Julgoldite-(Mg)
Chemical formula: Ca<sub>2</sub>MgFe<sup>3+</sup><sub>2</sub>(Si<sub>2</sub>O<sub>7</sub>)(SiO<sub>4</sub>)(OH)·H<sub>2</sub>O
A rare silicate from the pumpellyite group, forming dark green to black, acicular or fibrous aggregates.
Description
## Characteristics Julgoldite-(Mg) is a mineral belonging to the pumpellyite group, an hydrated calcium, magnesium, and iron silicate. It forms characteristic, slender crystals with prismatic, acicular, or capillary habits. It most commonly occurs as radial or spherical aggregates, as well as fibrous and felted masses. Its color ranges from dark green, through greenish-black, to almost black, depending on the iron content. ## Physical Properties This mineral has a hardness of approximately 6 on the Mohs scale, making it relatively scratch-resistant. It has a vitreous luster, which can be duller on fracture surfaces. It is translucent (in thin fragments) to completely opaque in thicker pieces. Its density is approximately 3.5-3.6 g/cm³. ## Colors and Varieties Julgoldite-(Mg) does not form color varieties in a commercial sense. Its color is relatively constant and falls within the range of dark greens and blacks. The name "Julgoldite-(Mg)" indicates that it is a member of the julgoldite solid solution series, in which magnesium (Mg) dominates over iron (Fe²⁺) in a specific structural site. Julgoldite-(Fe²⁺) and Julgoldite-(Fe³⁺), known as "pumpellyite-(Fe³⁺)", also exist. ## History and Name The name "julgoldite" was given in 1971 in honor of Julian Royce Goldsmith (1918-1999), a distinguished geochemist from the University of Chicago. The suffix "-(Mg)" was added later, in accordance with the International Mineralogical Association (IMA) nomenclature rules, to precisely specify the dominant cation in the mineral's crystal structure. The mineral was first described from the Sugashima locality in Japan. ## Uses Julgoldite-(Mg) has no industrial applications. Its significance is limited to the scientific community (as an indicator of metamorphic conditions) and the collection of rare minerals.
Diagnostic features
## Identification Identifying features include: dark green to black color, radial or fibrous habit of aggregates, and occurrence in a specific geological environment – altered volcanic rocks. The streak has a characteristic greenish-gray color. ## Distinguishing from Similar Minerals Julgoldite-(Mg) can be confused with epidote, actinolite, aegirine, or tourmaline (schorl). It usually differs from epidote by its darker color and more often forms radial rather than granular aggregates. Actinolite can be lighter and has a different internal structure. Distinguishing it from other members of the pumpellyite or julgoldite group based on visual characteristics is practically impossible and requires advanced chemical (e.g., EDS) or X-ray (XRD) analyses. ## Crystal Forms Crystals are elongated, prismatic, acicular, or capillary. They almost always occur in aggregates, forming radial masses resembling sea urchins, fans, sheaves, or spherulites. Fibrous and felted forms, filling cracks in the host rock, are less common.
Geological environment
## Genesis Julgoldite-(Mg) is a typical mineral for rocks that have undergone low-grade metamorphism, under zeolite and prehnite-pumpellyite facies conditions. It forms as a result of hydrothermal alteration of mafic igneous rocks, such as basalts, andesites, or gabbros. It crystallizes in gas cavities (amygdules), fractures, and veins cutting through the rock. ## Mineral Associations This mineral often co-occurs with other products of low-temperature alteration. The most common associated minerals include: prehnite, pumpellyite, epidote, chlorites, calcite, quartz, and various minerals from the zeolite group (e.g., laumontite, stilbite). ## Localities As a rare mineral, julgoldite-(Mg) is known from relatively few localities worldwide. The type locality is Sugashima, Toba, Mie Prefecture, Japan. Other significant occurrences, from which well-formed specimens originate, include the Wessels Mine in Hotazel (South Africa) and some occurrences in the Alps in Italy and Switzerland.
Rarity
Rare
Collector aspects
## Quality Criteria The most sought-after specimens by collectors are those exhibiting well-formed, radial aggregates with an intense, dark green color. Spherulites with sharply terminated needles, contrasting with a light host rock (e.g., with white calcite or prehnite), are highly valued. The size of the aggregates and the absence of mechanical damage also significantly increase the collectible value of the specimen. ## Popular Localities The best quality specimens come from historical finds in Japan (Sugashima) and from the Wessels Mine in South Africa, where julgoldite-(Mg) forms beautiful, spherical aggregates on matrix. These two localities have provided exemplary specimens of this mineral, which can be found in collections worldwide.
Care and storage
## Cleaning Specimens should be cleaned gently, using a soft brush to remove dust. If necessary, the mineral can be rinsed with distilled water. Strong rubbing should be avoided, especially for specimens with a fibrous structure. ## What to Avoid The mineral is sensitive to strong acids. Ultrasonic cleaners should not be used, as vibrations can damage delicate, acicular crystals and lead to the disintegration of aggregates. ## Storage Julgoldite-(Mg) is stable under household conditions. It should be stored in a dust-free environment and protected from mechanical damage. It is best kept in closed boxes or display cases.