Pabstite is 37.7% SiO2, 3.8% TiO2, 24.4% SnO2 and 33.2% BaO. However, Ti and Sn could vary from point to point by approximately ±0.5% TiO2 and ±1% SnO2. Pabstite is a tin bearing analog of benitoite. Although, and have similar charge and ionic size, it is uncommon to find them substituting each other.
Geologic occurrence
Pabstite commonly occurs as anhedral crystals and masses that vary in their color from colorless to white. They produce a pink tinge when they are freshly broken. Large amounts of pabstite were found in Santa Cruz as fracture filling and disseminated grains in recrystallized siliceous limestones. This is geologic evidence of contact metamorphism. In addition, pabstite can be found in Rush Creek in California when benitoite contains small amounts of tin. It is commonly occurs in rocks that contain calcite, quartz, tremolite, witherite, phlogopite, diopside, minor amounts of forsterite and taramellite. Pabstite can also be found associated with galena, cassiterite and sphalerite.
Structure
Pabstite is considered the tin analog of benitoite. It has a hexagonal crystal system with a P6*2C space group. Its dimensions are as follows, a= 6.7037, c= 9.824 Å3, C= 382.3 Å3, Z=2. Pabstite has the structure of the benitoite group of minerals. In the structure of pabstite, there are four oxygens surrounding the cations in a pseudo-tetrahedral arrangement. A three-membered cyclosilicate ring is formed by repeating the tetrahedron using space group symmetry. Quadrivalent cations connect the rings to form a three-dimensional framework. Since the silicate rings have a geometry that is identical in all directions, a solid rigid unit is formed in the structure. In a distorted hexagonal antiprism arrangement, Ba, which has a high symmetry, is bounded by 12 oxygens. Two different BaـــO distances are present. frameworks are connected by Ba cations which lead to a poor cleavage.
Physical properties
Finding pabstite in the field is hard and rare. When using a shortwave ultraviolet light, bluish white fluorescence is revealed from the specimens. This property is commonly used to identify pabstite. Pabstite is colorless to white. The diameter of pabstite grains is usually less than 2 mm and they contain minute fluid and solid inclusions. Its hardness is 6 on Mohs scale of mineral hardness. Its density is 4.03 g/cm3 and it is uniaxial. The interference colors of pabstite are anomalous blue-violet and golden yellow. The refractive indices of pabstite are ω = 1.685±0.002 and ε = 1.674±0.002 which result in low birefringence and absent dichroism.
