Scalenohedron — The Twelve-Faced Crystal Form of Calcite
Scalenohedron — The Twelve-Faced Crystal Form of Calcite
A trigonal-system form with twelve scalene-triangular faces, classically expressed by the dogtooth spar habit of calcite
A scalenohedron is a closed crystal form belonging to the trigonal and hexagonal crystal systems, in which the faces are twelve scalene triangles meeting at six pairs of edges and producing a wedge-like or pointed termination. The name comes from scalene, denoting a triangle with three unequal sides, distinguishing the form from the rhombohedron, which has six rhombic faces. Each face of a scalenohedron has different intercepts on the three crystallographic axes, producing the characteristic asymmetric profile when projected on a single face.
Calcite and the dogtooth habit
The scalenohedron is the classical and dominant habit of calcite (CaCO3) and is so closely associated with the species that well-formed calcite scalenohedra are colloquially known as dogtooth spar. The form arises in low-temperature hydrothermal veins, in carbonate-rich sedimentary cavities, and in geothermal precipitates, where slow growth from supersaturated solution allows calcite to develop characteristic pointed terminations. Famous localities producing fine calcite scalenohedra include Cumbria (England), Joplin (Missouri, USA), Elmwood (Tennessee, USA), and various deposits in central Europe.
Crystallography
In the trigonal scalenohedral class (point group 3̄2/m), the scalenohedron is the general form: a face with the indices (hkil) where the indices are all unequal generates the twelve-faced figure when symmetry-equivalent positions are added. The form can be described as a combination that resembles a doubly terminated wedge, with each face cutting the c-axis at a steep angle and the a-axes at unequal intercepts. Different specific scalenohedra — denoted by their Miller-Bravais indices, such as (2131) — produce wider or narrower wedge angles.
Distinction from related forms
The scalenohedron is distinct from the rhombohedron, which has six faces of equal rhombic shape and shorter terminations, and from the hexagonal pyramid, which has six faces meeting at a single point. Calcite commonly shows combinations of multiple forms — scalenohedron with rhombohedron, scalenohedron with prism, scalenohedron with basal pinacoid — and the modifications produce the morphological diversity for which calcite is famous. The hexagonal scalenohedron, a related but distinct form belonging to the hexagonal system rather than the trigonal, is rarer in mineralogical specimens.
Other species
Beyond calcite, the scalenohedron occurs as a habit in other trigonal-system carbonates such as siderite, rhodochrosite, and smithsonite, and in some sulphides including pyrargyrite and proustite. In quartz, the trigonal trapezohedron rather than the scalenohedron produces the characteristic chiral terminations, although the two forms are sometimes confused in older mineralogical literature.
In the trade
For collectors and gemmologists, scalenohedral crystals are most often encountered as calcite specimens in cabinet collections, including the strongly fluorescent material from the Franklin and Sterling Hill mines in New Jersey. As a faceted gemstone, calcite is uncommon — its softness (Mohs 3) and perfect rhombohedral cleavage make it impractical for jewellery use — but well-formed scalenohedral crystals are occasionally cabbed or featured in mineral-specimen jewellery for their distinctive form.