Wurtzite

WurtziteWurtzite will form slightly translucent, burnt brownish red crystals in two different crystal systems. The most common form of Wurtzite is dihexagonal pyramidal(A). Rarely a crystal will form a true ditrigonal pyramid(B). Dihexagonal pyramidal crystals can occasionally be misidentified as ditrigonal pyramidal crystals without careful examination. It is not uncommon for the crystal faces to be unequal or distorted creating an illusion of a ditrigonal pyramidal crystal(C).

In either case one of pyrimids is truncated or shortened causing an uneven ratio between the two pyramids. The truncated pyrimid can develop a termination or can remain incomplete. The photo to the left displays two crystals, one with a truncated pyrimid showing termination and another which is incomplete.

Although the ratio is never constant, both a fully terminated pyramid and a truncated pyramid are usually present in Wurtzite from this locality. Crystals do occur, however, for which this last statement is untrue and only one pyrimid develops.

Habit

Wurtzite crystals generally develop in three different habits in the Negley Septaria; well formed single crystals, radiating clusters, and non radiating crystal clusters. In each case, crystals are generally well formed with crystal face striations perpendicular to crystal growth. Often septaria with greater concentrations of Wurtzite will produce crystals in all three typical habits. Very rarely, crystals will form acicular rods or free standing fans, but neither are usually well developed.

Single Crystals

Single Wurtzite crystals typically measure 1 millimeter or less. Less commonly single crystals can measure upwards of 2 millimeters. Over 90 percent of all single crystals develop with one or more faces attached to the matrix(parallel growth).

Of the crystals which grow away from the matrix, most will grow with the truncated pyramid attached to the matrix(base to tip growth). Very rarely crystals grow in reverse with the fully terminated pyramid attached to the matrix(tip to base growth). The rarity of this occurrence combined with the brittle nature of such crystal growth makes trimming such a specimen improbable.

Radiating Wurtzite

Radiating crystal groups of Wurtzite occur prevalently in septaria from Negley. As a rule, Wurtzite in this habit are produced by multiple crystals simultaneously growing away from a single point and can occur in a plane or at an angle. Often, crystals in these radiating groups are of different size, pyramidal proportion and color.

It is also important to note that these groups frequently do not form perfect radials. A radiating group may consist of no more than 2 or 3 crystals and only form a small percentage of the radial. Classically, Wurtzite radials occur in a plane and are flush with the matrix.

Uncommonly, radiating crystal groups will develop perpendicular to the matrix and can be mistakenly identified as free-standing fans. However, Wurtzite fans are typically flattened and seldom show evidence of pyramidal growth. Individual crystals of Wurtzite are clearly visible in radiating crystal groups.


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