Inmineralogy,crystal habitis the characteristic external shape of an individualcrystalor aggregate of crystals. The habit of a crystal is dependent on itscrystallographic formand growth conditions, which generally creates irregularities due to limited space in thecrystallizingmedium (commonly inrocks).[1][2]
![](https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Garnet%2C_quartz%2C_feldspar_8.jpg/300px-Garnet%2C_quartz%2C_feldspar_8.jpg)
Crystal forms
editRecognizing the habit can aid in mineral identification and description, as the crystal habit is an external representation of theinternal 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
editFactors influencing habit include: a combination of two or more crystal forms; trace impurities present during growth;crystal twinningand growth conditions (i.e., heat, pressure, space); and specific growth tendencies such as growthstriations.Minerals belonging to the samecrystal systemdo not necessarily exhibit the same habit. Some habits of a mineral are unique to its variety and locality: For example, while mostsapphiresform elongate barrel-shaped crystals, those found inMontanaform stouttabularcrystals. Ordinarily, the latter habit is seen only inruby.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 istiger's eyequartz, crocidoliteasbestosreplaced bysilica.Whilequartztypically formsprismatic(elongate, prism-like) crystals, in tiger's eye the originalfibroushabit ofcrocidoliteis preserved.
List of crystal habits
edit[3][better source needed][4][better source needed][5][better source needed][6]
Aggregate habits
editHabit | 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 ofamethyst(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 ingeodesand somefossils | 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 inmatrixor other surface | andradite,bornite,scheelite,quartz,uvarovite | |||
Hopper | Outer portions of cubes grow faster than inner portions, creating a concavity similar to that of ahopper | 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 asstalactitesorstalagmites;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
editHabit | Image | Description | Common example(s) | ||
---|---|---|---|---|---|
Amygdaloidal | Like embeddedalmonds | 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
editHabit | 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 | Hexagonalprism (six-sided) | beryl,galena,quartz,hanksite,vanadinite | |||
Icositetrahedral | Icositetrahedron-shaped, 24-faced | analcime,spessartine | |||
Octahedral | Octahedron-shaped, squarebipyramid(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
editHabit | 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 | Irregularkidney-shaped spherical masses | cassiterite,chalcedony,chrysocolla,hematite,hemimorphitefluorite,goethite,greenockite,malachite,rhodochrosite,smithsonite,mottramite,wavellite |
See also
editReferences
edit- ^abKlein, 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,ISBN978-0471772774
- ^Wenk, Hans-Rudolph and Andrei Bulakh, 2004,Minerals: Their Constitution and Origin,Cambridge, first edition,ISBN978-0521529587
- ^"What are descriptive crystal habits".Archived fromthe originalon 2017-07-07.Retrieved2009-04-06.
- ^Crystal HabitArchived2009-04-12 at theWayback Machine
- ^"Habit".Archived fromthe originalon 2017-12-01.Retrieved2009-04-06.
- ^Hanaor, D.A.H; Xu, W; Ferry, M; Sorrell, CC (2012). "Abnormal grain growth of rutile TiO2induced by ZrSiO ".Journal of Crystal Growth.359:83–91.arXiv:1303.2761.Bibcode:2012JCrGr.359...83H.doi:10.1016/j.jcrysgro.2012.08.015.S2CID94096447.
Bibliography
edit- Kostov, Ivan;Kostov, Ruslan I. (1999).Crystal Habits of Minerals.Sofia: Academic Publishing House "Prof. Marin Drinov".ISBN9789546420602.OCLC488807766.