Fluoro-pargasite
Chemical formula: NaCa₂(Mg₄Al)(Si₆Al₂)O₂₂F₂
A black, almost opaque amphibole group mineral, the fluorine analog of pargasite, valued for its well-formed crystals.
Properties
- Mohs hardness
- 6
- Luster
- Vitreous
- Streak
- Grey to greenish grey
- Density
- 3.18
- Cleavage
- on {110}
- Fracture
- Conchoidal
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Fluoro-pargasite is most easily recognized by its black color, vitreous luster, and characteristic form of short, thick prismatic crystals with an almost square cross-section. A hardness of 6 on the Mohs scale and a gray to greenish-gray streak are also important diagnostic features. ## Distinguishing from Similar Minerals It can be confused with other black amphiboles, such as hornblende, or pyroxenes, e.g., augite. It is often distinguished from hornblende by its more regular, prismatic habit. Key to distinguishing it from augite is the examination of cleavage angles – amphiboles (including fluoro-pargasite) have cleavage at angles of approximately 56° and 124°, while pyroxenes exhibit cleavage at an angle close to 90°. Final differentiation from other pargasites requires advanced chemical analyses (EDS, WDS) to confirm the dominance of fluorine. ## Crystal Forms This mineral forms columnar, prismatic crystals, often with a short and thick appearance. The cross-section of the crystals is usually tetragonal or octagonal. It occurs as single, well-formed crystals embedded in the host rock (most often marble).
Geological environment
## Genesis Fluoro-pargasite is a metamorphic mineral. It forms under conditions of contact or high-grade regional metamorphism, in calcium- and magnesium-rich rocks, such as marbles (metamorphosed limestones and dolomites) and skarns. The presence of fluorine in the formation environment is crucial for its crystallization. ## Mineral Associations At its type locality in Edenville (New York, USA), it co-occurs with minerals such as calcite, diopside, chondrodite, spinel, and graphite. In other localities, it may also be associated with fluorite, scapolite, or vesuvianite. ## Localities The most important and historic occurrence, from which the best specimens originate, is its type locality: Edenville, Orange County, New York, USA. This mineral is also reported from several other places worldwide where suitable geological conditions prevailed, but specimens from these localities rarely match the quality of those from the USA.
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with sharp, well-formed crystals with smooth, lustrous faces. The absence of mechanical damage and a clear contrast with the light host rock (most often white calcite or marble) are important. Crystal size also affects value – larger, well-formed specimens are sought after. ## Popular Localities By far the most desirable and classic locality for this mineral is Edenville, New York, USA. Specimens from this location are considered exemplary for the species and fetch the highest prices in the collector's market.
Care and storage
## Cleaning Fluoro-pargasite specimens can be cleaned with a soft, dry brush to remove dust. If necessary, distilled water may be used. Ultrasonic cleaners should be avoided, as they can damage crystals along cleavage planes. ## What to Avoid The mineral is sensitive to strong acids, which can etch its surface. Avoid sudden temperature changes and prolonged exposure to direct sunlight, although the latter is not considered particularly harmful. ## Storage Fluoro-pargasite crystals are relatively hard but brittle. It is best to store them in individual display boxes to prevent chipping and scratching from other minerals. Specimens can be displayed in enclosed cabinets, protecting them from dust.