Crystal habit
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In mineralogy, crystal habit is the characteristic external shape of an individual crystal or aggregate of crystals. The habit of a crystal is dependent on its crystallographic form and growth conditions, which generally creates irregularities due to limited space in the crystallizing medium (commonly in rocks).<ref name="Klein">Klein, Cornelis, 2007, Minerals and Rocks: Exercises in Crystal and Mineral Chemistry, Crystallography, X-ray Powder Diffraction, Mineral and Rock Identification, and Ore Mineralogy, Wiley, third edition, Template:ISBN</ref><ref name="Wenk">Wenk, Hans-Rudolph and Andrei Bulakh, 2004, Minerals: Their Constitution and Origin, Cambridge, first edition, Template:ISBN</ref>
Crystal forms
Recognizing the habit can aid in mineral identification and description, as the crystal habit is an external representation of the internal ordered atomic arrangement.<ref name="Klein" /> Most natural crystals, however, do not display ideal habits and are commonly malformed. Hence, it is also important to describe the quality of the shape of a mineral specimen:
- Euhedral: a crystal that is completely bounded by its characteristic faces, well-formed. Synonymous terms: idiomorphic, automorphic;
- Subhedral: a crystal partially bounded by its characteristic faces and partially by irregular surfaces. Synonymous terms: hypidiomorphic, hypautomorphic;
- Anhedral: a crystal that lacks any of its characteristic faces, completely malformed. Synonymous terms: allotriomorphic, xenomorphic.
Altering factors
Factors influencing 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 such as 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, i.e. 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. Template:Clear
List of crystal habits
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Aggregate habits
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Acicular | Template:Image array | Needle-like, slender, and end-tapered prisms growing in a radial/globular fashion. | natrolite, scolecite, yuanfuliite |
| Arborescent | Template:Image array | Tree-like crystals growing similar to branches. | copper, gold, silver |
| Capillary/Filiform | Template:Image array | Hair-like or thread-like, extremely fine | byssolite, millerite |
| Colloform/Nodular/Tuberose | Template:Image array | Rounded, finely banded deposits with irregular concentric protuberances | agate, baryte, sphalerite |
| Concentric | Template:Image array | Circular ring aggregates around a center. This habit is found in cross-sections from reniform/mamillary habits, and also from elongated stalactites of amethyst (quartz), malachites, rhodocrosite, and others | agate, quartz, malachite, rhodocrosite |
| Dendritic | Template:Image array | Root-like, branching in one or more direction from central point | copper, gold, romanechite, magnesite, silver |
| Druse/Encrustation | Template:Image array | Aggregate of crystals coating a surface or cavity, usually found in geodes and some fossils | azurite, celestine, calcite, uvarovite, malachite, quartz |
| Fibrous/Asbestiform | Template:Image array | Extremely slender prisms forming muscle-like fibers | actinolite, asbestos, baryte, kyanite, gypsum, nitratine, stilbite, serpentine group |
| Foliated/Micaceous/Lamellar | Template:Image array | Layered crystal planes, parting into thin sheets | biotite, hematite, muscovite, lepidolite, molybdenite |
| Granular | Template:Image array | Aggregates of diminute anhedral crystals in matrix or other surface | andradite, bornite, scheelite, quartz, uvarovite |
| Hopper | Template:Image array | Outer portions of cubes grow faster than inner portions, creating a concavity similar to that of a hopper | bismuth (artificial), halite, galena |
| Oolithic | Template:Image array | Small spheres or grains (commonly flattened) that resemble eggs | aragonite, calcite |
| Pisolitic | Template:Image array | Rounded concentric nodules often found in sedimentary rocks. Much larger than oolithic | aragonite, bauxite, calcite, pisolite |
| Platy/Tabular/Blocky | Template:Image array | Flat, tablet-shaped, prominent pinnacoid | baryte, feldspar, topaz, vanadinite, wulfenite |
| Plumose | Template:Image array | Fine, feather-like scales | aurichalcite, okenite, mottramite |
| Radial/Radiating/Divergent | Template:Image array | Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths). | aenigmatite, atacamite, epidote, pyrophyllite, stibnite |
| Reticulated | Template:Image array | Crystals forming triangular net-like intergrowths. | cerussite, rutile |
| Rosette/Lenticular | Template:Image array | Platy, radiating rose-like aggregate (also lens shaped crystals) | gypsum, baryte, calcite |
| Stalactitic | Template:Image array | Forming as stalactites or stalagmites; cylindrical or cone-shaped. Their cross-sections often reveal a "concentric" pattern | calcite, chalcedony, chrysocolla, goethite, malachite, romanechite |
| Stellate | Template:Image array | Star-like, radial fibers found inside spherical habits, such as mamillary or reniform. | hematite, pectolite, shattuckite, wavellite |
Asymmetrical/Irregular habits
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Amygdaloidal | Template:Image array | Like embedded almonds | heulandite, stilbite, zircon |
| Hemimorphic | Template:Image array | Doubly terminated crystal with two differently shaped ends | elbaite, hemimorphite, olivine |
| Massive/Compact | Template:Image array | Shapeless, no distinctive external crystal shape | limonite, turquoise, cinnabar, quartz, realgar, lazurite |
| Sceptered | Template:Image array | Crystal growth stops and continues at the top of the crystal, but not at the bottom. Exceptional aggregates of this habit (such as quartz) are often referred as "Elestial". | baryte, calcite, marcasite, quartz |
Symmetrical habits
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Cubic | Template:Image array | Cube-shaped | fluorite, pyrite, galena, halite |
| Dodecahedral | Template:Image array | Dodecahedron-shaped, 12-sided. Central facet can vary. | garnet, pyrite |
| Enantiomorphic | Template:Image array | Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals | aragonite, gypsum, quartz, plagioclase, staurolite |
| Hexagonal | Template:Image array | Hexagonal prism (six-sided) | beryl, galena, quartz, hanksite, vanadinite |
| Icositetrahedral | Template:Image array | Icositetrahedron-shaped, 24-faced | analcime, spessartine |
| Octahedral | Template:Image array | Octahedron-shaped, square bipyramid (eight-sided) | diamond, fluorine, fluorite, magnetite, pyrite |
| Prismatic | Template:Image array | Elongate, prism-like: may or not present well-developed crystal faces parallel to the vertical axis | beryl, tourmaline, vanadinite |
| Rhombohedral | Template:Image array | Rhombohedron-shaped (six-faced rhombi) | calcite, magnesite, rhodochrosite, siderite |
| Scalenohedral | Template:Image array | Scalenohedron-shaped, pointy ends | calcite, rhodochrosite, titanite |
| Tetrahedral | Template:Image array | Tetrahedron-shaped, triangular pyramid (four-sided) | chalcopyrite, tetrahedrite, sphalerite, magnetite |
Rounded/Spherical habits
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Botryoidal | Template:Image array | Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other | calcite, chalcedony, halite, plumbogummite, smithsonite |
| Globular | Template:Image array | Isolated hemispheres or spheres | calcite, fluorite, gyrolite |
| Mammillary | Template:Image array | Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal and/or reniform, also concentric layered aggregates. | chalcedony, hematite, malachite |
| Reniform | Template:Image array | Irregular kidney-shaped spherical masses | cassiterite, chalcedony, chrysocolla, hematite, hemimorphite fluorite, goethite, greenockite, malachite, rhodochrosite, smithsonite, mottramite, wavellite |