HODGKINSONITE

Hodgkinsonite is a zinc manganese silicate hydroxide mineral. Several microprobe analyses from Dunn and Bostwick (1982) 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.
Hodgkinsonite is not only partially reflective of the chemical composition of these unique orebodies, but it is also one of Franklin's most esthetically pleasing minerals and has provided a moderate wealth of mineral specimens.
Hodgkinsonite was first described from Franklin by Palache and Schaller (1913); additional data were given by Palache (1914, 1928a, 1935). These morphological data were further studied by Hardie et al. (1964). The unit-cell and space group were given by Rentzeperis (1958), but Roberts and Quodling (1962) suggested an interchanging of the a and c axes. X-ray powder diffraction data were given by Graeber and Rosenzweig (1963) and republished by Dunn and Bostwick (1982), together with many new analytical data. Hodgkinsonite is known from both Franklin and Sterling Hill.
The crystal structure of hodgkinsonite was given by Rentzeperis (1963). A study by Solov'eva and Belov (1963a) confirmed the study of Rentzeperis, but they later (1963b) published another interpretation of the structure. According to Rentzeperis (1963), ZnO₄, and SiO₄ tetrahedra form a network (Zn₂SiO₄)_X, interlayered with an MnO₆ octahedral sheet, both of which are parallel.
Hodgkinsonite occurs as 1-15 mm, euhedral, lustrous crystals and as massive material up to 10 cm. The best crystals are elongate in habit, but some are equant, and distorted crystals are common. The crystal morphology has been well described, summarized, and illustrated by Palache (1935). Hodgkinsonite is commonly reddish pink, pink, or violet pink, but also occurs, without visible impurities, in hues of light orange, brown, and yellow. Clear yellow crystals described by Palache (1935) were confirmed as hodgkinsonite by [Dunn].
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.
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.
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.
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 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.
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. 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 in veins in willemite/franklinite ore.
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.
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.
Superb hodgkinsonite crystals of unusual habit (Palache, 1928a, 1935) are associated with fine tephroite, willemite, and pyrobelonite; 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. The specimens seen by [Dunn] 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. (Dunn, 1995)

Location Found:

Franklin (Type Locality) and Ogdensburg, unique to Franklin/Ogdensburg area

Year Discovered:

1913

Formula:

Mn²⁺Zn₂(SiO₄)(OH)₂

Essential Elements:

Hydrogen, Manganese, Oxygen, Silicon, Zinc

All Elements in Formula:

Hydrogen, Manganese, Oxygen, Silicon, Zinc

IMA Status:

Valid - first described prior to 1959 (pre-IMA) - "Grandfathered"

Fluorescent Mineral Properties

Mid wave UV light:

Weak cherry-red

Longwave UV light:

Weak cherry-red

Additional Information:

Not all hodgkinsonite is fluorescent. Fluoresces weaker at longwave

To find out more about this mineral at minDat's website, follow this link Hodgkinsonite

References:

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.370

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.61

The Picking Table References

	PT Issue and Page	Description / Comment
View Issue	V. 57, No. 2 - Fall 2016, pg. 20	Fluorescent Minerals of Franklin and Sterling Hill, N.J., Part 1, Richard C. Bostwick - Hodgkinsonite
View Issue	V. 37, No. 1 - Spring 1996, pg. 19	Closest-Packed Mineral Structures of Franklin-Ogdensburg: Kepler's Gift of the Snowflake, Part two of two parts, Paul B. Moore - Hodgkinsonite
View Issue	V. 35, No. 1 - Spring 1994, pg. 9	The Famous Late-Stage Oxysalt Minerals of Franklin-Ogdensburg: Closest-Packing and Oxidation-Reduction, Paul B. Moore, Hodgkinsonite as an Example
View Issue	V. 33, No. 2 - Fall 1992, pg. 10	The Check List of Franklin-Sterling Hill Fluorescent Minerals - Hodgkinsonite (Fluorescent Info, under LW not SW)
View Issue	V. 24, No. 1 - Spring 1983, pg. 14	Mineral Notes Research Reports, Hodgkinsonite
View Issue	V. 22, No. 1 - March 1981, pg. 3	Guerinite and Hawleyite from Sterling Hill, Pete J. Dunn, Hodgkinsonite
View Issue	V. 18, No. 2 - September 1977, pg. 14	The Fluorescent Minerals of Franklin and Sterling Hill, NJ by Richard C. Bostwick - Hodgkinsonite
View Issue	V. 13, No. 2 - August 1972, pg. 12	The Fluorescent Minerals of Franklin/Ogdensburg Area by Frank Z. Edwards - Hodgkinsonite (Fluorescent Info)
View Issue	V. 13, No. 1 - February 1972, pg. 8	Franklin Mineral Notes - Hodgkinsonite
View Issue	V. 12, No. 2 - August 1971, pg. 4	Geological Notes - Fluorescent Hodgkinsonite
View Issue	V. 9, No. 1 - February 1968, pg. 14	The Exclusive Minerals of Franklin/Ogdensburg, N.J. (as of January 1968) by Frank Z. Edwards - Hodgkinsonite (Short Note)
View Issue	V. 6, No. 2 - August 1965, pg. 10	Hodgkinsonite
View Issue	V. 5, No. 1 - February 1964, pg. 8	Mineral Notes - Hodgkinsonite (small article)
View Issue	V. 2, No. 1 - February 1961, pg. 4	Yellow "Hodgkinsonite" (small article about misidentification)

Images


Hodgkinsonite (lavender-pink) non-fluorescent, willemite (light green), calcite, franklinite from Franklin, NJ. From the collection of, and photo by JVF.


Hodgkinsonite (gemmy violet, beige, pink), willemite (light green), franklinite (black) and minor barite (white) from Franklin, NJ. From the collection of, and photo by Robert A. Boymistruk.


Hodgkinsonite (gemmy violet, pink), willemite (light green) and franklinite (black) from Franklin, NJ. From the collection of, and photo by Robert A. Boymistruk.		Hodgkinsonite, willemite and franklinite from Franklin, NJ under longwave UV light. The hodgkinsonite fluoresces dark cherry and willemite green, the franklinite is non-fluorescent. From the collection of, and photo by Robert A. Boymistruk.

Hodgkinsonite, willemite and franklinite from Franklin, NJ under shortwave UV light. The hodgkinsonite fluoresces cherry and willemite green, the franklinite is non-fluorescent. From the collection of, and photo by Robert A. Boymistruk.


Hodgkinsonite (lavender-pink), barite (white) and franklinite (black) from Franklin, NJ. Field of view 2". From the collection of, and photo by Robert A. Boymistruk.


Hodgkinsonite crystals (purple), willemite crystals (green) and franklinite (black) from Franklin, NJ. Field of view 1 5/8". From the collection of, and photo by Robert A. Boymistruk.