ZINCITE Chapter 22. Oxides and hydroxides

ZINCITE

ZnO
Hexagonal, P6₃mc, a = 3.24, c = 5.18 Å, Z = 2




	Figure 22-1. Crystal drawings of zincite from the Buckwheat Mine at Franklin. Drawings are from Palache (1935) who provided crystallographic data.

Zincite was first described from these deposits as red oxide of zinc by Bruce (1810a) in one of the first American papers on mineralogy. It has been known by a number of names, including red zinc ore (Vanuxem and Keating, 1824), ruby zinc (Alger, 1861), manganesian oxide of zinc (Berthier, 1819), sterlingite or stirlingite (Alger, 1844), brucite (Dufrénoy, 1847), spartalite (Brooke and Miller, 1852), and calcozincite Shepard (1876), and has even been called ruby, a term of locally obscure origin but understandable in light of its red color. It is notable that the term “ruby zinc,” as used locally, refers specifically to zincite and not to sphalerite as it does elsewhere. The name zincite was proposed by Haidinger (1845) and has persisted, although the sterlingite of Alger had priority.




	Figure 22-2. Crystal drawing of zincite from Franklin, New Jersey. Drawing is from Palache (1941a) who provided crystallographic data.

The crystal structure was described by Bragg (1920), and X-ray powder diffraction data were given by Berry and Thompson (1962). Zincite has been the subject of a substantial number of investigations, some of which are cited below; the older chemical and crystallographic work was summarized by Palache (1935). Salotti (1972) reported vapor-pressure studies of zincite, franklinite, and willemite and found support for the paragenetic sequence of Ries and Bowen (1922).

Description




	Figure 22-3. Superb zincite crystal from Sterling Hill. Field of view is 0.2 mm in maximum dimension. Photograph courtesy of Tom Peters and the Paterson Museum.

Zincite rarely occurs as euhedral crystals; most material is massive. Crystals are commonly pyramidal and hemimorphic in habit, with a pronounced pedion, {0001} (Figures 22-1, 22-2, and 22-10). The crystals may be equally developed in the a and c directions, but most are elongate on c (Figure 22-3); freely-formed crystals up to 25 mm are known, but exceedingly rare. Some crystals are distorted.




	Figure 22-4. Zincite crystals from a secondary vein assemblage from Sterling Hill. Field of view of left photograph is 0.07 mm; that of right photograph is 1 mm, both in maximum dimension.

A formless single-crystal measuring 5 x 6 cm was reported by Frondel (1935). Zincite crystals were first measured by Dana (1886). Crystal drawings and measurements of both common and complex pyramidal crystals were given by Palache (1910, 1935, 1941a); forms present are {0001}, {1010}, {5054}, {4045}, and {1011}.

Druses of crystals in secondary veins and recrystallized assemblages commonly have a microgranular appearance and may form attractive specimens; such crystals are commonly rough-surfaced, and some have the texture of crystals which may have grown rapidly or been etched (Figure 22-4). The crystal faces of etched crystals may be dull or rough. Uncommon platy zincite crystals, with hexagonal outline and a layered and crenulated texture when viewed normal to c (Figures 22-5 and 22-6), were described by Dunn (1979a).




	Figure 22-5. Platy, hexagonal zincite crystals described by Dunn (1979a) from Sterling Hill. Field of view is 0.8 mm in maximum dimension.		Figure 22-6. Side view of platy zincite crystals shown in figure 22-5 showing the crenulated and layered aggregate from Sterling Hill. Field of view is 0.4 mm in maximum dimension.

Most primary and secondary zincite is deep dark red. Early studies by Hayes (1845, 1850, 1872) had suggested the red color was due to minute scales of hematite disseminated in the zincite, but this was rejected by others. The red color was attributed to the presence of Mn³⁺ by Dittler (1925), and it is supported indirectly by the abundance of exsolved hetaerolite described below. Kuz’mina et al. (1973) found that Mn gave a red color and Ni a green one to synthetic preparations. The color is also to some extent affected by grain size; lighter hues are found in fine-grained material. Zincite which has been micro-fractured, greatly increasing the number of reflecting grain boundaries, commonly is orange or dull red.




	Figure 22-7. Columnar zincite crystals in parallel growth from Franklin. Field of view is 0.75 mm in maximum dimension.		Figure 22-8. Etched pyramidal remnants of zincite in solution vugs in common ore from Franklin. The radial tufts at right are willemite; see figure 15-83. Field of view is 0.4 mm in maximum dimension.

Secondary zincite may be red, orange, yellow-orange, yellow, light green, or rarely colorless. Such secondary zincite is commonly finely-crystallized and may add a false color when included in transparent minerals, such as in some apparently orange hodgkinsonite. Mücke (1970) reported hematite as oriented inclusions in zincite. Inclusions of other minerals occur within zincite, but are largely unstudied.

The luster of zincite varies substantially. Freshly broken single-crystals of zincite exhibit a submetallic to subadamantine luster, but much common zincite is seen to have subvitreous to greasy to resinous lusters. Iridescent false-lusters are uncommon and are generally associated with either cleavage planes or parting planes. Zincite has an imperfect cleavage on {1010}, but the principal planar feature on most red zincite specimens is a parting on {0001), which is so pronounced as to be readily confused with cleavage on casual observation. The parting is commonly defined by abundantly exsolved hetaerolite which occurs as thin black films, discussed in detail under hetaerolite. The writer has also verified jacobsite and, on one specimen of manganhumite, sparse manganosite as thin films on such zincite parting surfaces. The density of zincite is 5.66 g/cm³.

Zincite also occurs as mutual exsolutions with manganosite, as studied by Frondel (1940), who found zincite {0001}[1010] parallel to manganosite {111}[011]. McSween (1976) studied these minerals and reported that there were two differently colored zincites present: a red one with no exsolutions and a yellow one with exsolved hausmannite and manganosite. He suggested that the yellow zincite represented red material which had undergone exsolution of Mn, an observation supported by his chemical analyses.

Although its use as a gemstone is restricted to museum exhibits, fine stones have been cut; among these are gems of 20.1 and 12.3 carats in the Smithsonian Institution and 3.08 carats in the Harvard Mineralogical Museum.

Optically, zincite is uniaxial, positive, with w = 2.013 and e = 2.029 (Berman, 1927); pleochroism is imperceptible. In reflected light, zincite has abundant red-to-orange internal reflections. The reflectivity (470 - 700 nm) is R = 12.1 to 10.5% in air and 2.38 to 1.74% in oil (Criddle and Stanley, 1986, 1993). Most zincite does not fluoresce in ultraviolet; Bostwick (1982) has reported a moderate pale-yellow fluorescence in longwave ultraviolet for a granular aggregate of zincite, subsequently found by the writer to be separable into light yellow and colorless grains. Both are fluorescent and were shown to be zincite using X-ray methods.

Composition

Zincite is a zinc oxide mineral; most specimens contain some Mn, which does not exceed 7% MnO in the reported analyses and is generally much lower. Some Mn₂O₃ may be present. The material used for reflectance measurements by Criddle and Stanley (1986) contained ZnO 96.9, MnO 3.4, total = 100.3 wt. %. Very little Fe has been reported in the many analyses of zincite, except for that of Bruce (1810a) which might be in error due to inclusions. In general, secondary recrystallized zincite is closer to the ideal composition than primary material (Peretsman, 1981; present study).

The best recent analyses of zincite are of Sterling Hill material; 14 were given by Squiller (1976) and 4 were given by Johnson (1990). The range of compositions given is: MnO 0.3 - 2.4, FeO 0.1 to 2.2, ZnO 96.0 - 99.1, MgO 0.0 - 0.2, Al₂O₃ 0.0 - 0.9 wt. %. Squiller found that, in general, zincite from Franklin contains more Mn and less Zn than that from Sterling Hill. Squiller discussed two types of zincite: (1) discrete grains in franklinite/willemite ore and (2) fracture-fillings in franklinite. In the discrete grains, he found a strong positive correlation between Zn and Mn and a characteristically low Fe content, suggesting that the Mn content of these zincite grains was fixed during metamorphism by local bulk-rock compositions. Zincite occurring as fracture fillings in franklinite has a higher and variable Fe content. Trace impurities, including arsenic, were determined by Eliot and Storer (1860, 1861) and summarized by Palache (1935). The solubility of synthetic material was studied by Laudise and Kolb (1963).

Occurrence and paragenesis




	Figure 22-9. Columnar zincite crystals in subparallel growth from Franklin. Field of view is 0.75 mm in maximum dimension.

Zincite is the least abundant of the three principal ore minerals (willemite, franklinite, and zincite), but is the richest zinc-ore mineral of all here and worldwide. Zincite has approximately 80 wt. % Zn, as compared with about 19 wt. % Zn in franklinite and 58 wt. % Zn in pure willemite. Zincite comprises approximately 1% of the bulk ore at both deposits. It occurs as both primary material with franklinite and willemite and as secondary material in vein assemblages, associated with a large variety of associated species, many of which contain essential Zn.




	Figure 22-10. One of the best zincite crystals ever found at Franklin. Specimen is 2.5 cm in maximum dimension. Smithsonian Institution, #R17880. Photo by the author.

At Franklin, the relative abundance of zincite in the ore units varied irregularly, as did the other components (franklinite, willemite, and calcite). Most of the zincite occurred disseminated in the common ore. Palache (1935) noted that “zincite in rounded, shotlike grains or in scales, specks, and splinters is a normal constituent of the orebody over the whole extent of Mine Hill.” At Franklin, zincite also occurs as discrete layers in banded ore, intermediate to franklinite and willemite layers, and generally 0.1 to 3.0 mm in thickness. It occurs as highly irregular masses of varying size, many of which may have formed by the recrystallization of smaller units of primary material (Figures 12-33, 12-35, 12-36, and 12-37). Zincite is mobile under certain conditions, and it occurs as veins (Figures 12-7 and 12-8), coatings, blebs (Figures 12-14 and 12-17), and inclusions in other minerals. Frondel and Baum (1974) reported one lenticular ore-unit comprised predominantly of zincite and calcite, approximately 2 x 10 feet in diameter and hundreds of feet long, but this was anomalous.




	Figure 22-11. Pyramidal, hemimorphic crystals of zincite from Franklin. Field of view is 1.1 mm in maximum dimension.

At Sterling Hill, much zincite occurs as disseminated intergranular material; zincite also occurs preferentially with the olivine-group minerals, principally tephroite (Figure 12-33). Occurrences of obvious concentrations, relative to other part of the orebody, were used in naming two zones of the deposit: the “outer zincite zone” and the “central zincite zone” (Metsger et al., 1958). In the outer zincite zone in the east and west limbs of the ore deposit, zincite commonly occurs as shard-like fragments in disseminated ore and gneissic ore. Zincite occurs in numerous types of occurrences, including many similar to those at Franklin. The most common is as intergranular material in the franklinite/willemite ore. See discussion in the section entitled “Mineral assemblages.”




	Figure 22-12. Large curving crystals of dark red zincite in white calcite with a minor franklinite and willemite segregation at right center from Sterling Hill. Specimen is 17 cm in maximum dimension. Smithsonian Institution, #C17. Photo by the author.

The outermost part of the outer zincite zone at Sterling Hill contains a thick layer of exceedingly rich zincite ore. Here zincite occurs as a massive red to orange matrix hosting equant grains of franklinite; this was the “metalliferous porphyry” of Nuttall (1822) which, together with a similar, localized, but less prominent layer at Franklin, supported the “separate beds” arguments for many years. Assays of this exceptionally rich zone effectively supported excessive appraisals of the value of the ore deposits and served as an example with which to refute skeptics.

Zincite also occurs as finger-like masses (Figure 12-17), recrystallized masses (Figures 12-35, 12-36, and 12-37), and large, distorted, conical masses in calcite at both deposits (Figure 22-12), as well as in druses and varied vein assemblages which clearly postdate the common ore and in other habits.




	Figure 22-13. Franklinite (gray-black at bottom) with zincite (black at top) surrounded by massive leucophoenicite (gray) with willemite (whitish-gray, surrounding zincite) from Franklin. Specimen is 9 cm in maximum dimension. Mineralogical Museum, Harvard University, #109376. Photo by Chip Clark.		Figure 22-14. Subparallel growth of elongate zincite crystals from Sterling Hill. Field of view is 0.5 mm in maximum dimension.

The formation of zincite is one of the enigmatic aspects of these deposits. Metsger et al. (1958) proposed the formation of zincite by residual Zn when willemite is replaced by tephroite. Frondel and Baum (1974) favored the decarbonation of smithsonite or hemimorphite. These ideas were investigated at Sterling Hill by Squiller (1976), who agreed with the idea of primary zincite forming from the decarbonation of a Zn-Mn-Fe protore carbonate.




	Figure 22-15. Curved and rounded aggregates of zincite within white calcite from Franklin. Matrix is common ore consisting of zincite, tephroite, and franklinite; note the thick zone of tephroite (gray) between the common ore and the calcite-zincite area. Specimen is 14 cm in maximum dimension. Smithsonian Institution, #83937-2. Photo by the author.

Additionally, he found evidence for secondary zincite formed by the “tephroitization” or “serpentinization” of primary willemite. Evidence for zincite formation by “tephroitization” consists of “zincite finely-dispersed in willemite partly replaced by tephroite” (Squiller, 1976). This is particularly common in the east limb of the Sterling Hill orebody. On a small scale, zincite may have been formed by the replacement of franklinite by hematite. Additional work remains to be done on the problem of zincite genesis, especially with regard to zincite possibly transported from one part of the orebody to another.

At Sterling Hill, solution-vugs form in some zincite (Figure 22-8), leaving remnant layers of hetaerolite; hodgkinsonite is common as secondary crystals in such solution-vugs. Zincite alters surficially to hydrozincite, which commonly occurs as white films and coatings, and to other minerals.

Synthetic zincite is formed during some furnace operations and was reported by Blake (1852a). Fragments from flues and furnaces are occasionally found; it is also found in clinkers and slags. Much detail on hydrothermal growth and the effects of impurities on color and habit of synthetic zincite were given by Kuz’mina et al. (1973).

The occurrence of zincite is further discussed in the sections on the “Geology and structure of the zinc deposits” and “Mineral assemblages.”

Name

Zincite was named for its major metal, zinc.

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