Leucostaurite
Chemical formula: Pb<sup>2+</sup><sub>2</sub>B<sub>5</sub>O<sub>9</sub>Cl·0.5H<sub>2</sub>O
Leucostaurite is an extremely rare lead borate, forming colorless, microscopic crystals, often as cruciform twins.
Description
## Characteristics Leucostaurite is a very rare, hydrated chloro-borate of lead. It occurs as very small, tabular or prismatic crystals, rarely exceeding 0.3 mm in length. The most characteristic feature of the mineral is the formation of cruciform penetration twins, which gives them a unique, star-like appearance. Crystals are usually colorless to white and transparent. ## Physical properties The mineral is characterized by a vitreous luster, and a pearly luster on cleavage surfaces. It is transparent to translucent. Due to the small size of the crystals, its hardness has not been precisely determined. The density calculated based on the chemical composition and unit cell parameters is 4.61 g/cm³. ## Colors and varieties Leucostaurite is colorless or white. No colored varieties or trade names are known. ## History and name The mineral's name comes from the Greek words *leukos* (λευκός) meaning "white" and *stauros* (σταυρός) meaning "cross," which is a direct reference to its color and characteristic twin form. It was described as a new mineral species in 2002 by Stefan Graeser, Heinz Schwander, and Hidemichi Hori. ## Applications Due to its extreme rarity and microscopic size, leucostaurite has no practical application. It is solely an object of scientific interest and is valued by specialized mineral collectors (micromounters).
Diagnostic features
## Identification The most important diagnostic feature of leucostaurite is its characteristic cruciform penetration twins. In combination with its colorless or white color, vitreous luster, and occurrence in a specific fumarolic environment, these features allow for preliminary identification. However, final confirmation requires advanced analytical techniques, such as X-ray diffraction (XRD) or chemical microanalysis (EDS), due to the microscopic size of the crystals. ## Distinguishing from similar minerals Leucostaurite can be confused with other colorless minerals found in fumarolic environments, such as sassolite (H₃BO₃), with which it often co-occurs. However, sassolite forms pseudohexagonal, platy crystals with a pearly luster and does not exhibit cruciform twins. It is distinguished from other similar minerals by its unique crystal form and chemical composition (presence of lead and chlorine). ## Crystal forms Crystals are small, tabular or short-prismatic. The most typical and recognizable form is penetration twins, in which two crystals intergrow at an angle of approximately 90°, forming a characteristic cross shape.
Geological environment
## Genesis Leucostaurite is a fumarolic mineral. It forms by sublimation from hot volcanic gases at relatively high temperatures, estimated at 400-500°C. It crystallizes directly on the walls of fumarole channels in volcanic rocks, such as dacites. ## Mineral associations This mineral occurs in paragenesis with sassolite (boric acid in mineral form), which also crystallizes from volcanic exhalations. Other rare mineral phases formed under similar conditions may also accompany it. ## Localities The only confirmed locality of leucostaurite in the world is its type locality – the Toroku mine in Miyazaki Prefecture on Kyushu Island, Japan. It was found there in a fumarolic activity zone associated with a dacitic dome volcanic complex.
Rarity
Extremely rare
Collector aspects
## Quality criteria As a micromount mineral, leucostaurite is evaluated by different criteria than cabinet specimens. Samples with sharp, well-formed, and clearly visible cruciform twins are most highly valued. The transparency of the crystals and their aesthetic arrangement on the rock matrix are also important. Specimens where leucostaurite co-occurs with well-formed sassolite are additionally attractive. ## Popular localities The only source of leucostaurite specimens is its discovery site – the Toroku mine in Japan. All samples available worldwide come from this single locality, making them extremely difficult to acquire.
Care and storage
## Cleaning Leucostaurite specimens are extremely small and fragile. They should only be cleaned using a gentle stream of compressed air from a safe distance to remove dust. Any contact with water, chemical solutions, or tools (even soft brushes) risks irreversible damage or loss of crystals. ## What to avoid Avoid all vibrations, shocks, and impacts. The mineral is susceptible to mechanical damage. It should not be exposed to acids or other chemicals. Due to its lead content, avoid inhaling dust and wash hands after any contact. ## Storage Leucostaurite should be stored only in specialized, sealed micromount boxes, which protect it from dust, moisture, and mechanical damage. It should be kept in a stable environment, away from sources of vibration.