Mineral habit
The habit of a mineral is the external shape of a crystal or a group of crystal. This shape depends in first order on the crystal system of the mineral but also on the conditions of crystallization, the availability of space, the interaction with fluids or magma, and the competition with neighboring grains. When studying rocks in the field or under the microscope, the external shape of a crystal can thus provide information on the order of crystallization of minerals or on the conditions that were present in the environment of crystallization. Mineralogists and geologists have used several terms to describe the morphology of crystals and this resulted in a wide range of terms available to describe crystal habit. A full description is beyond the aims of this page, which will focus mostly on the terms that are more useful for the description of rocks.
Quality of crystal shape
The external shape of crystals depends on the internal arrangement of atoms but also on external factors that limit or restrict crystal growth, first and foremost the availability of space. A growing crystal can ideally form all its crystal faces only if it grows in a cavity with available space in all directions, a conditions that is uncommonly met in rocks, where crystal compete with one another for space. For this reason, defining the quality of the crystal habit, before of the morphology of crystals themselves, is important in geology:
- Euhedral (or idiomorphic, automorphic) crystals show all their crystal faces. The term comes from the Greek eu, ‘good’, and hedron, ‘face’.
- Subhedral (or hypidiomorphic, hypautomorphic) crystals show a partially developed habit with some crystal faces and some irregular surfaces.
- Anhedral crystals (from the Greek anhedron, ‘no faces’, synonym with allotriomorphic and xenomorphic) do not show any visible crystal face and are characterized by an irregular habit. Anhedral crystals may be bound by irregular surfaces or be found in between crystal faces of other minerals in the case of interstitial minerals.
Crystal habit of euhedral minerals
The habit of euhedral minerals is controlled uniquely by the symmetry of their crystal system. In this case, the same mineral can show different crystal habits that obey to its crystal system. For example, pyrite, a cubic mineral, can form cubes (cubic habit) or dodecahedrons with irregular pentagonal faces (a habit known as ‘pyritohedron’). Similarly, fluorite can crystallize with cubic habit or octahedral habit. The recognition of the habit of euhedral minerals allows to recognize the crystal system and can help the identification of the mineral. In general, cubic minerals tend to develop equant habits (cube, dodecahedron, octahedron). Uniaxial minerals (hexagonal, tetragonal, trigonal crystal systems) tend to develop a more or less elongated habit consisting of prisms (prismatic habit), regular pyramids (pyramidal habit) or double pyramids (dipyramidal habit) with a base that depends on the crystal system. For example, vanadinite, a hexagonal mineral, commonly forms short prism with hexagonal base. Tetrahedrite, a tetragonal mineral, commonly forms regular pyramids with four regular triangular faces (tetrahedrons). Orthorhombic minerals also form prismatic or pyramidal crystals but the three axes of the crystal system differ and, therefore, these minerals can develop cuboids, pinacoids (a prism with only two parallel faces), and rhombohedrons. Finally, minerals of the monoclinic and triclinic crystal system can develop a wide range of habits that deviate from the ones described above as one (monoclinic) or all (triclinic) of their axes are not orthogonal.
Nassau, K. (1978). The origins of color in minerals. American mineralogist, 63(3-4), 219-229.
Rossman, G. R. (1994). Colored varieties of the silica minerals. Reviews in Mineralogy, 29, 433-433.
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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