HEMIMORPHITE Chapter16. Sorosilicates and cyclosilicates

HEMIMORPHITE

Zn₄Si₂O₇(OH)₂.H₂O
Orthorhombic, Imm2, a = 8.370, b = 10.719, c = 5.120 Å, Z = 2.




	Figure 16-9. Typical bladed hemimorphite crystals from Franklin. Field of view is 2 mm in maximum dimension.

Hemimorphite was first described formally by Seymour (1868), but was surely known before then. Most early references, through the time of Palache’s monograph (1935), utilized the now-obsolete name calamine; but Cook (1868) described it as electric calamine, and Ingalls (1895) called it hydrowillemite. Most subsequent studies have used the now-correct name. Hemimorphite comprises, in part, together with clays and trivial smithsonite, a mixture called vaneuxemite by Shepard (1876). The early work is summarized by Palache (1935) and Frondel (1972). Thermal dehydration studies were provided by Faust (1951) and Taylor (1962), and infrared spectra are on file in the Smithsonian Institution. Unit-cell parameters, powder data, and optical data were given by Swanson (ICDD, PDF #5-555).

Description




	Figure 16-10. Chalky-appearing, botryoidal hemimorphite, stained in some areas, from Sterling Hill. Specimen is 13 cm in maximum dimension. Smithsonian Institution, #C3239-10. Photo by the author.

Local hemimorphite varies substantially in its appearance. Fine 1-10 mm, colorless crystals are known from both deposits and were illustrated by Pratt (1894). A typical cluster of Sterling Hill crystals is shown in figure 16-9. The bulk of the Sterling Hill material, however, occurs as secondary, subparallel to divergent aggregates of crystals, forming mamillary masses, slabs, and stalactitic and crested masses. These range in size from microscopic specimens to very large specimens weighing tens of kilograms. Chalky and fibrous aggregates are also known (Figure 16-10).




	Figure 16-11. Hemimorphite from Sterling Hill, illustrating the “maggot-ore” texture for which this locality is famous. Specimen is 9 cm in maximum dimension. Smithsonian Institution, #131996. Photo by the author.

The color of massive hemimorphite varies from colorless (apparent white in the aggregate) to white, very light yellow, very light green, and light blue; apparently-black crystals result from included oxide minerals. The luster is vitreous, and the density is 3.48 g/cm³. Sterling Hill hemimorphite is biaxial, positive, 2V = 40^o, with a = 1.611, b = 1.619, and g = 1.630. Much local hemimorphite is fluorescent in shortwave and longwave ultraviolet with a weak white color, which varies in intensity with zoning. Minor cabochon gems have been cut from blue concentrically- banded material.

Composition

Hemimorphite is a zinc silicate hydroxide hydrate mineral. There have been no modern analytical studies of local hemimorphite. The analysis of Clarke and Steiger (1900) is in superb agreement with the above formula for hemimorphite.

Occurrence and paragenesis

Sterling Hill is the source for the vast preponderance of local hemimorphite, but there have been no recent studies of the major occurrence. The bulk of the material was found in the old Passaic and Noble Mines, the open pits. On many specimens the terminations of the hemimorphite crystals are serrated and closely-spaced. Collectively the crystals resemble aggregates of maggots; hence the old miner’s name, maggot-ore (Figure 16-11). These specimens, abundantly preserved, are among the best hemimorphite specimens world-wide and are represented in most systematic collections. When cleaned, most are white, lustrous, and very attractive; others have depositional stains which are interesting in other ways. The occurrence was mined extensively in the 1870’s, and hemimorphite was at that time a chief ore of zinc from Sterling Hill.

A description of the surface occurrence from Mr. O. J. Conley, superintendent of the Noble Mine in 1878, is given by Palache (1935; pages 22-23) and reprinted herein in the section entitled “The geology and structure of the zinc deposits.” An occurrence with altered sulfides was reported by Jenkins and Misiur (1994). Alterations of hemimorphite have not been studied in situ; Roy and Mumpton (1956) and Takahashi (1960) provided experimental data on the solubility and alteration of synthetic hemimorphite.

At Franklin, hemimorphite is a moderately uncommon mineral, occurring in seams and as coatings and crystals in vugs. Fine specimens are known, but are scarce, occurring as druses and in vugs. It is known from the weathered northern end of the west limb which was exposed in the Precambrian.

Not all local hemimorphite is associated with the principal zinc deposits. Palache (1935) noted minor amounts he considered to be derived from sphalerite in late-stage sulfide veins. Hemimorphite was also reported and well-illustrated from the Buckwheat Dolomite by Peters et al. (1983).

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