Occurrence
At Franklin, zincite in rounded, shotlike grains or in scales, specks, and splinters is a normal constituent of the ore body over the whole extent of Mine Hill. It is intermixed with franklinite and willemite but is generally much less abundant than they. Its distribution is very irregular, some layers being richer in zincite than others and individual layers differing widely in different parts. In the early history of the mine the ore was thought to he divisible into two layers—one without zincite, called the "franklinite vein"; the other, called the "zinc vein", containing franklinite and zincite in about equal amount. This supposed distinction, which led to much of the prolonged litigation over the property, was, however, wholly fallacious.

Like the other primary minerals of the ore, zincite has undergone recrystallization in parts of the deposit, whereby larger masses were formed than are found in the normal ore, and rarely distinct crystals were produced. The most striking ore specimens found in the district came from such recrystallized masses; patches of bright-red zincite, crystals or masses of green or yellow willemite, and black octahedrons of franklinite, all embedded in snow-white calcite, make a striking color combination. Plate 3, D, shows an unusually rich specimen of zincite from the Trotter mine.

Crystals of zincite, so far as known, have been found only in the Buckwheat mine. They appear in three distinct types, all of which are found in secondary veins in the granular ore. In one type the crystals are large pyramids, some of them 4 inches long, based on massive zincite and embedded either in massive zincite or in manganiferous calcite. Plate 3, C, show such crystals. They are not very brilliant and are revealed only by most careful excavation of the surrounding matrix. The pyramid faces are commonly horizontally striated by oscillatory combination with the prism, but distinct prism faces are rarely seen. On crystals of this type, locally called "ruby zinc " or simply "ruby", of which the author has seen perhaps 20, there is no distinct basal face other than the cleavage plane. The upper base described by Phillips (197) is also believed by the author to be a cleavage plane The measurements recorded on page 38, except those of the last column, were all made on crystals of this type and are highly unsatisfactory, the reflecting faces being of the poorest. The different pyramids cannot be distinguished by the eye because the apparent angle of slope of a pyramid may be abnormal on account of oscillatory development with the prism.

On one tiny fragment, collected by Mr. Cahn, is a fairly well defined twin crystal. A crystal of ordinary type a quarter of an inch long has, interpenetrating it, several similar but smaller parallel crystals with a common vertical axis but m reverse position—a mode of twinning well known on artificial crystals of zincite but seemingly very rare on the natural mineral.

The crystals of the second type are found projecting into open veins an inch or two wide. The largest crystal of this type in the Holden collection is 2 inches long and 1 inch in diameter at the base. It is shown in plate 3, A. Calcite and a very fibrous silky form of willemite are the associated minerals in these veins. The zincite crystals are clean-cut and have smooth faces, but they are dull and etched. In one specimen fibrous willemite is replacing the zincite.

The crystals of the third type are found in association with pyrochroite in a remarkable secondary vein described under that mineral on page 50. The zincite crystals are minute and sharply formed, with generally dull faces, but some of the faces are bright enough to reflect good signals. As shown in figures 25 and 26, many of them, which stand quite free in the cavity, are sharply hemimorphic, with the pyramids a(4045), or a and p(1011), above and the base alone below. The measurements recorded in the table on page 38 were made on two such crystals. Their color is clear orange-yellow instead of the ordinary deep red of the mineral, probably indicating that they contain less manganese than most crystals of zincite. Massive zincite of the same color is seen in the walls of the cavity.

Figure 25 Crystal of zincite showing the pyramid (4045). Buckwheat mine.
	Figure 26 Crystal of zincite showing the pyramids (4045) and p(1011) above and the base c1(0001) below. Buckwheat mine.

At Sterling Hill, to judge from the specimens preserved in collections, zincite was more abundant in the outcrop ore than at Franklin. It was found in similar granular form intermixed with rounded granular franklinite. In some of the ore the bulk of the matrix was crystalline zincite, enclosed in which were rounded or sharply octahedral crystals of franklinite. It was to this phase of the ore, which appears to have cropped out in large masses at Sterling Hill, that Nuttall (7) applied the very apt designation "metalliferous porphyry", and its richness in zinc was the basis of extravagant estimates of the total zinc. content of the deposit. Another phase of the ore showed zincite in isolated masses in limestone unaccompanied by franklinite. Many of these zincite masses are large, and they generally consist of a single individual, but all are of rounded irregular form without a trace of crystal faces. They appear to have been rounded by solution or by resorption on the part of the matrix. The best specimens of this phase, as seen in the Canfield, Kemble, and Harvard collections, were taken from the hanging wall of the first New Jersey Zinc Company’s mine in 1850. Blake’s analysis of zincite was made on material of this sort. Plate 3, B, shows an excellent example in the Canfield collection.

Veinlike masses of recrystallized ore showing franklinite and troostite in well-formed crystals and large platy masses of zincite in very coarsely crystalline calcite, wholly similar to those still found in places at Franklin, are also preserved from Sterling Hill. Zincite crystals with definite form are, however, not known to have been found there.

Calcozincite
The name "calcozincite" was applied by Shepard (110) to mixtures of powdery yellow and reddish zincite and compact calcite in various proportions. Numerous embedded fibers of tremolite asbestos give the whole mass a columnar fibrous appearance.

Historical notes
Zincite was the first of the remarkable minerals constituting these deposits to excite scientific interest, and its description and analysis by Dr. Bruce in 1810 (1) constitute one of the earliest mineralogical investigations made in this country. He called it "red oxide of zinc" and gave a correct characterization of the substance. The name "zincite" was given by Haidinger in 1845 and was really preceded by "sterlingite", proposed by Alger (38), which, however, never became current and is now used for a different substance—one of the micas. Alger’s name "ruby zinc" (81) is still current at Franklin. Brooke and Miller in 1852 proposed the name "spartalite", based on the occurrence of zincite in glacial boulders at Sparta, N.J., several miles south of Franklin, but the name is not found outside their work.

The cleavage and crystal form of zincite were not determined by Bruce, and its hexagonal nature was first established by Hausmann (34) after a long controversy, induced, no doubt, by the variable perfection of the prismatic cleavage. Although so abundant in the Franklin district, zincite has been found nowhere else except in traces. It is reported from two localities in Russia as an oxidation product of zinc ores. Artificial zincite, however, is by no means uncommon as a product of zinc ovens.