Glossary of Mineralogical Terms and Habits

The following Glossary of Mineral and Crystal terms typically used to describe mineralogical properties and habits  in the Dakota Matrix mineral galleries.

Acicular

A crystal habit. Needle-like.

Adamantine

A brilliant luster with high reflectivity. Adamantine luster is extremely shiny. Hard minerals and lead bearing minerals typically have adamantine luster.

Amygdaloidal

A term usually applied to rocks with small hollow or partially filled voids. If filled with a mineral, these are nodules (solid) or small geodes typically composed of zeolite or silica.

Anhedral

A crystal that has no faces.

Bladed

A crystal habit. Flat and long, like a knife or Wrigley gum stick.

Blocky

A crystal habit. Blocky or equi-dimensional, brick-shaped.

Botryoidal

A crystal habit. In aggregate, forms grape-like groups.

Brittle

A mineral characteristic describing its tenacity. A brittle mineral is easily broken into many pieces with a slight blow of a hammer.

Capillary

A crystal habit. Hair-like. Filiform.

Chatoyancy

A non-determinative but indicative mineral property. The appearance of a mineral's surface which reflects a band of light which appears to move as the sample is rocked back and forth. These wavy reflections are an optical phenomenon created from tightly packed fibrous inclusions within the mineral.

Cleavage

A mineral property. The cleavage of a mineral is a smooth, definite plane produced when broken. Best seen and characterized under a light transmitting microscope, but can also be produced easily in a hand specimen. Cleavage is usually a reliable property which should be used in combination with other properties for determinative mineralogy. Cleavages are seen as good, distinct, to perfect.

Colloform

A crystal habit. Radiating individuals forming spherical or hemispherical shapes.

Columnar

A crystal habit. Several parallel columns in aggregate, much thicker than acicular.

Concentric

A general descriptive term for a mineral or rock. Spherical, hemispherical, or layers of banding, usually with sharp, distinctive boundaries and subtle color changes. The bands usually form around a common center.

Dendritic

A crystal habit. Branch-like forms.

Divergent

A crystal habit. Groups of crystals, usually acicular or prismatic, that spread outward like a fan.

Drusy

A general descriptive term. A surface covered with tiny crystals, such as "Drusy Quartz".

Ductile

A mineral characteristic describing its tenacity and is a metallurgical property. The ability of a metal to be mechanically deformed and elongated without fracturing when cold. Mineralogically, it is a relative term which only serves to differentiate a mineral from other tenacities such as brittle or elastic. In a practical, determinative sense, it is the same as malleable.

Dull

A luster with very little or no reflectivity. They are typically porous or fine grained.

Earthy

A luster which resembles dirt or clay.

Elastic

A mineral characteristic describing its tenacity. A mineral is elastic if it rebounds to its original position when bent, typically by using a knife, to test hardness or fracture.

Equant

A crystal habit. Said of a crystal which has a blocky shape and equi-dimensional in a-b-c axes.

Euhedral

A crystal with faces on all sides.

Fibrous

A crystal habit. Hair-like, flexible (not brittle) crystals in aggregate, often easily pulled apart like cotton candy.

Filiform

A crystal habit. Hair-like. Capillary.

Foliated

A crystal habit. Easily separated into sheets or leaves. See also Micaceous.

Fracture

A mineral property. The fracture of a mineral is the breakage surface produced when it does not yield to a parting or cleavage. Somewhat subjective property, but can be helpful in determinative mineralogy.

Globular

A crystal habit. Small spherical or hemispherical groups.

Granular

A general descriptive term. An aggregate of mineral or crystal grains, tightly packed.

Greasy

A luster which appears to be covered by a thin layer of oil.

Growth Hillocks

Said of a crystal which display upon the faces additional growth features which manifest as inward or outward geometric or rounded shapes.

Habit

Crystal "habit" is the general shape of a crystal. A general identifying feature of many minerals.

Hardness

The resistance of a mineral surface to being scratched by an object of known hardness. Hardness is measured on the Mohs scale of hardness which defines the scale of hardness with ten common minerals (aside from diamond).

Iridescence

An optical effect on the surface of a mineral which produces a play of colors similar to an oil spill on water, sometimes producing color bands similar to a rainbow. It is a non-determinative mineral property.

Lamelar

A crystal habit. Flat, plate-like individuals superimposed upon and adhering together.

Lath-like or Lath-shaped

A crystal habit. Much like a narrow, thin and long piece of wood.

Luster

A mineral property. Luster is the general appearance of a mineral's surface in reflected light, preferably sunlight. Luster is not a definitive mineral property and can vary between a mineral of the same type. It is also a subjective term, meaning everyone may see the luster a little differently. The two major classes of luster are Metallic and Nonmetallic.

Malleable

A mineral characteristic describing its tenacity and is a metallurgical property. The ability of a metal to be mechanically deformed by hammering or rolling into thin sheets. Mineralogically, it is a relative term which only serves to differentiate a mineral from other tenacities such as brittle or elastic. In a practical, determinative sense it is the same as ductile.

Mammilary

A crystal habit. Large, rounded masses.

Massive

A general descriptive term. A compact aggregate without distinctive forms.

Micaceous

A crystal habit. Easily separated into paper-thin sheets.

Pearly

An iridescent luster, much like a pearl.

Pisolitic

A general descriptive term. An aggregate of pea-shaped masses.

Pleochroic

An optical property of a mineral which produces different colors when light passes through at different angles. It is a determinative property.

Plumose

A crystal habit. Feather-like arrangement of fine scales.

Prismatic

A crystal habit. Elongated with a square, nearly square, or equi-dimensional cross section and rectangular or rounded outline. The long dimension is at least three times as that of the short dimension. Pencil-like. A prism by definition has vertical faces with parallel edges that are parallel to the c-axis, so any habit which is said to be prismatic should satisfy the definiton of a prism.

Pseudomorph

A replacement of one mineral by another, in part or as a whole.

Radiating

A crystal habit. Much like divergent but usually thought of as crystals, typically acicular, compact acicular, or compact fibrous radiating outward from a common center, often in a complete sphere.

Reniform

A crystal habit. kidney-shaped masses.

Resinous

A luster which resembles amber or colored plastics.

Reticulated

A crystal habit. Slender crystals arranged in a lattice-like array.

Sectile

The property of a mineral observed when scratched with a knife, will yield a wax-like cut sometimes showing curled shavings. It is best seen under a microscope. It is a determinative property, but not diagnostic as similar minerals can show sectility, i.e. some tellurides.

Sheaf-like

A crystal habit. Said of a group of crystals which resemble a sheaf - or bundle grouped in parallel aggregate or much like a stack of paper. Usually seen in crystals from a side view or cross-section.

Silky

A luster which gives a silky appearance due to numerous, compact, fibrous crystals.

Splendent

A luster with the highest degree of "shininess" sometimes referred to as "specular".

Stalactitic

A general descriptive term applied to cave formations. Cone-like cylinders.

Stellate

A crystal habit. Star-like in shape.

Stout

A crystal habit. Nearly equi-dimensional in cross-section where the long dimension is about one to two times greater than that of the shorter dimension and is usually applied to pyramidal crystals.

Streak

A mineral property. The streak of a mineral is produced when the specimen is rubbed on unglazed porcelain (a streak plate). The color of the streak is a non-determinative property of a mineral. It is typically consistent for each mineral, but may vary slightly from different occurrences.

Striations

Shallow, thin, prominent, parallel grooves generally less that 0.5mm on a crystal face and always parallel to a crystal edge.

Subadamantine

A luster which is a little less brilliant than adamantine. It is sometimes described as "splendent" or "specular".

Subhedral

A crystal that is partly faced. Not all the faces are present.

Tabular

A crystal habit. More equi-dimensional than bladed and like a tablet of paper or a domino.

Tenacity

A mineral characteristic measuring its relative tensile strength or cohesiveness. Simplified, it is how easily a mineral will break when hit with a hammer or scratched with a knife. The tenacity covers a broad range from easily broken (brittle) to malleable (tenacious).

Vitreous

A luster of a mineral which resembles that of glass.

You may also access Webmineral's website to learn more about mineral and crystal habits and properties at:

http://webmineral.com/help/Habits.shtml

Much of the information on the Dakota Matrix Minerals website has been borrowed from the Webmineral website.