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The
six crystal systems of Crystallography
describe the patterns
of the molecules of crystals. Crystals can
also be described by their external shape
or "habit". Terms for crystal
shapes include acicular (needlelike),
bladed (flat with sharp edges), equant (roughly
equal length sides), filiform (hairlike),
prismatic (elongated), pyramidal (shaped
like single or double pyramids) and tabular
(flat with flat, perpendicular edges).
The
way minerals grow is greatly affected
by the environment and temperature in which
they form. Minerals deposited in sedimentary
environments are usually earthy (masses
of densley packed powder), stalactitic (shaped
like stalactites), oolitic or (masses of
spherical grains) and sometimes massive
(solid and chunky). Igneous minerals are
usually crystalline (varioius crystal shapes),
massive (solid and chunky) and sometimes
cleavable (crystalline masses that can be
cleaved).
Below
is a more detailed list of crystal habits.
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List
of Crystal Habits (shapes)
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|
Habit
|
Description
|
Example
|
|
Acicular |
Needle-like,
slender and/or tapered |
Rutile
in Quartz |
|
Amygdaloidal |
Almond-shaped |
Heulandite |
|
Anhedral |
Poorly
formed, distorted |
Olivine (Peridot) |
|
Bladed |
Blade-like,
slender and flattened |
Kyanite |
|
Botryoidal
or globular |
Grape-like,
hemispherical masses |
Smithsonite |
|
Columnar |
Similar
to fibrous: Long, slender prisms often with
parallel growth |
Calcite |
|
Coxcomb |
Aggregated
flaky or tabular crystals closely spaced. |
Barite |
|
Dendritic
or arborescent |
Tree-like,
branching in one or more direction from
central point |
Magnesite
in Opal |
|
Dodecahedral |
Dodecahedron,
12-sided |
Garnet |
|
Drusy
or encrustation |
Aggregate
of minute crystals coating a surface |
Uvarovite |
|
Enantiomorphic |
Mirror-image
habit and optical characteristics; right-
and left-handed crystals |
Quartz |
|
Equant,
stout, stubby or blocky |
Squashed,
pinnacoids dominant over prisms |
Zircon |
|
Euhedral |
Well-formed,
undistorted |
Spinel |
|
Fibrous
or columnar |
Extremely
slender prisms |
Tremolite |
|
Filiform
or capillary |
Hair-like
or thread-like, extremely fine |
Natrolite |
|
Foliated
or micaceous |
Layered
structure, parting into thin sheets |
Mica |
|
Granular |
Aggregates
of anhedral crystals in matrix |
Scheelite |
|
Hemimorphic |
Doubly
terminated crystal with two differently
shaped ends. |
Hemimorphite |
|
Mamillary |
Breast-like:
intersecting large rounded contours |
Malachite |
|
Massive
or compact |
Shapeless,
no distinctive external crystal shape |
Serpentine |
|
Nodular
or tuberose |
Deposit
of roughly spherical form with irregular
protuberances |
Geodes |
|
Octahedral |
Octahedron,
eight-sided (two pyramids base to base) |
Magnetite |
|
Plumose |
Fine,
feather-like scales |
Mattramite
|
|
Prismatic |
Elongate,
prism-like: all crystal faces parallel to
c-axis |
Tourmaline |
|
Pseudo-hexagonal |
Ostensibly
hexagonal due to cyclic twinning |
Aragonite |
|
Pseudomorphous |
Occurring
in the shape of another mineral through
pseudomorphous replacement |
Tiger's
eye |
|
Radiating
or divergent |
Radiating
outward from a central point |
Pyrite
suns |
|
Reniform
or colloform |
Similar
to mamillary: intersecting kidney-shaped
masses |
Hematite |
|
Reticulated |
Acicular
crystals forming net-like intergrowths |
Cerussite |
|
Rosette |
Platy,
radiating rose-like aggregate |
Gypsum |
|
Sphenoid |
Wedge-shaped |
Sphene |
|
Stalactitic |
Forming
as stalactites or stalagmites; cylindrical
or cone-shaped |
Rhodochrosite (Argentina) |
|
Stellate |
Star-like,
radiating |
Pyrophyllite |
|
Striated/striations |
Surface
growth lines parallel or perpendicular to
c-axis |
Chrysoberyl |
|
Tabular
or lamellar |
Flat,
tablet-shaped, prominent pinnacoid |
Ruby |
|
Wheat
sheaf |
Aggregates
resembling hand-reaped wheat sheaves |
Zeolites |
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