Wikipedia
Crystal habit
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]
Smoky quartz with spessartine on top of feldspar matrix, featuring different crystal habits (shapes)
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.[1] 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.
List of crystal habits
[3][better source needed][4][better source needed][5][better source needed][6]
Aggregate habits
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Acicular | Natrolite | Needle-like, slender and/or tapered | natrolite, rutile |
| Bladed | Blade-like ends, slender and somewhat flattened | quartz, stilbite, kyanite | |
| Columnar | Similar to bladed and radial: Long, slender prisms often with parallel growth | calcite, gypsum/selenite | |
| 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 | quartz, malachite, rhodocrosite | |
| Coxcomb | Aggregated flaky or tabular crystals closely spaced. | barite, marcasite | |
| Dendritic/Arborescent | Tree-like, branching in one or more direction from central point | romanechite, magnesite, native copper | |
| Druse/Encrustation | Aggregate of crystals coating a surface or cavity, usually found in geodes | azurite, celestine, calcite, uvarovite, malachite, quartz | |
| Fibrous (including asbestiform) | Extremely slender prisms, muscle-like fibers | serpentine group, actinolite, kyanite, gypsum, nitratine, tremolite (i.e. asbestos) | |
| Filiform or capillary | Hair-like or thread-like, extremely fine | many zeolites, byssolite, millerite, okenite | |
| Foliated/Micaceous/Lamellar | Layered crystal structures, parting into thin sheets | muscovite, biotite, lepidolite, molybdenite | |
| Granular | Aggregates of diminute anhedral crystals in matrix or other surface | andradite, bornite, scheelite, quartz | |
| Hopper | Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity | halite, calcite, synthetic bismuth | |
| 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 | bauxite, gibbsite | |
| Platy | Flat, tablet-shaped, prominent pinnacoid | wulfenite | |
| Plumose | Fine, feather-like scales | aurichalcite, boulangerite, mottramite | |
| Radial/Radiating/Divergent | Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths) | atacamite, stibnite | |
| Reticulated | Crystals forming net-like intergrowths | cerussite | |
| 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, chrysocolla, goethite, malachite | |
| Stellate | Star-like, radial aggregates radiating from a "star"-like point to produce gross spheres (crystals are not or weakly separated and have similar lengths) | pyrophyllite, aragonite, wavellite, "pyrite suns" | |
| Tabular/Blocky/Stubby | More elongated than equant, slightly longer than wide, flat tablet-shaped | feldspar, topaz, vanadinite | |
| Wheat sheaf | Aggregates resembling hand-reaped wheat sheaves | stilbite |
Asymmetrical/Irregular habits
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Amygdaloidal | Like embedded almonds | heulandite, subhedral zircon | |
| Hemimorphic | Doubly terminated crystal with two differently shaped ends | hemimorphite, elbaite | |
| Massive/Compact | Shapeless, no distinctive external crystal shape | limonite, turquoise, cinnabar, quartz, realgar, lazurite | |
| Nodular/Tuberose | Deposit of roughly spherical form with irregular protuberances | agate (and other chalcedony) | |
| Sceptered | Crystal growth stops and continues at the top of the crystal, but not at the bottom | hedenbergite, quartz |
Symmetrical habits
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Cubic | Cube shape | fluorite, pyrite, galena, halite | |
| Dodecahedral | Dodecahedron-shaped, 12-sided | garnet, pyrite | |
| Enantiomorphic | Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals | gypsum, quartz, plagioclase, staurolite | |
| Equant/Stout | Length, width, and breadth roughly equal | apophyllite, olivine, garnet | |
| Hexagonal | Hexagonal prism (six-sided) | emerald, galena, quartz, hanksite, vanadinite | |
| Icositetrahedral | Icositetrahedron-shaped, 24-faced | spessartine | |
| Octahedral | Octahedron-shaped, square bipyramid (eight-sided) | diamond, fluorine, fluorite, magnetite, pyrite | |
| Prismatic | Elongate, prism-like: well-developed crystal faces parallel to the vertical axis | beryl, tourmaline, vanadinite, emerald | |
| Pseudo-hexagonal | Hexagon-like appearance due to cyclic twinning | aragonite, chrysoberyl | |
| Rhombohedral | Rhombohedron-shaped (six-faced rhombi) | calcite, rhodochrosite, siderite | |
| Scalenohedral | Scalenohedron-shaped, pointy ends | calcite, rhodochrosite, titanite | |
| Tetrahedral | Tetrahedron-shaped, triangular pyramid (four-sided) | tetrahedrite, spinel, sphalerite, magnetite |
Rounded/Spherical habits
| Habit | Image | Description | Common example(s) |
|---|---|---|---|
| Botryoidal | Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other | chalcedony, pyrite, smithsonite, hemimorphite | |
| Colloform | Rounded, finely banded | sphalerite, pyrite | |
| 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. It is almost synonymous with reniform. | chalcedony, hematite, malachite | |
| Reniform | Kidney-shaped masses | cassiterite, chalcedony, chrysocolla, hematite, fluorite, goethite, greenockite, malachite, wavellite, mottramite |
See also
References
- ^ a b 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, ISBN 978-0471772774
- ^ Wenk, Hans-Rudolph and Andrei Bulakh, 2004, Minerals: Their Constitution and Origin, Cambridge, first edition, ISBN 978-0521529587
- ^ . Archived from the original on 2017-07-07. Retrieved 2009-04-06.
- ^ Crystal Habit 2009-04-12 at the Wayback Machine
- ^ Habit
- ^ Hanaor, D.A.H; Xu, W; Ferry, M; Sorrell, CC (2012). "Abnormal grain growth of rutile TiO2 induced by ZrSiO". Journal of Crystal Growth. 359: 83–91. arXiv:1303.2761. Bibcode:2012JCrGr.359...83H. doi:10.1016/j.jcrysgro.2012.08.015. S2CID 94096447.