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).[1] [2]
Recognizing the habit can aid in mineral identification and description, as the crystal habit is an external representation of the internal ordered atomic arrangement. 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:
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.
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Acicular | Needle-like, slender, and end-tapered prisms growing in a radial/globular fashion. | natrolite, scolecite, yuanfuliite | |
| Arborescent | Tree-like crystals growing similar to branches. | copper, gold, silver | |
| Capillary/Filiform | Hair-like or thread-like, extremely fine | byssolite, millerite | |
| Colloform/Nodular/Tuberose | Rounded, finely banded deposits with irregular concentric protuberances | agate, baryte, sphalerite | |
| Concentric | 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 | Root-like, branching in one or more direction from central point | copper, gold, romanechite, magnesite, silver | |
| Druse/Encrustation | Aggregate of crystals coating a surface or cavity, usually found in geodes and some fossils | azurite, celestine, calcite, uvarovite, malachite, quartz | |
| Fibrous/Asbestiform | Extremely slender prisms forming muscle-like fibers | actinolite, asbestos, baryte, kyanite, gypsum, nitratine, stilbite, serpentine group | |
| Foliated/Micaceous/Lamellar | Layered crystal planes, parting into thin sheets | biotite, hematite, muscovite, lepidolite, molybdenite | |
| Granular | Aggregates of diminute anhedral crystals in matrix or other surface | andradite, bornite, scheelite, quartz, uvarovite | |
| Hopper | Outer portions of cubes grow faster than inner portions, creating a concavity similar to that of a hopper | bismuth (artificial), halite, galena | |
| Oolithic | Small cirumferences or grains (commonly flattened) that resemble eggs | aragonite, calcite | |
| Pisolitic | Rounded concentric nodules often found in sedimentary rocks. Much larger than oolithic | aragonite, bauxite, calcite, pisolite | |
| Platy/Tabular/Blocky | Flat, tablet-shaped, prominent pinnacoid | baryte, feldspar, topaz, vanadinite, wulfenite | |
| Plumose | Fine, feather-like scales | aurichalcite, okenite, mottramite | |
| Radial/Radiating/Divergent | Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths). | aenigmatite, atacamite, epidote, pyrophyllite, stibnite | |
| Reticulated | Crystals forming triangular net-like intergrowths. | cerussite, rutile | |
| Rosette/Lenticular | Platy, radiating rose-like aggregate (also lens shaped crystals) | gypsum, baryte, calcite | |
| Stalactitic | Forming as stalactites or stalagmites; cylindrical or cone-shaped. Their cross-sections often reveal a "concentric" pattern | calcite, chalcedony, chrysocolla, goethite, malachite, romanechite | |
| Stellate | Star-like, radial fibers found inside spherical habits, such as mamillary or reniform. | hematite, pectolite, shattuckite, wavellite | |
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Amygdaloidal | Like embedded almonds | heulandite, stilbite, zircon | |
| Hemimorphic | Doubly terminated crystal with two differently shaped ends | elbaite, hemimorphite, olivine | |
| Massive/Compact | Shapeless, no distinctive external crystal shape | limonite, turquoise, cinnabar, quartz, realgar, lazurite | |
| Sceptered | 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 | |
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Cubic | Cube-shaped | fluorite, pyrite, galena, halite | |
| Dodecahedral | Dodecahedron-shaped, 12-sided. Central facet can vary. | garnet, pyrite | |
| Enantiomorphic | Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals | aragonite, gypsum, quartz, plagioclase, staurolite | |
| Hexagonal | Hexagonal prism (six-sided) | beryl, galena, quartz, hanksite, vanadinite | |
| Icositetrahedral | Icositetrahedron-shaped, 24-faced | analcime, spessartine | |
| Octahedral | Octahedron-shaped, square bipyramid (eight-sided) | diamond, fluorine, fluorite, magnetite, pyrite | |
| Prismatic | Elongate, prism-like: may or not present well-developed crystal faces parallel to the vertical axis | beryl, tourmaline, vanadinite | |
| Rhombohedral | Rhombohedron-shaped (six-faced rhombi) | calcite, magnesite, rhodochrosite, siderite | |
| Scalenohedral | Scalenohedron-shaped, pointy ends | calcite, rhodochrosite, titanite | |
| Tetrahedral | Tetrahedron-shaped, triangular pyramid (four-sided) | chalcopyrite, tetrahedrite, sphalerite, magnetite | |
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Botryoidal | Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other | calcite, chalcedony, halite, plumbogummite, smithsonite | |
| Globular | Isolated hemispheres or spheres | calcite, fluorite, gyrolite | |
| Mammillary | Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal and/or reniform, also concentric layered aggregates. | chalcedony, hematite, malachite | |
| Reniform | Irregular kidney-shaped spherical masses | cassiterite, chalcedony, chrysocolla, hematite, hemimorphite fluorite, goethite, greenockite, malachite, rhodochrosite, smithsonite, mottramite, wavellite | |