Crystal habit

In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crystal group. A single crystal's habit is a description of its general shape and its crystallographic forms, plus how well developed each form is.

Pyrite sun (or dollar) in laminated shale matrix. Between tightly spaced layers of shale, the aggregate was forced to grow in a laterally compressed, radiating manner. Under normal conditions, pyrite would form cubes or pyritohedrons.

Recognizing the habit may help in identifying a mineral. When the faces are well-developed due to uncrowded growth a crystal is called euhedral, one with partially developed faces is subhedral, and one with undeveloped crystal faces is called anhedral. The long axis of a euhedral quartz crystal typically has a six-sided prismatic habit with parallel opposite faces. Aggregates can be formed of individual crystals with euhedral to anhedral grains. The arrangement of crystals within the aggregate can be characteristic of certain minerals. For example, minerals used for asbestos insulation often grow in a fibrous habit, a mass of very fine fibers.[1][2]

The terms used by mineralogists to report crystal habits describe the typical appearance of an ideal mineral. Recognizing the habit can aid in identification as some habits are characteristic. Most minerals, however, do not display ideal habits due to conditions during crystallization. Euhedral crystals formed in uncrowded conditions with no adjacent crystal grains are not common; more often faces are poorly formed or unformed against adjacent grains and the mineral's habit may not be easily recognized.[1]

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: this process is called 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.

The names of crystal habits are derived from:[citation needed]

  • Predominant crystal faces (prism – prismatic, pyramid – pyramidal and pinacoid – platy)
  • Crystal forms (cubic, octahedral, dodecahedral)
  • Aggregation of crystals or aggregates (fibrous, botryoidal, radiating, massive)
  • Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped))

List of crystal habitsEdit

Habit[3][4][5] Image Description Common example(s)
Acicular   Natrolite Needle-like, slender and/or tapered natrolite, rutile[6]
Amygdaloidal   Native copper Like embedded almonds heulandite, subhedral zircon
Bladed Actinolite Blade-like, slender and flattened actinolite, kyanite
Botryoidal or globular Malachite Grape-like, hemispherical masses hematite, pyrite, malachite, smithsonite, hemimorphite
Columnar Selenite (gypsum) Similar to fibrous: Long, slender prisms often with parallel growth calcite, gypsum/selenite
Coxcomb Marcasite Aggregated flaky or tabular crystals closely spaced. barite, marcasite
Cubic Fluorite Cube shape pyrite, galena, halite
Dendritic or arborescent Pyrolusite Tree-like, branching in one or more direction from central point romanechite, magnesite, native copper
Dodecahedral Garnet Rhombic dodecahedron, 12-sided garnet
Drusy or encrustation Quartz Aggregate of minute crystals coating a surface or cavity uvarovite, malachite, azurite
Enantiomorphic Gypsum Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals quartz, plagioclase, staurolite
Equant, stout Apophyllite Length, width, and breadth roughly equal olivine, garnet
Fibrous Byssolite Extremely slender prisms serpentine group, tremolite (i.e. asbestos)
Filiform or capillary Millerite Hair-like or thread-like, extremely fine many zeolites
Foliated or micaceous or lamellar (layered) Lepidolite Layered structure, parting into thin sheets muscovite, biotite
Granular Bornite Aggregates of anhedral crystals in matrix bornite, scheelite
Hemimorphic Hemimorphite Doubly terminated crystal with two differently shaped ends hemimorphite, elbaite
Hexagonal Corundum Hexagon shape, six-sided quartz, hanksite
Hopper crystals Halite Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity halite, calcite, synthetic bismuth
Mammillary Malachite Breast-like: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates malachite, hematite
Massive or compact Turquoise Shapeless, no distinctive external crystal shape limonite, turquoise, cinnabar, realgar
Nodular or tuberose Chalcedony Deposit of roughly spherical form with irregular protuberances chalcedony
Octahedral Diamond Octahedron, eight-sided (two pyramids base to base) diamond, magnetite
Platy Wulfenite Flat, tablet-shaped, prominent pinnacoid wulfenite
Plumose Aurichalcite Fine, feather-like scales aurichalcite, boulangerite, mottramite
Prismatic Tourmaline Elongate, prism-like: well-developed crystal faces parallel to the vertical axis tourmaline, beryl
Pseudo-hexagonal Aragonite Hexagonal appearance due to cyclic twinning aragonite, chrysoberyl
Radiating or radial or divergent Stibnite Radiating outward from a central point without producing a star (crystals are generally separated and have different lengths) stibnite
Reniform or colloform Mottramite Similar to botryoidal/mamillary: intersecting kidney-shaped masses hematite, pyrolusite, greenockite
Reticulated Cerussite Crystals forming net-like intergrowths cerussite
Rosette or lenticular (lens shaped crystals) Desert rose (barite) Platy, radiating rose-like aggregate gypsum, barite (i.e. desert rose)
Sphenoid Titanite Wedge-shaped sphene
Stalactitic Malachite Forming as stalactites or stalagmites; cylindrical or cone-shaped calcite, goethite, malachite
Stellate Pyrophyllite 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
Striated Pyrite Not a habit per se, but a condition of lines that can grow on certain crystal faces on certain minerals tourmaline, pyrite, quartz, feldspar, sphalerite
Tabular (also stubby or blocky) Oligoclase More elongated than equant, slightly longer than wide, flat tablet-shaped feldspar, topaz
Tetrahedral Tetrahedrite Tetrahedra-shaped crystals tetrahedrite, spinel, magnetite
Wheat sheaf Stilbite Aggregates resembling hand-reaped wheat sheaves stilbite

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 ZrSiO4". Journal of Crystal Growth. 359: 83–91. arXiv:1303.2761. Bibcode:2012JCrGr.359...83H. doi:10.1016/j.jcrysgro.2012.08.015. S2CID 94096447.