While the Diploidal form with trapezohedral modifications is the most common crystal form observed at the Duff Quarry, it is not exclusive. In a study of diploidal forms conducted for this article, the forms A-H represent one observable progression of crystal modifications.
Other progressions of this type can and do exist. Additional modifications are possible, however no diploidal crystals with expanded form beyond form H were discovered in this study. Crystals with modifications of unequal size are common.
For example, a diploidal crystal with octahedral modifications should have 8 equal sized symmetrical octahedral faces. However, one diploidal pyrite crystal with octahedral modifications may have 8 octahedral faces of varying size. Further, all 8 octahedral faces may not be present on every crystal.
Other progressions can and do exist. The progression of modifications above is as follows:
A diploid(d’), B trapezohedron(m), C cube(a), D pyritohedron(e), E octahedron(o),
F dodecahedron(b), G trisoctahedron(p), H trapezohedron(n).
Common Modified Trapezohedral crystal form
Although the diploidal habit is by far the most common crystal form observed at this locality, it is by no means the most complex. Trapezohedral crystals are occasionally encountered throughout the quarry. These trapezohedral crystals usually have pyritohedral and octahedral modifications.
True unmodified trapezohedrons are generally not encountered at this locality. The pyritohedral modification creates a kite shaped quadrilateral face between two trapezohedral faces. This pyritohedral face, labeled e, is visible in Figure E. to the upper left.
This face is commonly quite small in comparison to the trapezohedral faces, but occasionally has been observed to be of equal size to the trapezohedron. The photo to the left exhibits a clear example of this pyritohedral modification to the trapezohedron. In general, the pyritohedral face on this crystal is much larger than most from the locality.
Notice also that the four trapezohedral faces do not meet at a single point. This is more common in the trapezohedral form than in the diploidal form. The octahedral(o) modification is usually found in combination with the pyritohedral modification. On a perfect crystal, this modification makes an equilateral triangular face between three unrelated trapezohedral faces.
Very rarely, will the octahedral modification be found on a trapezohedral crystal without the pyritohedral modification. Figure F. to the lower left exhibits this form. Occasionally, this modification is enhanced with a second trapezohedral modification. In the photo to the left, the octahedral modification in the upper right corner has this additional modification.
Modifications to the Trapezohedron
Similar to the progression of diploidal crystal forms, Figure G. to the left, forms I-O to avoid confusion with the diploidal crystal forms, shows a possible progression. However, only forms J, K, M, and O were actually observed. It is logical that forms L and N exist along with other combinations.
True, unmodified trapezohedral pyrite crystals I, have not been observed or reported and likely do not exist. No predominately trapezohedral crystals were observed with additional modifications beyond form O, however it is conceivable that such crystals may exist. Forms J and K, as mentioned above, are without doubt the most common of trapezohedral forms to be found at this locality.
Like the diploidal crystals, these crystal forms are often somewhat irregular, especially in larger crystals. Although the faces of these crystals are generally sharp, similar faces of varying size are common.