Chambersite

Chambersite is a manganese borate mineral with formula Mn₃B₇O₁₃Cl. It is a member of the borate mineral series^[5] that includes other minerals such as ericaite (Fe₃B₇O₁₃Cl) and boracite (Mg₃B₇O₁₃Cl).^[6] When chambersite was first discovered, it was the second chemical analogue of boracite to be found in nature.^[7] It was discovered as a mineral at Barber's Hill salt dome in Texas in 1957 and in 1971 at the Dongshuichang deposit in Jixian, Tianjin, China.^[8] Chambersite occurs associated with the evaporite minerals halite, anhydrite, and gypsum.^[7]

Chambersite
Chambersite from Barber's Hill Salt Dome, Mont Belvieu, Texas USA, the type locality. Size: 0.7 cm × 0.6 cm × 0.6 cm (0.28 in × 0.24 in × 0.24 in).
General
Category	Tektoborates
Formula	Mn3B7O13Cl
IMA symbol	Cbs[1]
Strunz classification	6.GA.05
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-M symbol)
Space group	Pca21
Identification
Color	Colorless to deep purple
Crystal habit	Pseudocubic crystals
Twinning	Rare as interpenetration twins on [111]
Cleavage	None
Fracture	Subconchoidal to uneven
Mohs scale hardness	7
Luster	Vitreous
Diaphaneity	Transparent to opaque
Specific gravity	3.49
Optical properties	Biaxial (+)
Refractive index	nα = 1.732 nβ = 1.737 nγ = 1.744
Birefringence	δ = 0.012
2V angle	Measured: 83°
Dispersion	r > v
Alters to	Color darkens when exposed to sunlight
References	[2][3][4]

Chemical and physical properties

When chambersite was first discovered in Barber's Hill, the dominant form was the positive tetrahedron, but single morphological twinned crystal was found with interpenetrating tetrahedrons with the twinning axis.^[7] The ore discovered in China had formed in high grades that are spindle and granular that form micritic aggregates because the boron present is very pure and in low grades that had formed spheroids that are radially oriented.^[8] When chambersite in China was compared it had the same chemical oxide ratio as the samples found in Barber's Hill.^[8]

Oxide	Chemical composition of chambersite in oxide percent[7]
B2O3	49.50
MnO	41.87
Cl	6.34
FeO	1.28
MgO	0.05
CaO	trace
Total %	99.04

Geologic occurrence

Chambersite minerals were first found in 1957 in brine returns from a gas storage well in the Barber's Hill salt dome. The age of the dome is not definitely known, but it is at least dates back to the Cretaceous and possibly upper Jurassic.^[7] Other chambersite deposits with this similar geologic occurrence can be found in the area of the Gulf Coast of Mexico.^[9] The Venice dome in Plaquemines Parish, Louisiana is a site that has a shallow piercement salt dome, LPG storage and brine production.^[9] Other chambersite deposits can be found in the Penobsquis deposit in Southern New Brunswick, Canada, which has high grade salt deposits as well as a significant borate mineral concentration, including chambersite.^[5] Other chambersite deposits can be found as ores in Ji County, Tianjin, China, an area that is known to have had much magnetic activity and submarine volcanism in a subtidal lagoon.^[8]

Special characteristics

Chambersite was named after the county where it was first discovered, Chambers County, Texas, US.^[7] Optical examination and x-ray powder diffraction indicated the mineral to be related to boracite but different from any other published description.^[7]

See also

• Earth sciences portal

• List of minerals