Anomalous coloration is sometimes due to inclusions of finely-divided zincite (giving a bright orange color) or of pyrochroite or hetaerolite (giving a black color). Transparent crystals on colored matrixes can also give a false-color impression by transmittance of the color of the underlying mineral. Hodgkinsonite has a vitreous luster, a density of 4.07 g/cm³, and one perfect basal cleavage. 

Optically, hodgkinsonite is biaxial, negative, 2V = 50-60^o, with a = 1.724, b = 1.742, and g = 1.746. Pleochroism is distinct.

The fluorescence in ultraviolet is dull weak red in longwave, even weaker in shortwave, and has been confirmed on well-studied specimens. The fluorescence is easily masked by the fluorescence of strongly-fluorescing associated minerals, such as willemite.

Cut gemstones were reported by Bank and Henn (1990) to have optical data and density data similar to those given here.

	Figure 15-45. Typical terminations of Franklin hodgkinsonite crystals, at different magnifications. The one on the left is 0.6 mm wide at the base; the one on the right is 0.4 mm wide at the base.

Hodgkinsonite is distinguished from leucophoenicite by the latter’s lack of cleavage and fluorescence, by optical properties, and by differing solubilities in acid (Dunn and Bostwick, 1982). It is distinguished from rhodonite by the three cleavages of the latter. In color only, it resembles some Sterling Hill corundum, but it is easily distinguished by hardness and associated minerals.

	Figure 15-46. Typical habit for hodgkinsonite crystals from Franklin. Field of view is 1.0 mm in maximum dimension.		Figure 15-47. Typical hodgkinsonite crystals from Franklin. Field of view is 2 mm in maximum dimension.

Composition 

Hodgkinsonite is a zinc manganese silicate hydroxide mineral. Several microprobe analyses from Dunn and Bostwick (1982) are given in Table 3 and demonstrate that most hodgkinsonite conforms to the theoretical composition, with limited substitution of other octahedral cations for Mn. Some very light pink hodgkinsonite has approximately 1.6 wt. % CaO in substitution for Mn.

Occurrence and paragenesis

Hodgkinsonite is a mineral of the secondary veins. It is known from both Franklin and Sterling Hill, but the preponderance of fine specimens are from Franklin; those from Sterling Hill are inferior. Franklin hodgkinsonite is almost invariably associated with willemite and almost always associated with hetaerolite (Figures 22-65 and 22-66) or franklinite (both Zn-Mn bearing minerals). Associated calcite and zincite are sparse, and native copper is generally rare, but may be present in some veins. Barite and hetaerolite accompany many vein assemblages, especially vuggy ones. Much Franklin hodgkinsonite is massive. It occurs as veins, aggregates, and intergrowths with varying amounts of willemite and franklinite. Well-formed crystals occur singly and in druses on franklinite/willemite  ore; these have provided the best crystals. Franklin assemblages are numerous, and only some are mentioned here.

	Figure 15-48. The classic hodgkinsonite assemblage, consisting of hodgkinsonite (gray), barite (white), and franklinite (black, at right), from Franklin. Specimen is 11 cm in maximum dimension. Smithsonian Institution, #C4541-1. Photo by the author.

Clearly the most esthetic and prized specimens are those first described, in which hodgkinsonite occurs as pink massive material, with pre-existing, platy, white barite (Figure 15-48). Such specimens were once moderately abundant and many have been preserved in many systematic collections.  Apparent hodgkinsonite crystal faces in this assemblage are likely pseudo-faces, contact relict-faces from barite crystals.

Equally prized are the specimens of very typical crystals with white, rough-surfaced barrel-shaped calcite crystals (Figures 15-45, 15-46, 15-47, and 15-49) in veins in willemite/franklinite  ore.

	Figure 15-49. Superb hodgkinsonite crystals with barrel-shaped calcite crystal from Franklin. Field of view is 5 mm in maximum dimension.

An uncommon, yet significant assemblage is one of isolated, light-pink clusters of highly lustrous, yet poorly formed hodgkinsonite with clusters of barite, associated with willemite and fine 2-3 mm pyrochroite crystals on granular willemite/franklinite  ore. This assemblage also hosts some of the best Franklin pyrochroite.

Some superb hodgkinsonite crystal groups are found in an assemblage formed on a fine-grained, calcite matrix which has willemite and serpentine stringers randomly distributed through it. The surface of the specimens appears weathered and etched. Angular and platy chunks of hodgkinsonite and willemite are on this surface and appear to be in a relict texture, having once occupied interstitial spaces in a pre-existing, but now missing aggregate of platy crystals, perhaps calcite or barite. Second generation hodgkinsonite occurs, as the last mineral formed, in light grayish-pink 1-2 mm crystal aggregates with an irregular, subparallel arrangement.  

	Figure 15-50. Franklin hodgkinsonite (gray in center of veins) with willemite (white, at contact with ore), franklinite (black), and primary, granular franklinite-willemite ore. Specimen is 7 cm in maximum dimension. Smithsonian Institution, #C3191-5. Photo by the author.		Figure 15-51. Hodgkinsonite crystals of uncommon habit from Sterling Hill. Field of view is 0.3 mm in maximum dimension.

A few samples have been preserved of a rare assemblage, consisting of pink-to-orange hodgkinsonite, in elongate (1-2 cm), subparallel sprays of crystals, not unlike the subparallel fingers of a human hand. The crystals radiate from a light pink massive hodgkinsonite and have a light pink core throughout their length, a pink to reddish druse surface, and orange color on the terminal forms.

Palache (1935, p.127) described, as sussexite, an uncommon assemblage which was re-investigated by Dunn and Bostwick (1982). They found it to be an intergrowth of light-pink hodgkinsonite and sussexite and recalculated the prior analysis of this material as that of a mixture of these minerals. These specimens, few in number, consist of chert-like, 2 cm-thick aggregates, with a distinctly dull to satiny luster, and a fibrosity due to the included sussexite.          

A listing of all Franklin hodgkinsonite assemblages is beyond the scope of this effort, but a few rare and significant occurrences should be mentioned. Hodgkinsonite occurs, together with clinohedrite and willemite, in a vuggy recrystallized hetaerolite assemblage, providing the type specimen of franklinfurnaceite. Hodgkinsonite is found as pink crystals with larsenite which formed from the hydrothermal alteration of the esperite-glaucochroite assemblage. It also occurs commonly, but in small grains, at the feathery reaction zone where esperite replaces hardystonite crystals.

	Figure 15-52. Franklin hodgkinsonite showing an uncommon habit. Field of view is approximately 1 mm in maximum dimension. See figure 15-43 (bottom drawings) and figure 12-43.

Superb hodgkinsonite crystals of unusual habit (Palache, 1928a, 1935) (Figure 15-52) are associated with fine tephroite, willemite, and pyrobelonite (Figures 25-33 and 25-34); the assemblage is described in the section on tephroite. Hodgkinsonite from Franklin is associated with numerous common minerals, as noted above, and is also associated with rare species such as adelite, cahnite, and others.

Hodgkinsonite was first reported from Sterling Hill by Cook (1973) as a brownish botryoidal coating and as very small reddish-brown microcrystals (Figure 15-51). The specimens seen by the writer are varied, but generally have a matrix of calcite with varying amounts of minor willemite and franklinite. Most Sterling Hill hodgkinsonite occurs as druses, commonly associated with chlorophoenicite (Zn-Mn arsenate) and hetaerolite (Zn-Mn oxide) and also with zincite and numerous other arsenates in one-of-a-kind assemblages. Light brownish gray crystals with simple morphology, resembling barite, were found in 1980.

Name

Hodgkinsonite was named in honor of H. H. Hodgkinson, assistant underground superintendent of the Franklin Mine.

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