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 forms edit

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 edit

 
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 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 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

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 edit

References edit

  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". Archived from the original on 2017-12-01. Retrieved 2009-04-06.
  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.

Bibliography edit

  • Kostov, Ivan; Kostov, Ruslan I. (1999). Crystal Habits of Minerals. Sofia: Academic Publishing House "Prof. Marin Drinov". ISBN 9789546420602. OCLC 488807766.