a. One of the 32 crystallographically possible combinations or groups of
symmetry operations that leave one point, or origin, fixed. AGI
b. All minerals having the symmetry of one of the 32 point groups belong
to the same crystal class. CF:point group
Any deviation from perfect periodicity in a crystal structure. Some
defects depend on temperature, mainly point defects; others depend on the
specific history of the crystal. The presence of defects alters the
physical properties of crystals. CF:Frenkel defect; point defect;
line defect; volume defect.
Irregularities in a lattice structure that affect resistance to crushing.
Microdefects are due to irregular distribution of ions. Macrodefects are
incipient strain areas or discontinuities in an otherwise regular lattice.
Mosaic defects are orderly blocks of regular lattice that are packed
together to form a larger and imperfect particle. Pryor, 3
The abnormal ratio of magnetization to the magnetizing force responsible
for it, as observed in some crystals, such as those of bismuth.
Hess
a. One of the several flat or plane exterior surfaces of a crystal.
See also:crystal
b. A planar surface developed on a crystal during its growth. Crystal
faces tend to parallel planes of high lattice-point density (Bravais' law)
with the result that they make rational intercepts with the
crystallographic axes and may be assigned rational indices, e.g., (hkl).
CF:Miller indices
The floating of lighter-weight crystals in a body of magma.
CF:crystal settling
a. The form or shape in which crystals occur; the cube, the octahedron,
and others.
b. All crystal faces related by the symmetry elements of the point group
of the crystal structure belong to the same crystal form. Crystal forms
are designated by the indices of the unit face enclosed in braces, e.g.,
hkl . Forms are closed if they singly enclose a volume and open if two or
more forms are required. The terms "prism," "pyramid," "cube,"
"octahedron," and "tetrahexahedron" refer to crystal forms. A crystal form
is ideal when all faces are the same size. Syn:crystalline form
Magmatic differentiation resulting from the floating or settling, under
gravity, of mineral crystals as they form.
CF:fractional crystallization
AGI
The study of conditions for growing crystals experimentally, esp. in the
control of chemical and physical properties and in application to the
growth history of natural crystals. Also the microchemical and isotopic
study of crystals for the physical and chemical constraints on their
formation.
The forms typically appearing on specimens of a mineral species or group,
rarely all the forms permitted by its point group. Crystal habits range
from highly diverse, e.g., calcite, to almost never showing crystal faces,
e.g., turquoise. In addition to describing mineral habits with form names,
e.g., prismatic, pyramidal, or tetrahedral, other names for appearances
are used, e.g., fibrous, columnar, platy, or botryoidal. Intergrowths are
given by specific description. Pryor, 3
a. Numbers or other representations that indicate the inclination of a
crystal face to the crystal axes.
b. Numbers based on the rational intercepts of crystal faces with
crystallographic axes. The Miller index is the reciprocal of a face's
axial intercepts. Indices of crystal faces are enclosed in parentheses
(hkl), crystal forms in braces hkl, crystal directions in brackets [hkl],
and Bragg indices with no closure hkl. For crystals with hexagonal and
trigonal symmetry, Miller-Bravais indices (hkil) may be used although the
added intercept and index number are redundant (h+k+i=0). Not all
mineralogists follow this usage. CF:Miller indices
a. The regular and repeated three-dimensional arrangement of atoms that
distinguishes crystalline solids from all other states of matter.
Essentially the regularity displayed by a crystal lattice is that of a
three-dimensional mesh that divides space into identical parallelepipeds.
Imagine a number of identical atoms placed at the intersections of such a
mesh; then we have what is known as a simple lattice (synonymous with
Bravais lattice). AGI
b. A periodic array of points in three dimensions such that each point is
in an identical point environment. Fourteen possible lattices that are
used to describe the structural patterns are found in all crystalline
materials by assigning an asymmetric unit to each lattice point.
Syn:Bravais lattice; space lattice; direct lattice;
translation lattice. CF:reciprocal lattice
Producing or bearing crystals. Webster 3rd
Having a crystalline form. Standard, 2
a. Made of crystal. Webster 3rd
b. Resembling a crystal; clear, transparent, pure. AGI
c. Pertaining to or having the nature of a crystal, or formed by
crystallization; specif. having a crystal structure or a regular
arrangement of atoms in a space lattice. Ant: amorphous. Said of a mineral
particle of any size, having the internal structure of a crystal, but
lacking well-developed crystal faces or an external form that reflects the
internal structure. AGI
d. Said of a rock consisting wholly of crystals or fragments of crystals;
esp. said of an igneous rock developed through cooling from a molten state
and containing no glass, or of a metamorphic rock that has undergone
recrystallization as a result of temperature and pressure changes. The
term may also be applied to certain sedimentary rocks (such as quartzite,
some limestones, evaporites) composed entirely of contiguous crystals.
AGI
e. Said of the texture of a crystalline rock characterized by closely
fitting or interlocking particles (many having crystal faces and
boundaries) that have developed in the rock by simultaneous growth. A
crystalline rock. Term is usually used in the plural; e.g., the
Precambrian crystalline. This usage is not recommended. AGI
f. Referring to a homogeneous solid material that has long-range periodic
order of its atomic constituents. Crystalline materials distinctly
diffract X-rays.
g. Referring to a rock composed of crystalline minerals, e.g., granite.
h. Referring to the texture of a rock composed of contiguous mineral
crystals with or without crystal faces.
i. Referring to underlying rock with coarse texture as opposed to
overlying noncrystalline or finely crystalline rock, e.g., Precambrian
crystalline basement.
An aggregate of crystalline intergrowths, such as granite, that does not
show well-defined crystal forms.
See:flake graphite
a. The external geometrical shape of a crystal. CMD
b. See:crystal form
Minute crystals or crystalline particles which compose a granular
crystalline aggregate. Distinguished from minute fiberlike crystals which
compose fibrous crystalline aggregates.
A primary texture due to crystallization from an aqueous medium, as in
rock salt (halite), gypsum, and anhydrite.