Bettertonite

Bettertonite is a mineral of the arsenate category, named after John Betterton. He was a museum geologist and mineralogist at Haslemere Educational Museum in Surrey, England. Bettertonite is a white arsenate mineral with a formula of [Al6(AsO4)3(OH)9(H2O)5]・11H2O. Bettertonite is in the monoclinic system and has a heteropolyhedral layered structure type. It is a natural forming polyoxometalate. Bettertonite forms in clusters of radiating rectangular laths. Laths are thin and usually < 20 μm laterally. Laths are flat on {010}. Bettertonite is similar to penberthycroftyite and it transforms into penberthycroftyite at low temperatures (67-97 ℃).[1]

Bettertonite
General
CategoryArsenate
Formula
(repeating unit)		[Al6(AsO4)3(OH)9(H2O)5]・11H2O
Crystal systemMonoclinic
Crystal classPrismatic
Space group2/m
Unit cella = 7.773(2) Å; b = 26.991(5) Å; c = 15.867(3) Å; β = 94.22(3)°
Identification
Formula mass77.47 gm
ColorWhite
Crystal habitUltrathin rectangular laths (< 20 μm)
TwinningNot observed
CleavagePerfect cleavage on {010}
FractureNot observed
TenacityNot observed
Mohs scale hardnessNot observed
LusterVitreous, Silky, Pearly
StreakWhite
DiaphaneityTranslucent
Specific gravity2.02 g/cm3 (calculated)
Optical propertiesBiaxial positive α = 1.511 (1), β = 1.517 (1), γ = 1.523 (1)
Refractive indexNot observed
Birefringenceδ = 0.012
PleochroismNon-pleochroic
GroupArsenates
References[1][2]

Physical Properties

Bettertonite is a white and translucent mineral. It has a vitreous and silky to pearly luster. It has a calculated density of 2.02 g/cm3. Bettertonite has perfect cleavage on {010}. Bettertonite’s hardness and fracture were not observed. Bettertonite is nonpleochroic and biaxially positive with α = 1.511 (1), β = 1.517 (1), γ = 1.523 (1).[1][2]

Occurrence

Bettertonite was discovered in St. Hilary, Cornwall, UK, at the Penberthy Croft Mine. The Penberthy Croft Mine is known for the rare secondary CuPbFe arsenates that have been found there. Bettertonite most likely formed as a product of leaching and the replacement of Al to Fe in pharmacosiderite. Bettertonite infills isolated cavities in quartz veins and is associated with arsenopyrite, chamosite, liskeardite, pharmacoalumite, and pharmacosiderite.[1]

Structure, WDS Data, and XRD Data

Bettertonite forms in undulating layers that are parallel to (010). These undulating layers are made up of hexagonal rings of AlO6 octahedra. The rings are connected by AsO4 tetrahedra. The undulating stacking creates water-filled channels along [100]. Wavelength-Dispersive X-Ray Spectroscopy (WDS) and Powder X-ray Diffraction (XRD) were used on Bettertonite to understand its chemical composition. WDS analyses, which included an average of 4 electron probes, yielded the following results when H2O was calculated on structural grounds and the analyses was normalized to 100%: Al2O3 = 29.5, Fe2O3 = 2.0, As2O5 = 30.1, SO3 = 1.8, Cl = 0.5, H2O = 36.2. XRD results can be found in a table located below. The empirical formula (based on 9 metal atoms) is Al5.86Fe0.26(AsO4)2.65(SO4)0.23(OH)9.82Cl0.13(H2O)15.5.[1][2]

X-Ray Powder Diffraction Data
D-SpacingIntensity
13.648 Å(100)
13.505 Å(50)
7.805 Å(50)
7.461 Å(30)
5.880 Å(20)
5.622 Å(12)
3.589 Å(20)
2.857 Å(14)

|[2]

See also

References
  1. Grey, I.E., Kampf, A.R., Price, J.R., Macrae, C.M. (2015) Bettertonite, [Al6(AsO4)3(OH)9(H2O)5]·11H2O, a new mineral from the Penberthy Croft mine, St. Hilary, Cornwall, UK, with a structure based on polyoxometalate clusters. Mineralogical Magazine. Mag., 79(7), 1849–1858.
  2. Hudson Institute of Mineralogy, 2019, Bettertonite: https://www.mindat.org/min-46516.html (accessed December 2019)
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