Gypsum var. Selenite Crystals

Gypsum crystals

Macrodome Face (e) Development

Although the macrodome face (e), is always the most crude face on crystals from this locality, it is the only face which varies in proportion from crystal to crystal. In fact, while the matching pairs of prismatic faces, (l) and (m), and clinopinacoid faces (b) may be nearly identical, the two macrodome faces (e) on any one crystal are often slightly different.

It is important to understand the three categories of development for the macrodome face (e), as illustrated in Figure II, to the upper left. The size of the macrodome face (e) greatly changes the shape and appearance of Selenite crystals from this locality. The first degree is the Deficient form, labeled A. Any crystal in which the macrodome face (e) does not connect with the clinopinacoid face (b) fits this category.

Notice that the macroprism face (l) clearly separates the macrodome face (e) from the clinopinacoid face (b). Labeled B in Figure II., the Ideal category includes only those crystals where all four different faces of the crystal meet at one point. As can be seen in crystal drawing B, faces (e),(l) and (b) meet at a single point.

The unit prism, face (m), also meets at this point. This is the rarest single crystal to find at the locality. The last category, Amplified, encompasses all crystals where the macrodome face (e) and the clinopinacoid face (b) join at a line. This is the most common crystal form seen at the Ellsworth locality. In this form, the macrodome is proportionally larger than either of the prism faces. Also, the clinopinacoid face (b) is hexagonal in all crystals which exhibit this form.

No macrodome face (e) has been observed larger than 1/2 the size of the clinopinacoid face (b), in crystals from this locality. Typically, the macrodome face (e) will be heavily clouded with sediment. This feature is displayed in the photo to the near lower left.

Crowned Macrodome

As previously mentioned, the Macrodome is often concave, appearing as a slight to severe divot at the end of the adjoining macroprism faces (l). Occasionally, the clinopinacoid face (b) along with the adjoining macroprisms (l) will grow beyond the amplification of the macrodome.

In this form, although the the macrodome face (e) is in an amplified state, it never joins the clinopinacoid face (b). This form is commonly known as a crowned macrodome. The termination of the resultant crystal forms the outline of a crown.

This crystal form is completely dominated by the clinopinacoid face (b). This crowned form is exhibited in the the photo to the left. The clinopinacoid face (b) developed a 4 sided face despite the size of the amplified macrodome face (e). The photo to the Right displays another similar macrodome and more clearly exhibits the resultant crown shape.

 

Disproportionate Crystals

While perfectly proportioned text book crystals are occasionally recovered, most crystals from this locality are slightly to moderately disproportionate. There are two basic ways in which the crystal can be disproportionate. The first is prismatic disproportion, labeled A in Figure III to the lower left.. This crystal imperfection occurs when the left and right prisms of the crystal are of unequal size. On a perfect crystal, the left and right macroprism faces (l) and the left and right unit prism faces (m) should converge at a single point.

On crystals with this imperfection, these faces do not connect. This disproportion is frequently seen in the macroprism faces (l). Rare examples of disproportionate unit prism faces (m), have also been recovered from this locality. The crystal in the photo to the right is an example of prismatic disproportion.

disproportinate mineral

Fig. III. Disproportionate Gypsum crystals A: prismatic

This imperfection can be observed on most crystals from the locality and occurs when the macrodome face (e) is not symmetrical. Occasionally, but not always, the macroprism faces (l) will be misshapen to accommodate the macrodome face (e) imperfection.

This has been illustrated in example B, Figure III. Notice that the macrodome face (e) is not symmetrical and that the macroprism faces (l) have shifted to complete the crystal. Other crystal flaws have been noted, but are not as common.

 

 

January 5, 2017