Summary
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]
Crystal forms
editRecognizing 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
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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
edit[3][better source needed][4][better source needed][5][better source needed][6]
Aggregate habits
edit| 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 spheres 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 |
Asymmetrical/Irregular habits
edit| 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 |
Symmetrical habits
edit| 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 |
Rounded/Spherical habits
edit| 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 |
See also
editReferences
edit- ^ 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
- ^ "What are descriptive crystal habits". Archived from the original on 2017-07-07. Retrieved 2009-04-06.
- ^ Crystal Habit Archived 2009-04-12 at the Wayback Machine
- ^ "Habit". Archived from the original on 2017-12-01. Retrieved 2009-04-06.
- ^ 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.
Bibliography
edit- Kostov, Ivan; Kostov, Ruslan I. (1999). Crystal Habits of Minerals. Sofia: Academic Publishing House "Prof. Marin Drinov". ISBN 9789546420602. OCLC 488807766.