FMF - Friends of Minerals Forum, discussion and message board

[Pete Richards]

Recently I obtained some tiny but lovely samples of the isometric mineral zunyite Al₁₃Si₅O₂₀(OH,F)₁₈Cl from the Big Bertha Extension Mine in La Paz County, Arizona. One particularly attractive crystal (first image below) caught my eye - more transparent than the image suggests and a perfect cubeoctahedron.

But other crystals in the same material were quite different – with large hexagonal faces, smaller triangular ones, and even smaller narrow rectangular ones (second images). These offered a hint that zunyite had lower symmetry than I had presumed. In fact, zunyite, while indeed isometric, belongs not to the full-symmetry hexoctahedral class, but instead to the lower-symmetry hextetrahedral class, the same class to which sphalerite belongs. This class does not have an octahedron; rather it has two complementary tetrahedra, designated positive and negative, which if combined in equal balance create a form that looks like an octahedron. However, the faces of the two tetrahedra are structurally different, and as a result often have differences in luster or other properties that allow one to discern that they belong to two different forms.

Looking more closely at the non-cube faces of this same crystal, it is apparent that faces which are quite lustrous and smooth alternate around the top of the crystal with faces that are rather pitted (third set of images). These represent the positive and negative tetrahedra, though it is not possible to say which is which without x-ray studies. Traditionally, the form with the larger and/or more perfect faces is taken to be the positive tetrahedron.

Similarly, the large hexagonal faces on crystals of the other habit are glossy and nearly perfect, while the smaller triangular faces which belong to the other tetrahedron are rough-textured (fourth pair of images).

Sphalerite is another mineral in the same class, it often has a pseudo-octahedral habit composed of fairly balanced positive and negative tetrahedra, and it often forms twins that are geometrically like classic spinel twins. In the hexoctahedral class to which spinel belongs, these can be described as twins by reflection across an octahedral face, or by rotation through 180° of one half of the crystal about the three-fold axis perpendicular to the octahedral face. The two operations produce identical results. In the hextetrahedral class, however, these operations produce different results: twinning by reflection places faces of the same tetrahedron opposite each other across the twin plane, but twinning by rotation places faces of different tetrahedra opposite each other.

In some environments, the faces of one tetrahedron are frosted and/or irregularly developed, while faces of the other tetrahedron are smoother. Examination of twins from such environments always show a smooth face opposite a frosted one, demonstrating that the twinning is described by rotation, not reflection.

These are interesting examples of ways in which observable details of crystal morphology can lead to an enhanced understanding of the internal structure of the mineral.

[Elise]

Hi Pete,
This is really a great little essay - I really enjoy how you walk readers through these really interesting analyses. This is what makes FMF a wonderful resource as well as entertaining!
Very best wishes,
Elise