Crystal habit

Short description: Mineralogical term for the visible shape of a mineral

Smoky quartz with spessartine on top of feldspar matrix, featuring different crystal habits (shapes)

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]

## Contents

* 1 Crystal forms
* 2 Altering factors
* 3 List of crystal habits
* 3.1 Aggregate habits
* 3.2 Asymmetrical/Irregular habits
* 3.3 Symmetrical habits
* 3.4 Rounded/Spherical habits
* 4 See also
* 5 References

## 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

Goethite replacing pyrite cubes.

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] [4][better source needed]</ref>[better source needed][5]

### Aggregate habits

Habit  | Image  | Description  | Common example(s)

|
|
|

Acicular  | Script error: No such module "Image array". | Needle-like, slender, and end-tapered prisms growing in a radial/globular fashion.  | natrolite, scolecite, yuanfuliite
Arborescent  | Script error: No such module "Image array". | Tree-like crystals growing similar to branches.  | copper, gold, silver
Capillary/Filiform  | Script error: No such module "Image array". | Hair-like or thread-like, extremely fine  | byssolite, millerite
Colloform/Nodular/Tuberose  | Script error: No such module "Image array". | Rounded, finely banded deposits with irregular concentric protuberances  | agate, baryte, sphalerite
Concentric  | Script error: No such module "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  | Script error: No such module "Image array". | Root-like, branching in one or more direction from central point  | copper, gold, romanechite, magnesite, silver
Druse/Encrustation  | Script error: No such module "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 | Script error: No such module "Image array". | Extremely slender prisms forming muscle-like fibers  | actinolite, asbestos, baryte, kyanite, gypsum, nitratine, stilbite, serpentine group
Foliated/Micaceous/Lamellar  | Script error: No such module "Image array". | Layered crystal planes, parting into thin sheets  | biotite, hematite, muscovite, lepidolite, molybdenite
Granular  | Script error: No such module "Image array". | Aggregates of diminute anhedral crystals in matrix or other surface  | andradite, bornite, scheelite, quartz, uvarovite
Hopper | Script error: No such module "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  | Script error: No such module "Image array". | Small cirumferences or grains (commonly flattened) that resemble eggs  | aragonite, calcite
Pisolitic  | Script error: No such module "Image array". | Rounded concentric nodules often found in sedimentary rocks. Much larger than oolithic  | aragonite, bauxite, calcite, pisolite
Platy/Tabular/Blocky  | Script error: No such module "Image array". | Flat, tablet-shaped, prominent pinnacoid  | baryte, feldspar, topaz, vanadinite, wulfenite
Plumose  | Script error: No such module "Image array". | Fine, feather-like scales  | aurichalcite, okenite, mottramite
Radial/Radiating/Divergent  | Script error: No such module "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  | Script error: No such module "Image array". | Crystals forming triangular net-like intergrowths.  | cerussite, rutile
Rosette/Lenticular  | Script error: No such module "Image array". | Platy, radiating rose-like aggregate (also lens shaped crystals)  | gypsum, baryte, calcite
Stalactitic  | Script error: No such module "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  | Script error: No such module "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 | Script error: No such module "Image array". | Like embedded almonds | heulandite, stilbite, zircon
Hemimorphic  | Script error: No such module "Image array". | Doubly terminated crystal with two differently shaped ends  | elbaite, hemimorphite, olivine
Massive/Compact  | Script error: No such module "Image array". | Shapeless, no distinctive external crystal shape  | limonite, turquoise, cinnabar, quartz, realgar, lazurite
Sceptered  | Script error: No such module "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  | Script error: No such module "Image array". | Cube-shaped  | fluorite, pyrite, galena, halite
Dodecahedral  | Script error: No such module "Image array". | Dodecahedron-shaped, 12-sided. Central facet can vary.  | garnet, pyrite
Enantiomorphic  | Script error: No such module "Image array". | Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals  | aragonite, gypsum, quartz, plagioclase, staurolite
Hexagonal  | Script error: No such module "Image array". | Hexagonal prism (six-sided)  | beryl, galena, quartz, hanksite, vanadinite
Icositetrahedral  | Script error: No such module "Image array". | Icositetrahedron-shaped, 24-faced  | analcime, spessartine
Octahedral  | Script error: No such module "Image array". | Octahedron-shaped, square bipyramid (eight-sided)  | diamond, fluorine, fluorite, magnetite, pyrite
Prismatic  | Script error: No such module "Image array". | Elongate, prism-like: may or not present well-developed crystal faces parallel to the vertical axis  | beryl, tourmaline, vanadinite
Rhombohedral  | Script error: No such module "Image array". | Rhombohedron-shaped (six-faced rhombi)  | calcite, magnesite, rhodochrosite, siderite
Scalenohedral  | Script error: No such module "Image array". | Scalenohedron-shaped, pointy ends  | calcite, rhodochrosite, titanite
Tetrahedral  | Script error: No such module "Image array". | Tetrahedron-shaped, triangular pyramid (four-sided)  | chalcopyrite, tetrahedrite, sphalerite, magnetite

### Rounded/Spherical habits

Habit  | Image  | Description  | Common example(s)

|
|
|

Botryoidal  | Script error: No such module "Image array". | Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other  | calcite, chalcedony, halite, plumbogummite, smithsonite
Globular  | Script error: No such module "Image array". | Isolated hemispheres or spheres  | calcite, fluorite, gyrolite
Mammillary  | Script error: No such module "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  | Script error: No such module "Image array". | Irregular kidney-shaped spherical masses  | cassiterite, chalcedony, chrysocolla, hematite, hemimorphite fluorite, goethite, greenockite, malachite, rhodochrosite, smithsonite, mottramite, wavellite

## See also

* Abnormal grain growth
* Grain growth
* Crystal structure

## References

1. ↑ 1.0 1.1 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
2. ↑ Wenk, Hans-Rudolph and Andrei Bulakh, 2004, Minerals: Their Constitution and Origin, Cambridge, first edition, ISBN:978-0521529587
3. ↑ "What are descriptive crystal habits". http://www.galleries.com/minerals/property/habits.htm.
4. ↑ Crystal Habit
5. ↑ Hanaor (2012). "Abnormal grain growth of rutile TiO2 induced by ZrSiO". Journal of Crystal Growth 359: 83–91. doi:10.1016/j.jcrysgro.2012.08.015. Bibcode: 2012JCrGr.359...83H.

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Mineral identification

* Cleavage
* Crystal habit
* Crystal system
* Fracture
* Lustre
* Mohs scale
* Specific gravity
* Streak

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