From Wikipedia,
the free encyclopedia
In
mineralogy, shape and size
give rise to descriptive terms
applied to the typical appearance,
or habit of
crystals.
Pyrite sun (or dollar) in
laminated shale matrix.
Between tightly spaced
layers of shale, the
aggregate was forced to grow
in a laterally compressed,
radiating manner. Under
normal conditions, pyrite
would form cubes or
pyritohedrons.
The many terms used by
mineralogists to describe crystal
habits are useful in communicating
what specimens of a particular
mineral often look like.
Recognising numerous habits helps
a mineralogist to identify a large
number of minerals. Some habits
are distinctive of certain
minerals, although most minerals
exhibit many differing habits
which are influenced by certain
factors. Crystal habit may mislead
the inexperienced as a mineral's
crystal system can be hidden
or disguised.
Factors influencing a crystal's
habit include: a combination of
two or more
forms; trace impurities
present during growth;
crystal twinning and growth
conditions (i.e., heat, pressure,
space). Minerals belonging to the
same crystal system do not
necessarily exhibit the same
habit. Some habits of a mineral
are unique to its variety and
locality: For example, while most
sapphires form elongate
barrel-shaped crystals, those
found in
Montana form stout tabular
crystals. Ordinarily, the latter
habit is seen only in
ruby. Sapphire and ruby are
both varieties of the same
mineral;
corundum.
Some minerals may replace other
existing minerals while preserving
the original's habit: this process
is called
pseudomorphous replacement. A
classic example is
tiger's eye
quartz,
crocidolite
asbestos replaced by
silica. While quartz typically
forms euhedral
(well-formed), prismatic
(elongate, prism-like) crystals,
in tiger's eye the original
fibrous habit of crocidolite
is preserved.
List of crystal habits
| Habit: |
Description: |
Example: |
| Acicular |
Needle-like, slender
and/or tapered |
Rutile in quartz |
| Amygdaloidal |
Almond-shaped |
Heulandite |
| Anhedral |
Poorly formed, distorted |
Olivine |
| 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 |
Mottramite |
| 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 |
| 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 |
