Fluellite
Chemical formula: Al<sub>2</sub>(PO<sub>4</sub>)F<sub>2</sub>(OH)·7H<sub>2</sub>O
Hydrated aluminum phosphate, forming small, colorless, and highly lustrous dipyramidal crystals.
Description
## Characteristics Fluellite is a hydrated aluminum phosphate, prized by collectors for its perfectly formed, small crystals. They most often take the form of acute, double pyramids (dipyramids) that are highly lustrous, resembling miniature diamonds or Herkimer quartz. It usually occurs as fine coatings and crystalline druses on the surface of other minerals or in small rock fissures. It is a brittle mineral. ## Physical Properties Fluellite crystals are relatively soft, with a Mohs hardness of 3. They are characterized by a vitreous luster and a low density of approximately 2.17 g/cm³. They are transparent to translucent. ## Colors and Varieties Colorless or white crystals are most common. Less frequently, fluellite takes on pale yellow, greenish, or bluish hues. No distinct color varieties or commercial names are recognized. ## History and Name The mineral was first described in 1824 by William Phillips. Its name comes from one of its main chemical components – fluorine (Latin: *fluere*). The type locality (first finding place) was identified as the former Stenna Gwyn tin mine in Cornwall (Great Britain). ## Uses Fluellite has no industrial application and is solely an object of interest for collectors.
Diagnostic features
## Identification The most important diagnostic feature of fluellite is the form of its crystals – sharp, lustrous, double pyramids (dipyramids). Its low hardness (about 3 on the Mohs scale), vitreous luster, and occurrence environment, i.e., oxidation zones of deposits rich in phosphorus and fluorine, are also important. ## Distinguishing from Similar Minerals Fluellite is sometimes confused with small quartz crystals (including "Herkimer diamonds"), from which it differs by its significantly lower hardness (quartz has a hardness of 7). From other secondary phosphate minerals, such as wavellite or variscite, it differs primarily in its characteristic crystal shape. Wavellite forms radial aggregates, while variscite most often forms massive green aggregates. ## Crystal Forms Fluellite crystals almost always form as simple, rhombic dipyramids. It most often forms small druses and crystalline coatings, less frequently single, isolated crystals grown on the host rock.
Geological environment
## Genesis Fluellite is a secondary mineral, forming in the oxidation (weathering) zones of ore deposits that contain minerals rich in phosphorus and fluorine. It forms as a result of surface water acting on rocks containing, for example, apatite and fluorite. It can also crystallize on mine dumps and in old mine workings as a product of post-mining activity. ## Mineral Associations It often co-occurs with other secondary phosphates, such as wavellite, variscite, strengite, phosphosiderite, crandallite, and cacoxenite. It is also accompanied by minerals from the surrounding rocks, e.g., quartz, kaolinite, gypsum, or limonite. ## Localities Important global localities include historical sites in Cornwall (Great Britain), from where the first described specimens originated. Beautiful crystals were found in the Kapunda iron mine in South Australia. It also occurs in Krásno (Czech Republic), in the Willard mine in Nevada (USA), and in many other places around the world, though rarely in large quantities.
Rarity
Rare
Collector aspects
## Quality Criteria The most highly valued specimens are those with sharp, well-formed, transparent, and highly lustrous crystals. Their size is important – although fluellite forms small crystals, larger specimens are sought after. The attractiveness of a specimen is enhanced by the aesthetic arrangement of crystals on the rock matrix and color-contrasting associations with other minerals, such as green wavellite. ## Popular Localities Specimens from Cornwall (Great Britain) are considered classic and most desirable. Rich crystalline druses from the Kapunda mine in Australia are also highly valued. Specimens from other localities, such as the Czech Republic or the USA, are also valuable additions to specialized collections.
Care and storage
## Cleaning Fluellite specimens should be cleaned very carefully, using a soft brush and distilled water. Due to its low hardness, it is susceptible to scratching. Ultrasonic cleaners should not be used, as they can cause cracking or complete destruction of the crystals. ## What to Avoid The mineral is sensitive to high temperatures, which can lead to its dehydration (loss of water from the crystal lattice) and dulling. Contact with strong acids and other chemicals should be avoided. Protect from impacts and falls due to its brittleness. ## Storage It is recommended to store specimens in closed collector boxes that protect them from dust, mechanical damage, and sudden changes in humidity. It should not be exposed to direct sunlight or near heat sources.