Dissakisite-(Ce)

Chemical formula: CaCe³⁺MgAl₂(Si₂O₇)(SiO₄)O(OH)

A rare mineral from the epidote group, characterized by the presence of cerium and a complex silicate structure, found in metamorphic rocks.

## Characteristics Dissakisite-(Ce) is a complex silicate belonging to the epidote group, specifically to the allanite subgroup. It typically forms small, prismatic crystals with a tabular or columnar habit, often terminated by sloping faces. Specimens rarely exceed a few millimeters in length. It occurs as individual crystals or granular aggregates embedded in the host rock. Its color is usually dark, ranging from brown to almost black. ## Physical Properties This mineral is characterized by a hardness of 6.5-7 on the Mohs scale, making it relatively scratch-resistant. The luster is vitreous to resinous. It is a brittle mineral, with an uneven or conchoidal fracture. The density is quite high, approximately 3.79 g/cm³. It is weakly radioactive due to the content of cerium and other lanthanides that may accompany it. ## Colors and Varieties Dissakisite-(Ce) is typically dark brown to black. Some specimens may exhibit reddish-brown hues. No named color or commercial varieties are distinguished, and its identification is primarily based on chemical analysis. ## History and Name The mineral's name comes from the Greek word *dissakis* (δισσάκις) meaning "twice", which refers to the fact that its chemical formula contains two different silicate groups: [Si₂O₇] and [SiO₄]. The suffix -(Ce) indicates the dominant rare earth element in its composition – cerium. The mineral was first described in 1991 by Donald R. Peacor, Pete J. Dunn, William B. Simmons, and Robert A. Gain based on material from Balmat, New York, USA. ## Uses Due to its rarity and small crystal size, dissakisite-(Ce) has no industrial applications. It is of purely scientific and collector's interest, being an object of interest for specialists and advanced collectors of rare minerals.

Properties

Mohs hardness
6.5-7
Color
Pale yellow-brown in thin section.
Luster
Vitreous to resinous
Streak
White
Density
3.75
Cleavage
No cleavage was observed (type material), but by analogy to other members of the epidote group, there is probably a {001} cleavage.
Fracture
Uneven to conchoidal
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Field identification of dissakisite-(Ce) is practically impossible. It requires advanced analytical methods, such as X-ray microanalysis (EDS/WDS) to confirm its complex chemical composition, especially the presence of cerium as the dominant rare earth element. In a collection, it can be preliminarily recognized by its dark color, vitreous luster, prismatic crystal form, and occurrence in a specific geological environment (metamorphic rocks). ## Distinguishing from Similar Minerals Dissakisite-(Ce) is visually indistinguishable from other minerals in the allanite subgroup, such as allanite-(Ce), allanite-(La), or ferriallanite-(Ce). They differ in subtle proportions of iron, magnesium, and aluminum, which requires specialized analysis. It can also be confused with dark varieties of tourmaline (schorl), epidote, or pyroxenes, from which it is often distinguished by higher density and specific mineral associations. ## Crystal Forms Crystals are typically columnar or tabular, with a cross-section resembling a rhomb. They often exhibit striations on prismatic faces, parallel to the main crystal axis. They occur as single, embedded crystals or in granular aggregates.

Geological environment

## Genesis Dissakisite-(Ce) is a mineral typical of metamorphic rocks that have undergone transformation under high temperature and pressure conditions, corresponding to the amphibolite and granulite facies. It forms in skarns, gneisses, marbles, and amphibolites, often as a result of metasomatism – a process of chemical exchange of components between the rock and hot solutions. ## Mineral Associations This mineral co-occurs with other minerals typical of its formation environment. It is most often accompanied by: diopside, scapolite, phlogopite, titanite, apatite, calcite, dolomite, quartz, and other minerals from the epidote group. ## Localities The most important, confirmed occurrences of dissakisite-(Ce) are: - Balmat, St. Lawrence County, New York, USA - type locality. - Various localities in the Val d'Ossola region, Piedmont, Italy. - Jakobsberg mining district, Värmland, Sweden. - Localities in the Alps, on both the Italian and Swiss sides.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of dissakisite-(Ce) primarily depends on the quality and development of the crystals. The most valued specimens are those with sharply terminated, lustrous crystals that are clearly distinct from the surrounding rock (matrix). The color contrast between dark dissakisite and lighter host rock (e.g., white marble) significantly enhances the visual value of the specimen. Crystal size is also important, as most are very small – specimens with crystals exceeding a centimeter are exceptionally rare and sought after. ## Popular Localities The best localities for this mineral are considered to be in Italy, especially in the Val d'Ossola valley, from where well-formed crystals on light matrices originate. Specimens from the type locality in Balmat (USA) are mainly of historical and scientific significance.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If washing is necessary, distilled water is recommended, followed by gentle drying. Ultrasonic cleaners should be avoided, as they can damage brittle crystals. ## What to Avoid Avoid contact with strong acids and chemicals that may react with the mineral. It should not be heated or exposed to sudden temperature changes. Despite its weak radioactivity, it is recommended to store it away from radiation-sensitive materials and limit direct, prolonged contact. ## Storage It is best to store dissakisite-(Ce) in a separate, sealed display box to prevent abrasion from harder minerals. Specimens in host rock are more resistant but still require careful handling. The label should include information about its weak radioactivity.

External references

Sources

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