FRANKLIN AND STERLING HILL NEW JERSEY: THE WORLD'S MOST MAGNIFICENT MINERAL DEPOSITS
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SOROSILICATES

The epidote group

ALLANITE-(Ce)

CLINOZOISITE

EPIDOTE

HANCOCKITE

PIEMONTITE


Other sorosilicates

BARYLITE

BARYSILITE

CUSPIDINE

FERROAXINITE

GANOMALITE

HARDYSTONITE

HEMIMORPHITE

JUNITOITE

KENTROLITE

MANGANAXINITE

NASONITE

PUMPELLYITE-(Mg)

SAMFOWLERITE

THORTVEITITE

VESUVIANITE


CYCLOSILICATES

The tourmaline group

DRAVITE

SCHORL

UVITE

 

HARDYSTONITE

Ca2ZnSi2O7 
Tetragonal, P421m, a = 7.827. c = 5.014 Ĺ, Z = 2.

Hardystonite was first described from Franklin by Wolff (1899a) and Wolff and Melczer (1900). Ries and Bowen (1922) reported it occurring mainly in the northern end of the orebody. Hardystonite was synthesized by Segnit (1954, 1962), and X-ray powder data are given by the ICDD (PDF# 12-453). Modern microprobe analyses and a brief discussion of textural relations with esperite were given by Dunn (1985b). In spite of these works, the petrographic and genetical relations for hardystonite are little studied. Hardystonite is a common mineral at Franklin, and is second only to willemite as the most abundant zinc silicate mineral here. It has not been reported from Sterling Hill.  

Crystal structure

The crystal structure was described by Warren and Trautz (1930) and was refined by Louisnathan (1969), who described it as consisting of [Zn2Si2O7] sheets within which the nature of the bonding is dominantly covalent, with the adjacent sheets being held together by Ca2+ ions.

Description

Although massive material is predominant, crystals of hardystonite up to at least 9 cm have been found; most are 1-2 cm in size. Such elongate crystals are slightly resorbed and have not been measured; their morphology is unknown. These crystals occur for the most part in calcite; some occur in willemite; all are rock-locked; and none have been found growing free in vugs. Hardystonite is white to gray to light pink, but may be light yellow from included zincite (Ries and Bowen, 1922). It has a distinctly greasy to vitreous luster and a density of 3.39-3.44 g/cm3 (Palache, 1935). Cleavage is perfect on {001} and imperfect on {100}; the resultant crystal fragments can have a rectangular appearance. Exsolution textures are unknown, but late-stage willemite commonly permeates the cleavage traces, giving an apparent “exsolution” appearance seen under ultraviolet.

Optically, hardystonite is uniaxial, negative, with w = 1.669, and e = 1.657. The fluorescence in ultraviolet is bluish-violet to violet to violetish-blue in shortwave and longwave; see Newsome (1978) for details. In general, under the electron microprobe beam, the intensity of the cathodoluminescence is proportional to the Pb-content, being stronger with increasing Pb. Most hardystonite is easily distinguished by its fluorescence in ultraviolet.

Composition

Hardystonite is a calcium zinc silicate of the melilite group. Representative analyses are given in Table 3; these show that there is very limited solid solution towards melilite. Also evident is the small but ubiquitous Pb content. Hardystonite is, together with esperite, an original host for Pb in the primary ores. Bauer found 3.61 wt. % Pb in one specimen, but most have PbO values less than 1.8 wt. %. Ries and Bowen (1922) checked for native lead in thin section and found none. Lead was quite deleterious to the refining processes used by the New Jersey Zinc Company. Accordingly, it was carefully monitored, both by the manual extraction of Pb-containing minerals at the picking table and by constant sampling and chemical analysis of ores. See discussion in the section entitled “The lead problem.”

Occurrence and paragenesis

Hardystonite was found in abundance in the Franklin Mine, but not even as traces at Sterling Hill: an enigma that stands unaddressed. It occurred on a large scale, as a high-temperature primary mineral, in part a reactant between calcite-bearing ore and non-calcite-bearing ore. It occurred mostly in the northern end of the deposit, as deep as the 1150 level, and also in the south end on the 600 and 800 levels, in addition to many other occurrences. Zones of hardystonite several feet (0.6 meters) across were not uncommon. It occurs in a large number of assemblages associated with a great many minerals. Foremost among these are franklinite, willemite, and calcite; zincite and esperite are also associated, albeit less abundantly.

Hardystonite is a common component of symplectites, occurring intergrown with tephroite. Most franklinite occurring within hardystonite is rimmed by reactant willemite; zincite is not so rimmed (Figure  17-11). Hardystonite also occurs in veins in willemite- franklinite ore, commonly with calcite, and sometimes occurs as a reactant where willemite-rich veins form in calcite.

Hardystonite is common to many assemblages which have Ca-Zn-Si bulk compositions. They are too numerous to list here. Of particular interest is the association of hardystonite with esperite, willemite, and franklinite, with sparse calcite (Dunn, 1985b). Esperite replaces hardystonite along cleavage traces, and willemite, hodgkinsonite, and clinohedrite form at the feathery intergrowth of esperite and hardystonite. Such assemblages are prized as fluorescent specimens. Hardystonite also occurs in banded granular ore with franklinite, willemite, and rhodonite.

Hardystonite is readily altered by hydration to clinohedrite. The replacement is commonly partial, creating thin films on open fracture surfaces. However, it may be extensive and nearly complete. In the assemblage which serves as host for glaucochroite and cuspidine crystals, hardystonite has been almost wholly replaced by clinohedrite. Not all clinohedrite occurs in this manner. Hardystonite alters surficially to a brown rind.

Name

Hardystonite was named for Hardyston Township, the local political entity in which the Franklin Mine was located at the time of hardystonite’s discovery.

 

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Copyright © 1995 by Pete J. Dunn
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CHAPTER 16. SOROSILICATES AND CYCLOSILICATES