Wurtzite crystal clusters (non-radiating) occur in the septaria at Negley usually, but not always, in combination with radiating crystal groups and single crystals. This habit is caused by single crystal growth occurring in close proximity, but not from the same source as in radiating crystal groups. This allows the crystals to grow into each other instead of away from a single point forming clusters.
When single crystals develop in this way, many unusual and curious growth characteristics can be observed. A typical cluster showing both radiating and non radiating crystals is pictured to the left.
Notice the upper crystal has grown towards the face of the largest crystal instead of away from the focus of the radiating pair. The photo on top shows several unusual facets of crystal growth. First, what looks like a twinned crystal is more likely a product of two separate crystals, which at one point were unrelated, having grown together.
Although the photo does not clearly portray the development, the two crystals have actually grown at a 30 degree angle. In addition, the right crystal is curving toward the viewer and the left crystal curving away. Also, note the “twins” are a second generation growth on a larger 1st generation crystal showing heavy internal fracturing. Finally, and most importantly, the crystals displayed in this picture are fully terminated hexagonal prisms without truncated counterparts.
Hexagonal and Trigonal Pyramidal Wurtzite
As mentioned earlier, Wurtzite crystals generally will develop two pyramids; a fully terminated pyramid and a truncated pyramid. Often, dihexagonal pyramidal crystals with one very short truncated pyramid will be misidentified as being hexagonal pyramidal.
The photo illustrates this phenomenon. If only one pyramid is present it will typically be a fully terminated pyramid. Also, the base of a true hexagonal pyramidal Wurtzite crystal will appear to be hollow. Occasionally, a dihexagonal or ditrigonal crystal will develop with its fully terminated pyramid attached to the matrix. After full growth is completed, the crystal may no longer have a fully terminated pyramid or may only display one pyramid which can cause the crystal to appear as a truncated hexagonal pyramid.
Wurtzite Crystal Mutations
After careful study of Wurtzite crystals found in the septaria at Negley, several different mutations can be readily identified. The most typical mutations are illustrated in the figure to the right. Mutations occur in both trigonal and hexagonal crystal systems and can be observed in several habits. Notch mutations(1) are a most common occurrence in Wurtzite from this locality.
Crystals displaying this mutation appear to have had a stunted growth, meaning that crystal development stopped and then was later resumed resulting in an indentation parallel to the striations on the pyramid faces. A rounded or teardrop mutation(2) can occur in dihexagonal and ditrigonal pyramidal crystals.
In this mutation the crystal faces of both pyramids are curved, but the axis remains similar to a normal Wurtzite crystal. A similar sloped or horn mutation(3) can occur in hexagonal and trigonal crystals. In a horn mutation, the axis again remains typical, but crystal faces curve outward towards the base.
In the teardrop mutation, the crystal faces begin to curve outward away from the termination, but then curve inwards towards the base. When the crystal axis is curved, an axis mutation(4) is formed. In this mutation, the crystal faces are curved to accommodate the curved crystal axis resulting in a bent crystal. It is not uncommon for a crystal to display more than one mutation.
Of all the Wurtzite specimens locked inside the septaria at Negley, acicular crystals are by far the most difficult to locate and properly trim. This is caused, primarily, by the fact that acicular Wurtzite is exceedingly rare. In addition, the needle like crystal shape, along with the brittle nature of Wurtzite, leaves most acicular crystals in a fragile state.
Finally, the largest acicular crystals rarely reach much more than .4 millimeter in length. All of these factors make micro mounting an acicular Wurtzite crystal unlikely. The photo below shows an acicular Wurtzite crystal which developed out of an angled radiating group. The crystal is hexagonal in nature and has grown away from the focus of the radial.
The crystals in the radiating group formed at an unusually steep angle creating a “cone” of radiating Wurtzite crystals. It seems possible that the steep angle formed by the radiating group spawned the acicular crystal.
Septaria and concretions are abundant and can be easily collected off the shale spoil piles around the open cuts and pits at Negley. The photo to the right shows a typical quantity of concretions in shale at Negley. A bucket, rock hammer, and hand lens are necessary for collecting. Since the percentage of concretions with wurtzite is low, it is wise to break open the nodules and cull only those with wide shrinkage cracks.
A quick rinse in water will remove the clay which develops in the shrinkage cracks concealing the minute crystals. While collecting, a quick puff of air can help to remove some of this for examination, but a light rinse will more thoroughly expose the crystals. For further information and a more detailed analysis on the occurrence of Wurtzite in Ohio, read the Hollenbaugh and Carlson articles.