Dissakisite-(La)

Chemical formula: CaLaMgAl<sub>2</sub>(Si<sub>2</sub>O<sub>7</sub>)(SiO<sub>4</sub>)O(OH)

Dissakisite-(La) is a rare, brownish-black mineral from the epidote group, distinguished by the presence of lanthanum in its chemical composition.

## Characteristics Dissakisite-(La) is a mineral belonging to the epidote group. It forms prismatic, elongated crystals, which usually occur as granular or radial aggregates. Its color is dark, most often brownish-black to black. It is an opaque mineral with a vitreous or resinous luster. ## Physical Properties This mineral is characterized by a hardness of 6.5-7 on the Mohs scale. It has a relatively high density, approximately 3.79 g/cm³. It exhibits weak cleavage in one direction and an uneven fracture. ## History and Name The name "dissakisite" comes from the Greek word *dissakis* meaning "twice four," which refers to the fact that its crystals have eight faces in the prismatic zone. The suffix -(La) indicates the dominance of lanthanum as a rare earth element in its structure. The mineral was first described in 1991 by Donald R. Peacor, Pete J. Dunn, William B. Simmons, and Robert A. Fitz, based on material from Balmat, New York, USA.

Properties

Mohs hardness
6.5-7
Luster
Vitreous to resinous
Streak
Gray
Density
3.79
Cleavage
Weak on {001}
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Characteristic features of dissakisite-(La) include its dark, brownish-black color, prismatic crystal habit, high hardness (6.5-7), and occurrence in a specific geological environment - skarns. Vitreous to resinous luster is also helpful in identification. ## Distinguishing from Similar Minerals Dissakisite-(La) can be confused with other dark minerals from the epidote group, such as allanite, as well as tourmaline (schorl) or pyroxenes. Final differentiation from allanite and other rare earth minerals requires advanced chemical analyses (e.g., EDS) to confirm the dominance of lanthanum. It is often distinguished from tourmaline by its crystal habit and occurrence environment. ## Crystal Forms Crystals are usually prismatic, elongated along the b-axis. They often form granular, radial, or fibrous aggregates intergrown with other minerals, mainly calcite.

Geological environment

## Genesis Dissakisite-(La) is a metamorphic mineral. It forms under conditions of high-grade contact metamorphism, in magnesian skarns that developed at the contact of granitoid intrusions with dolomitic marbles. It is a product of reactions between magnesium-, calcium-, and aluminum-rich minerals in the presence of rare-earth-element-saturated fluids. ## Mineral Associations This mineral coexists with minerals such as: calcite, diopside, pargasite, phlogopite, spinel, anhydrite, pyrite, and other minerals from the epidote and humite groups. ## Localities The most important and classic localities for this mineral are Mine #4 in Balmat, St. Lawrence County, New York, USA (type locality) and the phosphorus and rare earth element deposit in Trimouns, near Luzenac, France.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp, and lustrous crystals, clearly contrasting with a light background (e.g., white calcite), are most valued by collectors. The size of the crystals is important – the larger, the more valuable. Specimens from the type locality in Balmat are particularly sought after due to their historical significance.

Care and storage

## Cleaning It is recommended to clean specimens only with a soft, dry brush to remove dust. The use of water, and especially ultrasonics, is not advisable due to potential internal fractures and the rarity of the mineral. ## What to Avoid Avoid contact with acids and other chemicals that may react with the mineral. Do not heat the specimen or expose it to rapid temperature changes. Due to its dark color, prolonged exposure to intense sunlight is not recommended, although there is no evidence of fading. ## Storage Dissakisite-(La) specimens should be stored in stable conditions, in separate display boxes, to avoid abrasion and mechanical damage. It is best to keep them away from dust and moisture.

Sources

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