disphenoid
nounEtymology From di- (“twice, double”) + sphenoid (“wedge-shaped crystal or bone of the skull”).
1
Geometry, Mathematics, Sciences A non-regular tetrahedron whose four faces are congruent acute-angled triangles.
- The mineral chalcopyrite commonly crystallizes in disphenoids that are very difficult to distinguish from tetrahedrons.1949, Edward Salisbury Dana, Minerals and How to Study Them, page 32:
- 1973, H. S. M. Coxeter, 3rd Edition, unnumbered page, To make a model of a disphenoid, cut out an acute angled triangle and fold it along the joins of the mid-points of the sides. The disphenoid is said to be rhombic or tetragonal according as the triangle is isosceles or scalene.
2 more examples
- If you rotate the [tetragonal] disphenoid 90° around its 2-fold axis and then perform the operation of inversion through the center-point of the object, it will occupy its original position again.1977, Elizabeth A. Wood, Crystals and Light: An Introduction to Optical Crystallography, Revised edition, page 8:
- In addition it should be remarked that there are two types of disphenoids, with different symmetries: the tetragonal disphenoid (having isosceles facets) and the rhombic one.1993, Horst Martini, “A Hierarchical Classification of Euclidean Polytopes with Regularity Properties”, in Tibor Bisztriczky, Peter McMullen, Rolf Schneider, Asia Ivic Weiss, editors, Polytopes: Abstract, Convex and Computational, Proceedings of the NATO Advanced Study Institute, page 83:
- Synonyms
- bisphenoid, equifacial tetrahedron, isosceles tetrahedron
- Derived terms
- rhombic disphenoid, snub disphenoid, tetragonal disphenoid