Selenite

The most common twinned Selenite crystal found at the Ellsworth locality is the “butterfly” twin. This type of twin occurs when the two halves of a symmetrical crystal develop in the same plane. The butterfly twin takes on a completely different crystal form than that of the crossing twin. In the butterfly twin, the terminations of the two halves have completely joined together, creating one large clinopinacoid face. The resultant crystal resembles a pair hexagonal prisms joined by a “false” twinning plane. A small notch is created by the macropsims on each side of the false twinning plane.

selenite
Twinned Gypsum var. Selenite crystal in matrix

This  twinned crystal is also striated along the broad clinopinacoid faces perpendicular to the false twinning plane. Striations of this type are common in sharper twins. These striations are more clearly visible in the photo to the upper left. An elongate and flattened macrodome face terminates each half of the twin. As pictured to the upper left, an X shaped pattern of clay inclusions, can generally be found in butterfly twin Selenite crystals from the Ellsworth locality. This X shaped pattern clearly indicates these to be penetrating twins.

Occasionally, and more commonly in corroded crystals, this pattern is visible on the surface of the clinopinacoid faces. It is not uncommon for this type of twin to be incomplete. In this case, only half of the twinned crystal will appear as a butterfly twin. The crystal displayed in the photo to the lower left, is an exceptionally large and sharp example of this occurrence for the locality. Notice that the lower half of the crystal resembles a single Selenite crystal, while the upper half has taken the form of a butterfly twin.

Nearly half of the twinned crystals from this locality will be incomplete in this manner. This is also common in compound twinned crystals. While crossing twins will exhibit a similar fluorescence to single crystals, complete butterfly twins are not as impressive under ultraviolet light. A thin portion, directly beneath the elongate macrodome face is slightly fluorescent, but the classic hourglass patterns are no longer present. This is not completely true for incomplete twins. The incomplete half, which resembles a single crystal, will typically have the fluorescent hourglass pattern, while the completed half will not.

Compound Selenite Twins

compound selenite

There are many ways in which compound twinning occurs in Selenite from this locality. A compound twin consists of more than two twinned halves. Although this habit is far more common in butterfly twins, it has also been observed in crossing twins. The most common way compound twinning occurs is through a series of planar twins. A basic illustration on how this form of twinning occurs can be seen in figure V to the lower left. In the illustration, each X shape represents one twinned pair of crystals.

In general, the additional twinned pairs are smaller in size than the central twin. For instance, the crystal pictured to the left exhibits a compound butterfly twin. Notice that the additional twinned pairs are smaller than the central twin. In a regular butterfly twin, a notch was created by the macroprism on each side of the false twinning plane. In compound butterfly twinned crystals, a series of notches representing the number of twins will be always be clearly

Collecting Status

Because the clay banks at the Ellsworth locality are continually growing new Selenite crystals, this locality will likely continue to produce specimens for many years to come. Unfortunately, gaining access to the locality for future collecting is far more uncertain. Currently, the clay bank locality is situated on property owned by two separate individuals. A crude access road to the locality, is owned by a third individual.

Permission to cross and collect must be granted by all three landowners. The information provided in this article is meant for locality reference only and not intended to be used as a guide for future collecting. For this reason, contact information has not be provided in this article, nor will exact directions to the locality be revealed. An approximate locality description can be found in the introduction. In general, this locality is considered closed to collecting.

However, persons conducting research or collecting educational samples may be allowed to visit the locality. Interested collectors should inquire locally for contact information. Permission was obtained to visit and photograph the locality and collect a number of specimens for use in this article.