For most minerals, the crystals look the same at both vertical ends. When perfectly formed they are mirror images of each other. In hemimorphic minerals however, a bottom is unlike a top with different terminations at each end; hence the description “half-formed”. One termination, the "bottom" is rather blunt being dominated by a pedion face while the opposite end, the "top" is terminated by the point of a pyramid. Only a few other minerals show hemimorphic character such, but none show it as well as hemimorphite.
Hemimorphite is a sorosilicate. The crystal structure contains tetrahedrons of ZnO3 OH, interlocked with Si2 O7 groups and water molecules. The zinc is at the center of the tetrahedron while the three oxygens, along with an OH group, are at the four points of the tetrahedron. These tetrahedrons are all aligned in the same direction with one face parallel to the pedion termination and the "top" of the tetrahedrons pointing toward the pyramidal termination.
The hemimorphic asymmetry also accounts for hemimorphite's most striking characteristic: an electrical charge induced by changes in temperature or pressure. The phenomena is known as pyroelectricity where heated crystals will develop positive and negative charges at opposite ends of the crystal. Changes in pressure will result in similar charges and the phenomena is known as piezoelectricity. With positive and negative poles developing from pressure and temperature changes, some specimens - such as those subject to the daily, considerable temperature change of electrically lit display cases in museums - attract dust to both ends, visibly demonstrating this phenomenon.
Clusters of hemimorphite that show well shaped crystals do not always show the hemimorphic character because the crystals of a single specimen tend to grow outward with either the "top" or the "bottom" as the overall orientation for that specimen. In order to see the hemimorphic character either a doubly terminated specimen is necessary or two different clusters with different orientations will be needed.
Hemimorphite occurs in veins and beds in stratified calcareous rocks. Associated minerals include limonite, aurichalcite, calcite, smithsonite, and chalcopyrite.
Specimens of hemimorphite tend to be of two very different forms; the bladed crystal form and the botryoidal habit. The crystalline habit reveals elongated, flat, very glassy, clear or white, thin, bladed crystals, often well formed showing many crystal faces. The terminations are different at each end. Many times these crystals are arranged in fan shaped aggregates. The other common form is botryoidal producing a grape bunch like texture. This form produces a blue to blue-green botryoidal crust that resembles smithsonite or prehnite. Often hemimorphite will show rough crystal ridges or "cock's comb" structures over top of the basic botryoidal crust.
Other habits include: massive - uniformly indistinguishable crystals forming large masses; stalactitic - shaped like pendant columns as stalactites or stalagmites; and mammillary - larger "breast-like" rounded forms resembling botry
Cleavage is perfect in one direction, and the fracture is conchoidal to subconchoidal. The hardness is slightly less than 5, and the specific gravity is approximately 3.4+. The streak is white. Some examples from the Congo seem to weakly fluorescent in bluish shades while Mexcan crystals may show an orange or red response.
Although hemimorphite is a name relatively new in mineralogy, familiarity with this zinc silicate goes back to ancient history. Previous names included calamine, galmei, and zinc silicate.
Because of similarities in color and habit, hemimorphite was often confused with smithsonite. Smithsomite is a zinc carbonate ZnCO3 and hemimorphite a zinc silicate Zn4Si2O7(OH) 2.H2O.
Part of the confusion springs from the similarity of the botryoidal shapes that both minerals may assume, despite the difference in their crystal morphology. Pure zinc carbonate (smithsonite) and zinc silicate (hemimorphite) are both colorless or white. Chalcopyrite (copper iron sulfide), commonly found in association with zinc ores, provides the trace of copper that gives the bluish and greenish tints to the secondary knobby growths developed in either species. Though almost identical in appearance, simple tests will help to separate them. The carbonate (smithsonite) dissolves in warm acid with some effervescence, while the silicate (hemimorphite) dissolves slowly and quietly, leaving gelatinous silica. Smithsonite has a higher density as well a shimering luster that causes a play of light across the rounded surfaces. The two species also show differences in the way they cleave. Smithsonite knobs break with a ready convex (rhombohedral) cleavage not characteristic of hemimorphite. Hemimorphite is slightly harder than smithsonite.
Treatment
Most hemimorphite is treated to improve its’ stability. Natural hemimorphite almost always contains hairline fractures which inhibit the lapidary’s ability to work with this rare material. We immerse our material in a very strong and permeable colorless glue-like material which fills any hairline fractures and strengthens, toughens and improves the transparency of the material at the same time. The treatment also allows us to produce larger sizes. We do not add any color at all. Other treatments may vary but the information is proprietary and may also be undisclosed in some cases.
Occurrences
Important occurrences of hemimorphite include: Franklin and Sterling Hill New Jersey, New Mexico, Montana, Arizona, U.S.A; Congo; Nerchinak in Transbaikalia, Siberia; Zambia; Santa Eulalia and Mapimi, Mexico; England; Wenshan, Yunnan Province, China.
Well crystallized facetable and completely transparent hemimorphite occurs at two localities in Mexico, Mapimi, Durango; and Santa Eulalia in the Mexican state of Chihuahua. Many examples display the long white blade like habit. The knobby material, suggestive of cabochons is more common at other deposits.
Most recently, bright blue gem quality hemimorphite was found about 200km from Brazzaville in southern Congo. Specimens from this locality attain weights of 10-20 kgs. but only small portions are suitable for cutting cabochons. Unfortunately, there has been no additional production for several months due to armed conflict between rebels and government forces in the area.
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