Crystal habit

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]

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

Crystal formsEdit

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 factorsEdit

 
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 habitsEdit

[3][better source needed][4][better source needed][5][better source needed][6]

Aggregate habitsEdit

Habit Image Description Common example(s)
Acicular   Natrolite Needle-like, slender and/or tapered natrolite, rutile
Bladed
 
Quartz
Blade-like ends, slender and somewhat flattened quartz, stilbite, kyanite
Columnar
 
Selenite (gypsum)
Similar to bladed and radial: Long, slender prisms often with parallel growth calcite, gypsum/selenite
Concentric
 
Quartz
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
 
Marcasite
Aggregated flaky or tabular crystals closely spaced. barite, marcasite
Dendritic/Arborescent
 
Copper
Tree-like, branching in one or more direction from central point romanechite, magnesite, native copper
Druse/Encrustation
 
Celestine
Aggregate of crystals coating a surface or cavity, usually found in geodes azurite, celestine, calcite, uvarovite, malachite, quartz
Fibrous (including asbestiform)
 
Kyanite
Extremely slender prisms, muscle-like fibers serpentine group, actinolite, kyanite, gypsum, nitratine, tremolite (i.e. asbestos)
Filiform or capillary
 
Byssolite
Hair-like or thread-like, extremely fine many zeolites, byssolite, millerite, okenite
Foliated/Micaceous/Lamellar
 
Muscovite
Layered crystal structures, parting into thin sheets muscovite, biotite, lepidolite, molybdenite
Granular
 
Quartz
Aggregates of diminute anhedral crystals in matrix or other surface andradite, bornite, scheelite, quartz
Hopper
 
Halite
Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity halite, calcite, synthetic bismuth
Oolithic
 
Calcite
Small cirumferences or grains (commonly flattened) that resemble eggs aragonite, calcite
Pisolithic
 
Bauxite
Rounded concentric nodules often found in sedimentary rocks. Much larger than oolithic bauxite, gibbsite
Platy
 
Wulfenite
Flat, tablet-shaped, prominent pinnacoid wulfenite
Plumose
 
Aurichalcite
Fine, feather-like scales aurichalcite, boulangerite, mottramite
Radial/Radiating/Divergent
 
Atacamite
Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths) atacamite, stibnite
Reticulated
 
Cerussite
Crystals forming net-like intergrowths cerussite
Rosette/Lenticular
 
Baryte
Platy, radiating rose-like aggregate (also lens shaped crystals) gypsum, baryte, calcite
Stalactitic
 
Chrysocolla
Forming as stalactites or stalagmites; cylindrical or cone-shaped. Their cross-sections often reveal a "concentric" pattern calcite, chrysocolla, goethite, malachite
Stellate
 
Wavellite
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
 
Vanadinite
More elongated than equant, slightly longer than wide, flat tablet-shaped feldspar, topaz, vanadinite
Wheat sheaf
 
Stilbite
Aggregates resembling hand-reaped wheat sheaves stilbite

Asymmetrical/Irregular habitsEdit

Habit Image Description Common example(s)
Amygdaloidal
 
Native copper
Like embedded almonds heulandite, subhedral zircon
Hemimorphic
 
Hemimorphite
Doubly terminated crystal with two differently shaped ends hemimorphite, elbaite
Massive/Compact
 
Turquoise
Shapeless, no distinctive external crystal shape limonite, turquoise, cinnabar, quartz, realgar, lazurite
Nodular/Tuberose
 
Agate
Deposit of roughly spherical form with irregular protuberances agate (and other chalcedony)
Sceptered
 
Quartz
Crystal growth stops and continues at the top of the crystal, but not at the bottom hedenbergite, quartz

Symmetrical habitsEdit

Habit Image Description Common example(s)
Cubic
 
Halite
Cube shape fluorite, pyrite, galena, halite
Dodecahedral
 
Pyrite
Dodecahedron-shaped, 12-sided garnet, pyrite
Enantiomorphic
 
Gypsum
Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals gypsum, quartz, plagioclase, staurolite
Equant/Stout
 
Olivine
Length, width, and breadth roughly equal apophyllite, olivine, garnet
Hexagonal
 
Emerald
Hexagonal prism (six-sided) emerald, galena, quartz, hanksite, vanadinite
Icositetrahedral
 
Spessartine
Icositetrahedron-shaped, 24-faced spessartine
Octahedral
 
Fluorine
Octahedron-shaped, square bipyramid (eight-sided) diamond, fluorine, fluorite, magnetite, pyrite
Prismatic
 
Beryl
Elongate, prism-like: well-developed crystal faces parallel to the vertical axis beryl, tourmaline, vanadinite, emerald
Pseudo-hexagonal
 
Aragonite
Hexagon-like appearance due to cyclic twinning aragonite, chrysoberyl
Rhombohedral
 
Siderite
Rhombohedron-shaped (six-faced rhombi) calcite, rhodochrosite, siderite
Scalenohedral
 
Rhodochrosite
Scalenohedron-shaped, pointy ends calcite, rhodochrosite, titanite
Tetrahedral
 
Sphalerite
Tetrahedron-shaped, triangular pyramid (four-sided) tetrahedrite, spinel, sphalerite, magnetite

Rounded/Spherical habitsEdit

Habit Image Description Common example(s)
Botryoidal
 
Chalcedony
Grape-like, large and small hemispherical masses, nearly differentiated/separated from each other chalcedony, pyrite, smithsonite, hemimorphite
Colloform
 
Sphalerite
Rounded, finely banded sphalerite, pyrite
Globular
 
Gyrolite
Isolated hemispheres or spheres calcite, fluorite, gyrolite
Mammillary
 
Chalcedony
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
 
Mottramite
Kidney-shaped masses cassiterite, chalcedony, chrysocolla, hematite, fluorite, goethite, greenockite, malachite, wavellite, mottramite

See alsoEdit

ReferencesEdit

  1. ^ 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
  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". Archived from the original on 2017-07-07. Retrieved 2009-04-06.
  4. ^ Crystal Habit Archived 2009-04-12 at the Wayback Machine
  5. ^ Habit
  6. ^ 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.