Crystal Habit

In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. 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:>
b>Description: Example:
cicular Needle-like, slender and/or tapered Rutile in quartz
mygdaloidal Almond-shaped Heulandite
nhedral Poorly formed, distorted Olivine
laded Blade-like, slender and flattened Kyanite
otryoidal or globular Grape-like, hemispherical masses Smithsonite
olumnar Similar to fibrous: Long, slender prisms often with parallel growth Calcite
oxcomb Aggregated flaky or tabular crystals closely spaced. Barite
endritic or arborescent Tree-like, branching in one or more direction from central point Magnesite in opal
odecahedral Dodecahedron, 12-sided Garnet
rusy or encrustation Aggregate of minute crystals coating a surface Uvarovite
nantiomorphic Mirror-image habit and optical characteristics; right- and left-handed crystals Quartz
quant, stout, stubby or blocky Squashed, pinnacoids dominant over prisms Zircon
uhedral Well-formed, undistorted Spinel
ibrous or columnar Extremely slender prisms Tremolite
iliform or capillary Hair-like or thread-like, extremely fine Natrolite
oliated or micaceous Layered structure, parting into thin sheets Mica
ranular Aggregates of anhedral crystals in matrix Scheelite
emimorphic Doubly terminated crystal with two differently shaped ends. Hemimorphite
amillary Breast-like: intersecting large rounded contours Malachite
assive or compact Shapeless, no distinctive external crystal shape Serpentine
odular or tuberose Deposit of roughly spherical form with irregular protuberances Geodes
ctahedral Octahedron, eight-sided (two pyramids base to base) Magnetite
lumose Fine, feather-like scales Mottramite
rismatic Elongate, prism-like: all crystal faces parallel to c-axis Tourmaline
seudo-hexagonal Ostensibly hexagonal due to cyclic twinning Aragonite
seudomorphous Occurring in the shape of another mineral through pseudomorphous replacement Tiger's eye
adiating or divergent Radiating outward from a central point Pyrite suns
eniform or colloform Similar to mamillary: intersecting kidney-shaped masses Hematite
eticulated Acicular crystals forming net-like intergrowths Cerussite
osette Platy, radiating rose-like aggregate Gypsum
phenoid Wedge-shaped Sphene
talactitic Forming as stalactites or stalagmites; cylindrical or cone-shaped Rhodochrosite
tellate Star-like, radiating Pyrophyllite
triated/striations Surface growth lines parallel or perpendicular to c-axis Chrysoberyl
abular or lamellar Flat, tablet-shaped, prominent pinnacoid Ruby
heat sheaf Aggregates resembling hand-reaped wheat sheaves Zeolites

 

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