Bettertonite

Bettertonite
General
Category	Arsenate
Formula; (repeating unit)	[Al6(AsO4)3(OH)9(H2O)5]・11H2O
Crystal system	Monoclinic
Crystal class	Prismatic
Space group	2/m
Unit cell	a = 7.773(2) Å; b = 26.991(5) Å; c = 15.867(3) Å; β = 94.22(3)°
Identification
Formula mass	77.47 gm
Color	White
Crystal habit	Ultrathin rectangular laths (< 20 μm)
Twinning	Not observed
Cleavage	Perfect cleavage on {010}
Fracture	Not observed
Tenacity	Not observed
Mohs scale hardness	Not observed
Luster	Vitreous, Silky, Pearly
Streak	White
Diaphaneity	Translucent
Specific gravity	2.02 g/cm3 (calculated)
Optical properties	Biaxial positive α = 1.511 (1), β = 1.517 (1), γ = 1.523 (1)
Refractive index	Not observed
Birefringence	δ = 0.012
Pleochroism	Non-pleochroic
Group	Arsenates
References	[1][2]

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]

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 Al_5.86Fe_0.26(AsO₄)_2.65(SO₄)_0.23(OH)_9.82Cl_0.13(H₂O)_15.5.[1][2]

X-Ray Powder Diffraction Data

D-Spacing	Intensity
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]

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References

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.
Hudson Institute of Mineralogy, 2019, Bettertonite: https://www.mindat.org/min-46516.html (accessed December 2019)

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[Grey-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.

[mindat-2] Hudson Institute of Mineralogy, 2019, Bettertonite: https://www.mindat.org/min-46516.html (accessed December 2019)