Habit: The descriptive term used in mineralogy.
This description includes all the crystal’s visible characteristics, general shape and its crystallographic forms, as well as how developed each form is.
In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernable habit seen). Recognizing the habit may help in identifying a mineral. Descriptions incorporate the names of the crystal’s faces (for example, prismatic or pyramidal) and the name of its form (for example, cubic or octahedral). It also includes more general descriptive terms, such as bladed or dendritic. The description of crystal habit can relate to a single crystal, or to an assemblage of inter-grown crystals, known as an aggregate. Various terms are used to describe aggregates, such as massive or radiating.
The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing numerous habits helps a mineralogist to identify a large number of minerals. Some habits are distinctive of certain minerals, although most minerals exhibit many differing habits (the development of a particular habit is determined by the details of the conditions during the mineral formation/crystal growth). Crystal habit may mislead the inexperienced as a mineral's internal crystal system can be hidden or disguised.
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 is replaced by silica. While quartz typically forms prismatic (elongated, prism-like) crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.
List of Crystal Habits:
Acicular- Needle-like, slender and/or tapered (Natrolite, Rutile)
Amydaloidal- Almond-shaped (Heulandite, subhedral Zircon)
Bladed- Blade-like, slender, and flattened (Actinolite, Kyanite)
Botyoidal or Globular- Grape-like, hemispherical masses (Hematite, Pyrite, Malachite, Smithsonite, Hemimorphite, Adamite, Variscite)
Columnar- Similar to fibrous: Long, slender prisms often with parallel growth (Calcite, Gypsum/Selenite)
Coxcomb- Aggregated flaky or tabular crystals closely spaced (Barite, Marcasite)
Cubic- cube shape (Pyrite, Galena, Halite)
Dendritic or Arborescent- Tree-like, branching in one or more directions from central point (Pyrolusite and other Mn-oxide minerals, Magnesite, Native Copper)
Dodecahedral- Dodecahedron (twelve-sided) (Garnet)
Druzy or Encrustation- Aggregate of minute crystals coating a surface or cavity (Uvarovite, Malachite, Azurite, Quartz)
Enantiomorphic- Mirror-image habit (i.e. crystal twinning) and optical characteristics; right- and left-handed crystals (Quartz, Plagioclase, Staruolite)
Equant or Stout- Length, width, and breadth roughly equal (Olivine, Garnet)
Fibrous- Extremely slender prisms (Serpentine group, Tremolite (i.e. Abestos)
Filiform or Capillary- Hair-like or thread-like, extremely fine (many Zeolites)
Foliated or Micaceous or Lamellar- Layered structure, parting into thin sheets/books (Mica (Muscovite, Biotite, etc.)
Granular- Aggregates of anhedral crystals in matrix (Bornite, Sheelite)
Hemimorphic- Doubly terminated crystal with two differently shaped ends (Hemimorphite, Elbaite)
Hexagonal- Hexagon shape (six-sided) (Quartz, Hanksite)
Hoppers- Like cubic, but outer portions of cubes grow faster than inner portions, creating a concavity (Halite, Calcite, sythetic Bismuth)
Mamillary- Breastlike: surface formed by intersecting partial spherical shapes, larger version of botryoidal, also concentric layered aggregates (Malachite, Hematite)
Massive or Compact- Shapeless, no distinctive external crystal shape (Limonite, Turquoise, Cinnabar, Realgar)
Nodular or Tuberose- Deposit of roughly spherical form with irregular protuberances (Chalcedony, various Geodes)
Octahedral- Octahedron, eight-sided (two pyramids base to base) (Diamond, Magnetite)
Plumose- Fine, feather-like scales (Aurichalcite, Boulangerite, Mottramite)
Prismatic- Elongated, prism-like, semi-cylidrical crystal faces parallel to c-axis that are well-developed (Tourmaline, Beryl)
Pseudo-hexagonal- Hexagonal appearance due to cyclic twinning (Aragonite, Chrysoberyl)
Radiating or Divergent- Radiating outward from a central point (Wavellite, Pyrite Suns)
Reniform or Colloform- Similar to botryoidal/mamillary: intersecting kidney-shaped masses (Hematite, Pyrolusite, Greenockite)
Reticulated- Crystals forming net-like inter-growths (Cerussite)
Rosette or Lenticular (lens shaped)- Platy, radiating rose-like aggregate (Gypsum, Barite (ie. Desert Rose)
Sphenoid- Wedge-shaped (Sphene)
Stalactitic- Forming as stalactites or stalagmites; cylindrical or cone-shaped (Calcite, Goethite)
Stellate- Star-like, radiating (Pyrophyllite, Aragonite)
Striated- 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)
Stubby or Blocky or Tabular- More elongated than equant, slightly longer than wide, flat tablet shaped (Feldspar, Topaz)
Platy- Flat, tablet-shaped, prominent pinnacoid (Wulfenite)
Tetrahedral- Tetrahedra-shaped crystals (Tetrahedrite, Spinel, Magnetite)
Wheat Sheaf- Aggregates resembling hand-reaped wheat sheaves (Stilbite)
The names of crystal habits are derived from:
In most crystals some faces are more developed than others. As a crystal grows, the faces that grow most quickly eventually eliminate themselves, while those that grow slowly become more prominent. There are three types of crystal faces, which are determined by their relationship to the crystallographic axes:
Prism faces: A prism is a set of faces that run parallel (meaning they never converge) to an axes in the crystal. There can be three, four, six, eight or even twelve faces that can form a prism. All the faces of one prism must be separated form each other by a specific amount of degrees or they are not of the same prism.
Pyramid faces: The tetrahedron has only four equilateral triangular faces (unless modified), four points and six edges and when sitting on one face looks like a trigonal pyramid. But every point of the tetrahedron can serve as the top of the pyramid because all four points are identical.
Pinacoid faces: A Pinacoid is an open 2-faced form made up of two parallel faces. Where pinacoid faces predominate the crystal, the crystal habit may be describes as platy.
A face at the end of a crystal may be described as a termination face.
Some crystal habits derive their names purely from their crystal forms: for example, cubic, crystallizing in the form of cubes; dodecahedral, crystallizing in the form of dodecahedrons; and rhombohedral, crystallizing in the form of rhombahedrons. If crystals of one system crystallize in forms that appear to be the crystals of another system, (pseudomorphous replacement) the habit name is preceeded by the word “pseudo”. For example, if cyclic twins of orthorhombic aragonite appear to form hexagonal prisms, they are described as pseudohexagonal. If the terminations (end faces) of the crystal are different from each other, the habit is known as hemimorphic.
Aggregates are groups of intimately associated crystals. They differ from clusters in that in crystal clusters there are a number of individuals growing together, but there is not an intimate intergrowth as in aggregates. The type of aggregation is often typical of the particular mineral species. Terms used to describe aggregates include: granular, fibrous (consists of slender, parallel or radiating fibers), radiating (radiate from a common center), botryoidal (globular aggregates, resembling a bunch of grapes), stalactitic (form in cylindars or cones resembling icicles), concentric (forming roughly spherical layers around a common center), geode (partially filled rock cavity lined by minerals), oolitic (spherical grains composed of concentric layers), and massive (a mass of crystals that cannot be seen individually).
Some crystal habits are descriptions of the general appearance of a crystal.
Tabular (predominantly large, flat, paralleled faces)
Bladed (elongated crystals flattened like a knife blade)
Stalactitic (describes crystal aggregates that have grown in the shape of a stalactite)
Lamellar (flat, plate-like individual crystals arranged in layers)
Lenticular (lens shapes)
Dendritic (form in slender, divergent, somewhat plant-like branches)