence of the following four symmetry
elements:
|
|
An
axis of rotation is an imaginary line drawn
through the center of a crystal around which the crystal may be rotated
so that it appears the same as it did before the rotation.
Depending on the number of degrees through which the crystal must be
rotated to produce this effect, a crystal may look the same from 2, 3,
4, or 6 different positions; such a crystal is described as having
n-fold symmetry (e.g.: "2-fold symmetry") along that axis of
rotation. |
|
A
rotoinverison axis is similar to an axis of
rotation, except that in addition to being rotated around an imaginary
line the crystal must also be turned end to end around the center point
of the axis. A rotoinversion 3-fold or 4-fold axis is
distinguished from a 3-fold or 4-fold rotoinversion axis by a horizontal
line above the 3 or 4. That horizontal line is not supported on a
computer and so the rotoinversion axis is indicated on these pages by
highlighting the rotoinversion axis in yellow. |
|
A
crystal is said to have a
center of symmetry if for each and every
crystal edge, crystal plane or point on its surface, there is another
edge, plane, or point equidistant in the opposite direction from the
center. Another way of expressing this is to say that a mineral
has a center of symmetry if an imaginary line can be run from any
point on the surface of a crystal through its center to an identical
point on the opposite face which is the same distance beyond the center. |
|
A
mirror plane is an imaginary plane that
divides a crystal into halves that are mirror images of each other. |
A crystal may have
more than one of these symmetry elements, and a given symmetry element may occur
more than once in a given crystal. All crystals can be grouped into one of
32 classes, based what symmetry elements are present and their relationship to
each other.
Another concept
that can be used to classify crystals is that of crystallographic
axes. These are a set of imaginary lines
drawn through the center of a crystal. The 32 mineral classes are subdivided
into six crystal systems, based on the number
of crystallographic axes (always either 3 or 4), their relative lengths, and
whether the angles between them are perpendicular or oblique. Each system
is briefly described below; the hyperlinks lead to more detailed descriptions.
|
Isometric
system:
Three mutually perpendicular axes of equal lengths (a1, a2,
a3). |
|
Tetragonal
system:
Three mutually perpendicular axes (a1, a2, c),
with axes a1 and a2 of equal length
and axis c of a different length. |
|
Hexagonal
system:
Four axes (a1, a2, a3, and c).
Axes a1, a2, and a3 are of equal length
(a1 = a2 = a3) and intersect at 120o,
while axis c is at right angles to the plane formed by the other three
axes and may be of any length. |
|
Orthorhombic
system: Three mutually perpendicular
axes (a, b, c), all of which are different lengths. |
|
Monoclinic
system: Three axes (a, b, c) of
different lengths, two of which are inclined to each other at an
oblique angle (an angle other than 90o),
with the third axis perpendicular to the plane formed by the other two. |
|
Triclinic
system:
Three unequal axes (a, b, c) of different lengths, all intersecting at
oblique angles. |
Crystal
habit refers to
the general shapes of individual crystals or aggregates of crystals, which are
governed by the development of their faces. Crystal
habit depends not only on atomic structure and crystal symmetry, but also on on
the environment in which the crystals grew, and so a given mineral species may
exhibit different crystal habits at different localities. Since conditions
are seldom ideal for free, unimpeded crystal growth, beautiful, well-formed
mineral crystals are rare. Minerals that crystallize in open spaces filled
with fluid or gas are much more likely to be well formed. Some mineral
species very rarely develop crystals bounded by well defined planar faces.
Terms used to
describe the crystal habit of single crystals are listed below. Click on
the small images to see enlarged views.
 |
Acicular
Elongated, thin, needlelike crystals. |
 |
Bladed
Elongated, flattened prisms shaped like
a knife blade. |
 |
Dendritic
Arborescent, or shaped like a tree, having a branching form. |
 |
Filiform
or capillary Hairlike crystals, thinner
and more delicate than acicular crystals. |
 |
Platy
Thin plate-shaped or sheet-like crystals. |
 |
Prismatic
A crystal with one dimension markedly longer than the other two,
prism-shaped. |
A large number of
terms are used to describe the crystal habit of parallel or radiating groups of
crystals. Some of the most common ones are listed here.
 |
Amygdaloidal
Mineral-lined or mineral-filled cavities, typically gas holes, that
occur in some extrusive volcanic rocks. These crystals are
secondary minerals, precipitated from aqueous solutions which circulated
through cooling submarine lavas. |
 |
Asbestiform
or fibrous An aggregate of thin fibers. |
 |
Botryoidal,
reniform, mammillary An aggregate
composed of intergrown spherical shapes resembling the outside
surface of a bunch of grapes. The three terms differentiate
between spheres of different sizes. The specimen shown here is
"botryoidal" (the smallest). "Mammillary" is
the largest. |
 |
Concentric
More or less spherical layers around a common center or centers. |
 |
Drusy
A layer or encrustation of small (less than 2 mm), fairly well-formed
crystals. The tiny quartz crystals that line the cavity in the center of
the blue agate at the bottom of this page are another example of drusy
crystals. |
 |
Massive
A homogeneous texture or fabric characterized by an absence of layering,
foliation, cleavage, or any other directional structure or preferred
orientation of constituent minerals. If a massive group of
crystals has a preferred direction of cleavage, it is due to aligned
fractures or other closely spaced planes of weakness produced by
deformation or metamorphism. Foliation
is the planar structure that results from flattening and/or separation
of the constituent minerals of a metamorphic rock into light quartz- and
feldspar-rich layers and dark mica-rich layers. |
 |
Micaceous
An aggregate of very thin sheets, typically of the mica minerals
(biotite, muscovite, phlogopite, chlorite). |
 |
Radiating
Groups of elongated crystals that extend from a common central point. |
 |
Stalactitic
Icicle-like shapes that form through the precipitation of evaporating
aqueous solutions in caves and large, air-filled solution cavitites.
Cutting through the stalactite perpendicular to the length typically
reveals a concentric cross section, as shown in this rhodochrosite
sample. |
 |
Tabular
A crystal form characterized by having one dimension markedly smaller
than the other two. |
 The
Individual crystals in an monomineralic (composed of one mineral) aggregate may
be so small that their crystalline nature can be determined only with the aid of
a microscope (called microcrystalline) or by
X-ray diffraction (called cryptocrystalline).
Jade, a highly prized gemstone, is an aggregate of microcrystalline jadeite (a
pyroxene group mineral), or actinolite and tremolite (amphibole group
minerals). Chalcedony is a cryptocrystalline aggregate of quartz.
Agate (banded chalcedony), flint (dark colored and nonbanded), and chert (light
colored and nonbanded) are common names for varieties of chalcedony.
 Although
all crystalline substances have a regular, ordered, internal structure,
different planes and directions within the crystals have different atomic
environments. The isometric mineral halite illustrates this
observation. The figure to the left is a diagram of an imaginary plane
cutting through a halite crystal, showing the regular arrangement sodium (s) and
chlorine (c) ions. The lines marked 1, 2, 3 and 4 represent four of the
many different paths that a ray of light traveling through the crystal might
take. The light ray would encounter many more ions of sodium and chlorine
along Line 1 than it would along Lines 2, 3, and 4. These differences in atomic
environment can lead to differences in index of refraction, hardness, heat and
electrical conductivity, thermal expansion, growth and solution rates, and
others. The importance of these differences (called vectorial
properties) will be discussed in more
detail in later sections.
 Twinning
is the symmetrical intergrowth of two or more crystals of the same
substance. Three special symmetry elements are used to describe types of
twinning: 1) reflection across a mirror plane referred to as the twin
plane, 2) rotation about a twin axis
that is common to both of the twinned individuals, and 3) inversion about a
point referred to as the twin center. Twinned
crystals can be described as contact twins
or penetration twins. The individual
crystals in a contact twin meet along a plane referred to as the composition
surface. The individual crystals in a penetration twin pass
through each other, and share a common twin axis. The twinned microcline
shown on the right is a penetration twin in which the c-axis is the twin
element.
Repeated twinning
is characteristic of some minerals, particularly plagioclase feldspar, calcite,
rutile, and chrysoberyl. In plagioclase and calcite, the multiple
composition planes parallel each other, producing polysynthetic
twinning. A cyclic twin
results when the composition planes are not parallel, as in rutile and
chrysoberyl.
 Twins
are also classified by their cause. Primary
twinning occurs during crystal growth, and is thought to occur as a
result of nucleation errors, due to the addition of a few atoms or ions in the
wrong place on the surface of a growing crystal. Transformation
twinning occurs in response to changes in crystal structure and
symmetry, as a result of falling temperature. An example of transformation
twinning is Dauphine twinning in quartz. Deformation
twinning results from the application of an applied mechanical
stress. Calcite and other carbonate minerals twins unusually easily under
stress; the deformation produced caused by cutting and polishing thin sections
of rocks for microscopic examination typically is enough to cause twinning in
these minerals.
The notch on the
end of the long bladed selenite gypsum crystal shown above is an example of
primary twinning. This particular kind of twin in selenite gypsum is called a
swallowtail twin because of its shape. The swallowtail twin is a contact
twin. The two individuals in the twinned crystal are on either side
of a mirror plane.
|
