CALCITE

CaCO₃
Hexagonal

	Figure 23-4. Crystal drawings of calcite. That at upper left is of Buckwheat Mine calcite; those at upper right and lower left are of calcite from the Parker Shaft area; and the others are of calcite from Sterling Hill. Drawings are from Palache (1935) who provided crystallographic data.

Calcite is very common locally. A number of obscure names have been assigned to local calcite, among them are manganocalcite and calcimangite (Shepard, 1865) and spartaite (Breithaupt, 1858); the roepperite of Kenngott (1872), referred to under calcite by Palache (1935), is likely kutnahorite.

Description

Calcite from Franklin and Sterling Hill is mostly massive; fine crystals are uncommon and generally do not exceed several cm in size. Crystal habits and form development vary substantially; fine crystals occurred at the Buckwheat Mine and the Parker Mine (Palache, 1935) (Figure 23-4). Prismatic, equant, and tabular crystals occur; rhombohedral habits are common (Figure 23-7), and pseudo-octahedral habits are known (Knoll, 1972; Peters et al., 1983). Franklin crystals were studied and illustrated by Gordon (1923) and Palache (1935).

	Figure 23-5. Rare curved habit of calcite from Franklin; such crystals occur associated with gageite. Field of view is 0.7 mm in maximum dimension.		Figure 23-6. Equant crystal of calcite on willemite from Franklin. Field of view is 0.5 mm in maximum dimension.

Calcite is colorless, white, orange, or gray for the most part; highly manganoan material is light pink, and locally other colors are found. Manganoan calcite is commonly surficially altered to a grayish-black to dull-black color; this matter has not been investigated in detail. The color of common orebody calcite is generally white to grayish-white. However, the color can vary substantially and is sometimes enhanced or affected by inclusions of other minerals, transmission of color from underlying colored minerals, and thin films and druses of other minerals or substances.

	Figure 23-7. Sharp rhombohedral calcite crystals with “frosted” edges from Sterling Hill. Field of view is 0.1 mm in maximum dimension.		Figure 23-8. Rough-surfaced, barrel-shaped calcite crystal with hodgkinsonite crystals from Franklin. These calcite crystals are opaque white. Field of view is 5 mm in maximum dimension.

The texture of massive material varies from fine-grained to very coarse-grained, columnar (Levison, 1916), granular, and others. The cleavage is rhombohedral and perfectly developed, and striations likely caused by pressure twinning are commonly evident. The luster is vitreous in general, and pearly, fibrous, or silky locally. The density, ranging from 2.73-3.06 g/cm³ (Frondel and Bauer, 1955), and the indices of refraction vary with the extent of manganese substitution, as noted by Krieger (1930). Calcite is easily confused with kutnahorite and light-colored rhodochrosite.

The most notable of the properties of much orebody-related calcite is its fluorescence in ultraviolet; the earliest report of this is by Kunz and Baskerville (1903). Calcite from the Franklin Marble fluoresces but rarely. Calcite from both orebodies, and from the Franklin Marble in contact with them, is manganoan and fluoresces with a strong orange-red response color in shortwave; it is generally weaker in longwave. The fluorescence is activated by Mn and Pb concentrations (Schulman et al., 1947), and Zn may play a role as well (Gies, 1975).

Correlation of specific fluorescence colors with wavelength was discussed by Smith and Parsons (1938), and the effects of thermal change were given by Metsger et al. (1953), Nichols et al. (1918), Kulp et al. (1949), and McDougall (1952). Brown (1934) and Blazek (1973) provided a correlation between the Mn-content of Franklin calcite and the intensity of its fluorescence; Brown found the intensity maximized at approximately 3-4 mole % MnCO₃. 

The pink or red basic fluorescence color and its intensity vary substantially, likely as a result of compositional variation sometimes within a single hand-specimen. Much material commonly has a very brief, slightly orange-pink phosphorescence. Other fluorescent colors are seen locally. These include white, pale blue, orange, yellow, and others (R. C. Bostwick, pers. comm.). The luminescence in infrared was studied by Barnes (1958). Franklin calcite was found to have white to yellow thermoluminescence, and trace amounts of Pb, Sn, Mg, and Fe (Northup and Lee, 1940).

Composition

	Table 22. Chemical analyses of carbonate minerals.

Calcite is a calcium carbonate mineral, polymorphous with aragonite. Material from the Franklin Marble is virtually Mn-free (Frondel, 1972), but that from the orebodies and from near contacts with the orebodies is manganoan. A substantial number of analyses have been published; Palache cited those of Nason (1890a) and Kümmel (1905) and presented seven additional analyses, and Squiller (1976) gave four analyses of Sterling Hill calcite from ores. Additional discussion is given in the section entitled “Franklin Marble.”

Among recent compositional studies are those of Frondel and Bauer (1955). Fe and Zn substitutions are minimal, and Mg substitutes up to 11 mole % MgCO₃, but such substitution is not common; the extant analyses show most analyses to have much less than 6 mole % MgCO₃. Physical mixtures with dolomite are common, and the analyst must be aware of these before commencing work. The dominant substituent for Ca is Mn and, as noted by Frondel and Bauer (1955) (see Table 22), the Mn content may vary considerably, as discussed herein under rhodochrosite. Rare-earth analysis of a Franklin calcite was given by Gies (1975), and Buis (1983) reported the presence of La, Ce, and Nd in Sterling Hill calcite. Isotopic data were given by Johnson (1990) and Davis (1993).

Occurrence and paragenesis

	Figure 23-9. Parallel-growth array of calcite crystals from Franklin. Field of view is 1.2 mm in maximum dimension.

Calcite is the most abundant mineral locally. It comprises most of the Franklin Marble and is abundant as the dominant gangue mineral in both ore deposits. The percentages in the ores vary widely, but Frondel and Baum (1974) reported 20.7% calcite (Franklin) and 39.4% calcite (Sterling Hill) for crude ore milled in 1930-1934 and in 1935, respectively. However, these values are strongly affected by mining practices and should not be given great import in a geological sense.

	Figure 23-10. Stout calcite crystals with prismatic gageite crystals hosting tiny franklinite crystals. Field of view is 1.5 mm in maximum dimension.

In the Franklin Marble, calcite is coarse-grained in general; cleavage surfaces up to 30 cm are known, and 10 cm crystal-units are not uncommon. Calcite in the Franklin Marble is graphite- bearing except in close proximity to the orebodies; that within about 5-6 feet (1.5-1.8 meters) of both deposits is graphite-free in general (Frondel and Baum, 1974). The marble also contains dolomite locally, abundant calcium silicates, and numerous accessory minerals. It is discussed in more detail in the section entitled “Franklin Marble.”

At both localities, calcite occurs in the ore-units and in the calcium-silicate units. It forms interstitially with franklinite and willemite, forms masses of relatively pure calcite, and is common in secondary vein assemblages. It is nearly ubiquitous, occurring in nearly all mineralogic niches available.  Mentioning even a few is a forced and biased emphasis, useful only as partial and very incomplete documentation of the hand-specimen record.  

At Franklin, aside from its occurrence in primary and recrystallized ores, calcite forms large, bluish, cleavage rhombohedra, associated with sphalerite, lennilenapeite, and magnesioriebeckite and colored by inclusions of the latter (Figure 17-40). It also occurs as chevron-like growths with an orange-brown unanalyzed garnet, as curved-crystal aggregates with barylite and copper, as salmon-pink and orange massive material, as fine crystals in the Buckwheat Dolomite, and in many other assemblages.

At Sterling Hill, calcite is much more abundant within the high-grade ores than it is at Franklin. Aside from its ore occurrences, fine disc-shaped crystals, up to l cm in size, occur in groups with manganese oxides. At both deposits, calcite crystals are moderately common in vein assemblages, commonly 2-5 mm in size, and quite varied in habit. Secondary calcite from an oxidized sulfide occurrence below the 700 level was described by Jenkins and Misiur (1994).

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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