- Crystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates.
In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernable habit seen).
The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing 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 (the development of a particular habit is determined by the details of the conditions during the mineral formation/crystal growth). Crystal habit may mislead the inexperienced as a mineral's internal crystal system can be hidden or disguised.
Factors influencing a crystal's habit include: a combination of two or more crystal forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space); and specific growth tendencies like growth striations. 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 prismatic (elongate, prism-like) crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.
The names of crystal habits are derived from:
Predominant crystal faces (prism - prismatic, pyramid - pyramidal and pinacoid - platy). Crystal forms (cubic, octahedral, dodecahedral). Aggregation of crystals or aggregates (fibrous, botryoidal, radiating, massive). Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped)).
List of crystal habits
Habit Image Description Common Example(s) Acicular Needle-like, slender and/or tapered Natrolite, Rutile Amygdaloidal Almond-shaped Heulandite, subhedral Zircon Bladed Blade-like, slender and flattened Actinolite, Kyanite Botryoidal or globular Grape-like, hemispherical masses Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite Columnar Similar to fibrous: Long, slender prisms often with parallel growth Calcite, Gypsum/Selenite Coxcomb Aggregated flaky or tabular crystals closely spaced. Barite, Marcasite Cubic Cube shape Pyrite, Galena, Halite Dendritic or arborescent Tree-like, branching in one or more direction from central point Pyrolusite and other Mn-oxide minerals, Magnesite, native copper Dodecahedral Dodecahedron, 12-sided Garnet Drusy or encrustation Aggregate of minute crystals coating a surface or cavity Uvarovite, Malachite, Azurite Enantiomorphic Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals Quartz, Plagioclase, Staurolite Equant, stout Length, width, and breadth roughly equal Olivine, Garnet Fibrous Extremely slender prisms Serpentine group, Tremolite (i.e. Asbestos) Filiform or capillary Hair-like or thread-like, extremely fine many Zeolites Foliated or micaceous or lamellar (layered) Layered structure, parting into thin sheets Mica (Muscovite, Biotite, etc.) Granular Aggregates of anhedral crystals in matrix Bornite, Scheelite Hemimorphic Doubly terminated crystal with two differently shaped ends. Hemimorphite, Elbaite Hexagonal Hexagon shape, six-sided Quartz, Hanksite Hopper crystals Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity Halite, Calcite, synthetic Bismuth Mamillary Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates Malachite, Hematite Massive or compact Shapeless, no distinctive external crystal shape Limonite, Turquoise, Cinnabar, Realgar Nodular or tuberose Deposit of roughly spherical form with irregular protuberances Chalcedony, various Geodes Octahedral Octahedron, eight-sided (two pyramids base to base) Diamond, Magnetite Plumose Fine, feather-like scales Aurichalcite, Boulangerite, Mottramite Prismatic Elongate, prism-like, semi-cylidrical: crystal faces parallel to c-axis well-developed Tourmaline, Beryl Pseudo-hexagonal Hexagonal appearance due to cyclic twinning Aragonite, Chrysoberyl Radiating or divergent Radiating outward from a central point Wavellite, Pyrite suns Reniform or colloform Similar to botryoidal/mamillary: intersecting kidney-shaped masses Hematite, Pyrolusite, Greenockite Reticulated Crystals forming net-like intergrowths Cerussite Rosette or lenticular (lens shaped crystals) Platy, radiating rose-like aggregate Gypsum, Barite (i.e. Desert rose) Sphenoid Wedge-shaped Sphene Stalactitic Forming as stalactites or stalagmites; cylindrical or cone-shaped Calcite, Goethite Stellate Star-like, radiating Pyrophyllite, Aragonite Striated Not a habit per se, but a condition of lines that can grow on certain crystal faces on certain minerals Tourmaline, Pyrite, Quartz, Feldspar, Sphalerite Stubby or blocky or tabular More elongated than equant, slightly longer than wide, flat tablet shaped Feldspar, Topaz Platy Flat, tablet-shaped, prominent pinnacoid Wulfenite Tetrahedral Tetrahedra-shaped crystals Tetrahedrite, Spinel, Magnetite Wheat sheaf Aggregates resembling hand-reaped wheat sheaves Stilbite
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