Bavenite
Colourless bavenite with some minor fluorite
General
Category	Mineral
Formula (repeating unit)	Ca4Be2Al2Si9O26(OH)2
IMA symbol	Bvn
Strunz classification	9.DF.25
Dana classification	70.5.3.1
Crystal system	Orthorhombic
Crystal class	Pyramidal H-M Symbol: mm2
Space group	Am2a
Unit cell	2,250.52
Identification
Color	White, Green, Pink, Brown
Cleavage	Perfect on {001} Fair on {100}
Fracture	Uneven
Mohs scale hardness	5.5
Luster	Vitreous - Pearly
Streak	White
Diaphaneity	Transparent to translucent
Density	2.7
Optical properties	Biaxial (+)
Refractive index	nα = 1.578 - 1.586 nβ = 1.579 - 1.586 nγ = 1.583 - 1.593
Birefringence	0.005 - 0.007
2V angle	22° - 60°
Dispersion	Moderate r < v
Ultraviolet fluorescence	None
Solubility	Unsoluble in acids

Bavenite is a calcium beryllium aluminosilicate, and it is a part of the Bavenite-Bohseite series. Its name originates from its type locality, which is Baveno, Italy. This mineral is approved by the IMA, and got grandfathered, meaning it is still believed to refer to a valid mineral species.^[1] It was discovered in 1901 in a pink granite mined in Lago Maggiore. When bavenite was discovered, it was considered as a member of the zeolite series. Later it was removed from the series as unlike zeolites, bavenite loses the water stored in its crystal lattice in a way higher temperature, between 210 and 320 °C. It is a cheap mineral considering its rarity.^[2]

Properties

Bavenite mainly consists of oxygen (48.11%), silicon (27.75%) and calcium (17.65%), and otherwise contains aluminum (3.48%), beryllium (2.81%) and hydrogen (0.22%). This mineral is weakly piezoelectric, and is not radioactive. It is a lamellar mineral, meaning it grows in layers that part into thin sheets. However, it is massive, meaning it is shapeless, so singular crystals can't be distinguished. It can also appear in radial aggregates, meaning it has a center from which crystals radiate without producing a stellar form. Crystals that grow in radial aggregates are separated and have different lengths generally.^[3] In this case, crystal needles can grow from 1 mm to a few cms in size. Bavenite can be determined only with the help of infrared or x-ray. It is white usually, but other color variants can occur as well.^[2] Due to the variation of the chemical composition of bavenite, different substitution mechanisms had been brought up. However, Canille managed to solve its structure thanks to Berry, who suggested direct Be ⇌ Al with electroneutrality. The T(3) site is occupied by Si and Be, and Be and Al were assigned to the T(2) and T(4) tetrahedra. Cannillo also proposed additional minor Be to replace Al at T(4).^[4]

Environment

Bavenite is usually associated with beryl, phenakite, bertrandite, quartz, epidote, stibnite, albite, orthoclase, titanite, clinozoisite and tremolite. Although it was discovered in igneous rocks, later it was found in pegmatites as well. Bavenite can also occur in the Alps, where it was created as a product of the hydrothermal weathering of beryllyium-bearing minerals (mostly beryl) through metamorphism. In the latter case, the mineral occurs with zeolite and prehnite.^[2] It can also form by low-temperature alteration of beryl and beryllium-bearing minerals, and generally forms in more alkali environments.^[5] It can occur on quartz and feldspar in miarolitic pegmatites as a coating. It also occurs as a pseudomorph of beryl. There's a known case of it occurring from pneumatolytically altered syenites, where it grew into singular crystals on albite, up to 1.5 cms of size per crystal. The mineral was not pseudomorphic in this case.^[4]

References