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(Ca,Na)(Mg,Fe,Mn,Zn,Al)Si2O6
Monoclinic
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| Figure 17-1. Crystal drawings of augite (jeffersonite) from Sterling Hill. Drawings are from Palache (1935) who provided crystallographic data. | ||
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Augite, although validated from both deposits, has been little studied except for the paper by Frondel and Ito (1966b). Jeffersonite was first described and named from Franklin by Vanuxem and Keating (1822a, 1822b); its identity as augite was noted by Troost (1823a), and this relation was further supported by Seybert (1824). A review and morphological description was given by Palache (1935) (Figure 17-1). The preponderance of the material from Franklin and Sterling Hill formerly labeled jeffersonite is augite. Historical notes were provided by Palache (1935).
Frondel and Ito (1966b) described, from Franklin, greenish-black augite as indistinct crystals embedded in pale pink calcite. This augite has a density of 3.55 g/cm3 and a well developed parting; Palache (1935) described it occurring in platy masses. Optically, it is biaxial, positive, 2V = 74o, with a = 1.714, b = 1.723, and g = 1.774.
Augite is a calcium sodium magnesium silicate mineral of the pyroxene group; samples from Franklin and Sterling Hill contain much Fe, Mn, and Zn. An analysis of the Franklin augite of Frondel and Ito (1966b) is given in Table 10. Also given is one of dark greenish brown 1.0 cm crystals in rhodonite with minor andradite from Franklin (#143654), and one of dark green, nearly black, massive material with pronounced parting (#14388) from Franklin; material visually identical to the latter was studied by Marzolf (1967) using Mössbauer spectroscopy. An augite from Sterling Hill was analyzed by Sandhaus (1981); the data are given in Table 10, but nothing is known of the host assemblage. Analyses of the dark, well-formed crystals from Sterling Hill formerly known as jeffersonite are likewise presented and show moderately uniform compositions and low sodium contents.
At Franklin, augite has been reported associated with pink calcite (Frondel and Ito, 1966b), and herein with rhodonite. Little is known of the assemblages of augite from Franklin, but Palache (1935) noted it at the Trotter Mine and the Parker Mine.
Augite from Sterling Hill, however, has a well-documented literature, in which it is largely referred to as jeffersonite. Most of the black-surfaced, well-formed crystals collected at the open pits at Sterling Hill, and referred to in the literature as jeffersonite, are augite. This material, some occurring as enormous crystals to 30 cm, was locally abundant and was collected over a period of many years, as recently as the 1970’s. The crystals are invariably blackish brown; the coating is likely one of Mn-oxides, but has not been studied. These augite crystals are associated with well-crystallized fayalite, gahnite, spessartine, and apatite and with other species such as galena and sphalerite. Some crystals have been completely altered to a pith-like mixture called anomalite by Koenig (1879) and mentioned by Palache (1935). Magnesian hastingsite is common as oriented overgrowths of tiny lustrous blackish crystals on these augite crystals.
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| Copyright © 1995 by Pete J. Dunn |
Website
by Herb Yeates
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This
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