Julgoldite-(Fe^3+^)

Chemical formula: Ca<sub>2</sub>Fe<sup>3+</sup>Fe<sup>3+</sup><sub>2</sub>(Si<sub>2</sub>O<sub>7</sub>)(SiO<sub>4</sub>)O(OH)·H<sub>2</sub>O

Dark green to black silicate from the pumpellyite group, forming characteristic radial aggregates of acicular crystals.

## Characteristics Julgoldite-(Fe³⁺) is a mineral from the pumpellyite supergroup, an hydrated calcium and iron silicate. Its most typical and recognizable form consists of spherical or fan-shaped aggregates, composed of fine, acicular or tabular crystals arranged radially. Specimens range in color from dark green, through greenish-black, to almost completely black. The luster of the crystals is vitreous, which, combined with the dark color and specific aggregate form, gives it an attractive appearance. ## Physical Properties This mineral is characterized by a hardness of approximately 4.5 on the Mohs scale, making it relatively brittle. It has a density of about 3.59 g/cm³. It is translucent in thin fragments, while thicker pieces or compact aggregates are opaque. It exhibits good cleavage in one direction. ## Colors and Varieties The dominant colors are dark green and black. The name Julgoldite-(Fe³⁺) indicates that it is a member of the julgoldite group in which trivalent iron (Fe³⁺) dominates at a specific position in the crystal structure. Other minerals in this group include julgoldite-(Fe²⁺) and julgoldite-(Mg). ## History and Name The name "julgoldite" was given in honor of Julian Goldsmith (1918–1999), a distinguished geochemist from the University of Chicago. The suffix -(Fe³⁺) was added in accordance with the International Mineralogical Association (IMA) naming conventions to distinguish it from other members of the group based on the dominant ion. The mineral was formally described as a distinct species in 1971. ## Uses Julgoldite-(Fe³⁺) has no industrial applications. However, it is a valued and sought-after collector's mineral, especially specimens with well-formed, spherical aggregates.

Properties

Mohs hardness
4.5
Luster
Vitreous
Streak
Greenish-black
Density
3.59
Cleavage
Good on {001}
Fracture
Uneven, splintery
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of julgoldite-(Fe³⁺) is its mode of occurrence: spherical, fan-shaped, or radial aggregates composed of very fine, acicular or bladed crystals. The dark green to black color and vitreous luster are also characteristic. ## Distinguishing from Similar Minerals Julgoldite-(Fe³⁺) is sometimes confused with black tourmaline (schorl), which is, however, much harder (7-7.5 on the Mohs scale) and has its typical triangular crystal cross-section. It can also be confused with aegirine, which forms similar aggregates but usually occurs in a different geological environment (alkaline rocks). Distinguishing it from other minerals of the pumpellyite supergroup is usually impossible without advanced chemical analysis. ## Crystal Forms Crystals are most often acicular or thin-bladed (lamellar) and almost always occur in radial aggregates, forming spherical (spherulites), fan-shaped, or sheaf-like forms.

Geological environment

## Genesis Julgoldite-(Fe³⁺) is a secondary mineral, formed during low-temperature hydrothermal processes or under low-grade metamorphic conditions (greenschist facies). It is most commonly found in gas vesicles (amygdules) and fractures in mafic volcanic rocks, such as basalts and diabases, as well as in skarns. ## Mineral Associations It often co-occurs with minerals such as calcite, apophyllite, hematite, quartz, and minerals from the zeolite group. In the famous Kalahari manganese deposits in South Africa, it is associated with sugilite, among others, creating very striking associations sought after by collectors. ## Localities The most important localities yielding collector-quality specimens are the Wessels and N'Chwaning mines in the Kuruman area (Kalahari Manganese Fields, South Africa). The type locality for the mineral is the Jakobsberg mine in Nordmark (Filipstad, Sweden). It also occurs in the USA (e.g., in the Franklin area, New Jersey).

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued specimens are those with complete, well-formed spherical or fan-shaped aggregates of intense, dark color and strong luster. An attractive, contrasting matrix, such as white calcite, adds significant value. Specimens from the Kalahari Manganese Fields, where julgoldite forms associations with purple sugilite or other rare minerals, are particularly sought after. ## Popular Localities By far the most famous and prized specimens come from the Wessels and N'Chwaning mines in South Africa. They provide specimens of exceptional aesthetic quality, which set the standard for this mineral in the collector's market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used. Ultrasonic cleaners should be avoided, as they can irreversibly damage delicate, acicular crystals. ## What to Avoid The mineral is sensitive to acids. Due to its relatively low hardness and brittleness, it should be protected from impacts, falls, and contact with harder minerals that could cause scratches or breakage of the crystals. ## Storage It is recommended to store specimens in separate, padded boxes or display cases to protect them from dust and mechanical damage. Delicate, radial aggregates are particularly prone to crumbling and should therefore not be subjected to shocks or touch.

Sources

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