Hodgkinsonite

Characteristics

• Chemical Formula: MnZn₂(SiO₄)(OH)₂
• Crystal System: Monoclinic
• Color: Pink, red-purple, purple, brownish-red
• Luster: Vitreous to resinous
• Streak: Light pink or white
• Hardness (Mohs): 4.5–5
• Cleavage: Perfect on {100}
• Fracture: Uneven to conchoidal
• Specific Gravity: 3.9–4.0
• Diaphaneity: Transparent to translucent
• Fluorescence: May exhibit weak fluorescence under ultraviolet light (variable)

Etymology and Discovery

Hodgkinsonite was first discovered in 1913 at the renowned Franklin Mine in Sussex County, New Jersey, USA. It was named in honor of H.H. Hodgkinson, who served as the superintendent of the Franklin Furnace and Mine at the time of its discovery. The Franklin and Sterling Hill mines are celebrated for their unique assemblage of zinc and manganese minerals, many of which are found almost exclusively in these localities.

Physical Properties

Hodgkinsonite typically forms as prismatic or tabular crystals, often occurring in radiating aggregates or as granular masses. Its striking pink to red-purple color, attributed to the presence of manganese, is one of its most defining features. The mineral exhibits a vitreous to resinous luster and a light pink or white streak. With a Mohs hardness of 4.5 to 5, it is a relatively soft mineral. It displays perfect cleavage on one plane and has an uneven to conchoidal fracture. Hodgkinsonite is also notably dense, with a specific gravity ranging from 3.9 to 4.0. Some specimens may show a weak, dull red or orange fluorescence under ultraviolet light, although this property is not consistent across all samples.

Occurrence and Formation

Hodgkinsonite is a rare secondary mineral found predominantly in metamorphosed zinc ore deposits. Its type locality, Franklin, New Jersey, remains the most significant source, where it occurs in association with a unique suite of other rare zinc and manganese minerals. It forms under specific conditions of metamorphism and hydrothermal alteration within these zinc-rich environments. While the Franklin and Sterling Hill mines are the primary localities, hodgkinsonite has also been reported, though much more rarely, from a few other places globally, including Långban, Sweden, and localities in Germany.

Associated Minerals

In its primary occurrences, Hodgkinsonite is commonly found alongside other minerals characteristic of the Franklin-Sterling Hill deposits, such as:

• [[Franklinite]]
• [[Willemite]]
• [[Zincite]]
• [[Rhodonite]]
• [[Bustamite]]
• [[Tephroite]]

Significance

Due to its vibrant coloration and rarity, hodgkinsonite is primarily sought after by mineral collectors. Well-formed crystals are highly prized for their aesthetic appeal and scientific significance, providing insights into the complex mineralogy of its unique ore deposits.