Mccrillisite
Chemical formula: NaCs(Be,Li)Zr<sup>4+</sup><sub>2</sub>(PO<sub>4</sub>)<sub>4</sub>·1-2H<sub>2</sub>O
An extremely rare cesium and zirconium phosphate, forming small, pyramidal crystals in granitic pegmatites.
Description
## Characteristics Mccrillisite is a very rare mineral from the phosphate group, chemically classified as a hydrated sodium, cesium, zirconium, beryllium, and lithium phosphate. It occurs as small, well-formed crystals, usually not exceeding a few millimeters. Most often, it forms steep, tetragonal bipyramids with sharp edges. It is colorless to pale pink and has a vitreous luster. ## Physical Properties Its Mohs hardness is approximately 4.5, making it a relatively soft and scratch-prone mineral. It is transparent, and its density, calculated based on chemical composition and structure, is 3.29 g/cm³. It exhibits indistinct cleavage and conchoidal fracture. ## Colors and Varieties This mineral is usually colorless, rarely taking on pale pink hues. No color varieties or commercial varieties have been identified. ## History and Name It was recognized as a new mineral species by the International Mineralogical Association (IMA) in 1990. Its name honors Paulen H. McCrillis (1930-2011), a renowned American mineral collector from Maine, who specialized in pegmatite minerals and discovered many rare species. Mccrillisite was first identified in material from the Dunton Gem Quarry in Newry, Maine, USA. ## Applications Due to its extreme rarity and small crystal sizes, mccrillisite has no industrial applications. It is solely an object of scientific interest and a prized acquisition for specialized collectors of systematic minerals and micromounts.
Diagnostic features
## Identification Identification of mccrillisite is based on its characteristic crystallographic form – sharp, tetragonal bipyramids. Important features also include vitreous luster, hardness of about 4.5, and color (colorless to pale pink). A key indicator is its occurrence environment – miarolitic cavities in cesium- and lithium-rich granitic pegmatites. Final confirmation of identity requires advanced chemical analyses (e.g., EDS/WDS) due to its similarity to other rare pegmatite minerals. ## Distinguishing from similar minerals It can be confused with other colorless minerals found in pegmatites, such as quartz, apatite, herderite, or danburite. It differs from quartz by its lower hardness and tetragonal crystal form (quartz is trigonal). Apatite crystallizes in the hexagonal system. Herderite has similar hardness but crystallizes in the monoclinic system. Key is to examine crystal form and verify chemical composition. ## Crystal forms It forms almost exclusively single, well-formed crystals in the shape of steep, tetragonal bipyramids {101}. Crystals are usually very small, rarely exceeding 1-2 mm, which means most specimens are micromounts.
Geological environment
## Genesis Mccrillisite is a hydrothermal mineral, crystallizing in the late stage of the evolution of complex granitic pegmatites, rich in rare elements such as lithium, cesium, and beryllium. It forms in miarolitic voids and cavities, where it crystallizes from residual hydrothermal solutions at relatively low temperatures. ## Mineral associations At the type locality (Dunton Gem Quarry), mccrillisite coexists with minerals such as albite, quartz, elbaite (tourmaline), eosphorite, manganapatite, childrenite, hydroxylherderite, and siderite. ## Localities This is an extremely rare mineral, known practically only from one place in the world – the Dunton Gem Quarry in Newry, Oxford County, Maine, USA. This is its type locality and the main source of the best specimens.
Rarity
Extremely rare
Collector aspects
## Quality criteria The most highly valued mccrillisite specimens are those with sharp, fully formed, transparent, and undamaged crystals. Due to their small size, perfection of form is crucial. Specimens where crystals are set on a contrasting matrix (e.g., on albite or quartz) are more desirable. Size is an important factor – any crystal over 2 mm is considered exceptionally large for this species. ## Popular localities The only source of collectible mccrillisite specimens is its type locality: the Dunton Gem Quarry in Maine, USA. Material from this location is extremely difficult to obtain and appears on the market very rarely.
Care and storage
## Cleaning Mccrillisite specimens should only be cleaned mechanically, using a soft brush to remove dust. If necessary, distilled water can be used. Ultrasonic cleaners should be avoided, as they can damage delicate crystals. ## What to avoid The mineral is relatively soft (4.5 on the Mohs scale), so it should be protected from contact with harder minerals to avoid scratches. As a phosphate, it may be sensitive to acids. Rapid temperature changes and strong chemicals should also be avoided. ## Storage It is recommended to store specimens in separate, padded collector's boxes (micromount type) to prevent mechanical damage and abrasion. It should be protected from dust and stored in stable humidity and temperature conditions.