GAHNITE

ZnAl₂O₄ 
Cubic, Fd3m, a = 8.116 Å, Z = 8

	Figure 22-39. Crystal drawings of gahnite; the one on the left is of a crystal from the Buckwheat Mine in Franklin. Drawings are from Palache (1935) who provided crystallographic data.

The occurrence of what is now called gahnite was noted early by Nuttall (1822) and also by Vanuxem and Keating (1822b), who named it dysluite. This material was later shown by Alger (1846) to be the same as automolite from Franklin (the name automolite was originally applied to zinc spinel from Falun, Sweden, and the local adoption of the name was by Vanuxem, 1822). Abich (1831) studied a Franklin spinel rich in Zn and recognized it as gahnite, the name with priority and that which is currently accepted. “Zinc spinel” is a common informal name in the non-mineralogical literature. X-ray powder data were given by Berry and Thompson (1962) for Sterling Hill gahnite, and compositional data were given by other investigators, as noted below. The spinel of the Franklin Marble is true spinel and is discussed under that heading.

Description

Gahnite commonly occurs in idiomorphic crystals which are well-formed, equant, and predominantly octahedral in habit; some have dodecahedral and, more rarely, cubic modifications. Predominantly cubic gahnites have been reported.

	Figure 22-40. Gahnite crystals from Franklin; the dominant form is the cube, modified by the octahedron and other forms. Field of view is 0.5 mm in maximum dimension.

Crystals may be up to 12 cm in size, but most are less than 2 cm, and few exceed 1 cm. The general priority of forms is {111} >>> {011} >> {100}. Common crystal habits were illustrated by Palache (1935) (Figure 22-39), and a photomicrograph of gahnite crystals is shown in figure 22-40. Crystals may be twinned by the spinel law, and surficial striations and trigons on {111} are common, but not diagnostic. Additionally, gahnite is known in massive aggregates, as anhedral 1-3 mm crystals intergrown with rhodonite, as very fine-grained to saccharoidal masses, and in other textures, as discussed below under exsolution.

The color is quite variable with green, blue, yellow, yellow-brown, greenish black, and intermediate hues all known; the most common color is dark green. The luster is vitreous; the density varies from 4.5 to 4.9 g/cm³; and the index of refraction, also varying with composition, is commonly near 1.82. Reflectance data, given by Sandhaus (1981) for 546 and 460 nm, respectively, are R = 8.3 and 9.5 %. There is no discernible fluorescence in ultraviolet. Gahnite is generally identified by its octahedral crystals, luster, transparency or translucency, superior hardness, and occurrence within the orebodies.

By far the most interesting and significant textural aspects of gahnite are the exsolution relations described by Frondel and Klein (1965). They found two types of gahnite exsolution in franklinite: one as isolated 6-micron-thick lamellae and the other as 30- micron rounded crystals arranged in rows. The exsolution relations and equilibria investigations were described by Squiller (1976), Carvalho (1978), and Carvalho and Sclar (1988); much detail on the different exsolution relations was given by Carvalho; and these are the definitive studies.

Squiller (1976), as part of a broad study, found lamellae of gahnite parallel to {100} of the host franklinite and noted that these intergrowths are quite regular but show local coarsening. The observed gahnite:franklinite ratio is 1:12, and the intergrowths occur principally in franklinite which contains > 2 wt. % Al₂O₃. This writer’s studies in part confirm this observation. Squiller also observed the two types of gahnite exsolution noted by Frondel and Klein (1965), and discussed single-stage and two-stage cooling hypotheses for its formation.

Carvalho (1978) focused on the gahnite and franklinite intergrowths and found that the original homogeneous spinel-structure phase was as aluminous as franklinite₈₀gahnite₂₀. He determined the miscibility gap in the gahnite-franklinite series using hydrothermal methods and approached equilibrium by exsolution of synthetic homogeneous spinels and by reaction of the spinels in the mechanical mixtures (Carvalho and Sclar, 1988). Additional chemical data were provided by Davis (1993), but the analyses of the associated franklinite are of poor quality.

Composition

	Table 21. Chemical analyses of gahnite and hercynite.

Gahnite is a zinc aluminum oxide mineral of the spinel group. Representative analyses are given in Table 21; many fine analyses are given by Squiller (1976), Carvalho (1978), Sandhaus (1981), and Craig et al. (1985). In general, most gahnite from Sterling Hill has 85-89 mole % ZnAl₂O₄; gahnite from Franklin is less well studied, but extant analyses are in the same compositional range. Local gahnites studied to date are quite simple solid solutions; substitution of Mg (spinel component) and Mn (galaxite component) are quite limited; MnO does not exceed 1.0 wt. % in discrete crystals, and Mg is present only in minor amounts. The principal substituent is iron, as Fe²⁺ in solid solution toward hercynite and as Fe³⁺ in solid solution toward franklinite. Carvalho (1978) found that gahnite can contain up to 15 mole % franklinite in solid solution at low temperatures and that it approaches a maximum of 20 mole % at high temperatures. Carvalho also discussed the unmixing of opaque magnetite-franklinite  in gahnite crystals with decreasing temperature.

Occurrence and paragenesis

In general gahnite occurs within the zinc orebodies and common spinel occurs in the Franklin Marble, but gahnite is also found in the marble near contacts with the ores (Figures 12-28 and 12-29).

	Figure 22-41. Octahedral gahnite crystals with calcite (white) and rhodonite (light gray) from Franklin. Specimen is 9 cm in maximum dimension. Mineralogical Museum, Harvard University, #96205. Photo by Chip Clark.

At Franklin gahnite is found as both large and small crystals, associated with a large number of minerals; the most common of these are rhodonite, franklinite, calcite, andradite, and willemite (Figures 22-41 and 22-42). A number of specific Franklin occurrences are noted by Palache (1935). Among the most notable of these are the sapphire-blue crystals with pyroxene, titanite, and loellingite found in the Trotter Mine in 1896 and the cubic gahnite crystals associated with apatite, loellingite, and fayalite which were found in the tunnel to the Wallkill River (Brush, 1871). It has been found on the Buckwheat Dump in fine-grained masses with microcline. Although few specific Franklin occurrences have been documented, the mineral is not rare, providing a ready host for Al in the zinc ores.

	Figure 22-42. Rhodonite (gray) with gahnite (black) and calcite (white) from Franklin. Specimen is 10 cm in maximum dimension. Smithsonian Institution, #47896-1. Photo by the author.

At Sterling Hill, gahnite has long been known in association with augite (jeffersonite), spessartine, and other species in the weathered part of the east limb of the orebody and in the pits. Some giant crystals were recovered from these areas. Gahnite was reported in the second pyroxene zone by Metsger et al. (1958), and this observation was confirmed in detail by Squiller (1976) and Carvalho (1978), who reported their gahnite-franklinite  intergrowths as coming from the pyroxene zones. As at Franklin, gahnite has been found in large crystals, but most are badly fractured and crumble upon removal from the orebody. The associated species are quite variable, including most of the common ones noted above and a number of rare minerals such as pyroxmangite, pyrophanite, and manganpyrosmalite. A colorful and significant skarn assemblage was found in the 1980’s on the 700 and 800 levels, consisting of spessartine, tirodite, calcite, rhodonite, franklinite, and gahnite.         

Among the more chemically interesting gahnite occurrences in the Franklin Marble is one of chromian gahnite, (Zn,Fe)(Al,Cr)₂O₄, rimming a core of chromian hercynite, (Fe,Zn)(Al,Cr)₂O₄ (Table 21). These gahnite crystals are a millimeter or less in size, dark red, and associated with margarite, anorthite, thortveitite, rutile, corundum, arsenopyrite, goldmanite, pyrite, and calcite (Dunn and Frondel, 1990). The occurrence is described under margarite. The enrichment of Zn toward the rims of the crystals is in agreement with the observations of Spry (1987) for zoned gahnites in general and is consistent with retrograde metamorphism.

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