Metarauchite

Metarauchite is a member of the autunite group, found at the Jáchymov ore district (type locality), Czech Republic and in Schneeberg, Germany. The autunite group is a group of structured uranyl phosphates and arsenates; the other members of the group are autunite, bassetite,^[2] heinrichite, kahlerite, nováčekite-I, nováčekite-II, rauchite,^[3] sabugalite,^[4] saléeite, torbernite, uranocircite, uranospinite, and zeunerite. The mineral is named after Czech mineral collector Luděk Rauch, who died in the Jáchymov mines during mineral prospecting.^[5]

Metarauchite
Metarauchite
General
Category	Uranyl Arsenates
Formula	Ni(UO2)2(AsO4)2·8H2O
IMA symbol	Mrau[1]
Strunz classification	08.EB.05
Dana classification	40.02a.17.02
Crystal system	Triclinic
Crystal class	Pinacoidal H-M symbol: 1
Space group	P1
Unit cell	a = 7.194(4) b = 9.713(5) c = 13.201(9) α = 75.79(5)° β = 83.92(3)° γ = 81.59(4)°
Identification
Color	Yellow to light greenish yellow
Crystal habit	Tabular crystals
Twinning	Multiply twinned along {011}
Cleavage	Perfect on (011)
Fracture	Uneven
Tenacity	Very brittle
Mohs scale hardness	2
Luster	Vitreous – pearly
Streak	Light green to pale yellow
Diaphaneity	Transparent to translucent
Density	3.81 gm/cc
Optical properties	Biaxial (−)
Birefringence	δ = 0.024
Pleochroism	Non-pleochroic
2V angle	Measured: 23–52°
Ultraviolet fluorescence	None
Solubility	Cold 10% HCl

Occurrence

Metarauchite occurs as a secondary mineral in strongly oxidized polymetallic vein material. Metarauchite is found primarily in two localities, the northern part of the Jáchymov ore district, northwestern Bohemia, Czech Republic and the Heber mine, Neustädtel, Germany. It is commonly associated with metazeunerite, erythrite, and gypsum (Jáchymov) and Ni-bearing metanováčekite,^[6] metazeunerite, pharmacosiderite (Schneeberg). Both localities have a surface containing strongly altered aggregates of primary ore with relics of uraninite, arsenopyrite and nickelskutterudite and traces of native bismuth at the Schweitzer vein in the Eduard mine.^[5]

Physical properties

Metarauchite exhibits yellow to light greenish-yellow crystals, up to 0.8 mm in size, these crystals are transparent to translucent and display vitreous to pearly luster. It exhibits a hardness of 2 on the Moh's Hardness Scale.^[7] Displays thick tabular crystals, with a prevalent pinacoid (011). Metarauchite is very brittle and shows perfect cleavage along the {011} plane.^[8][5] Metarauchite does not exhibit fluorescence either in short- and long-wavelength ultraviolet radiation. The density of metarauchite is not easily measured because of the relatively small size of most of its crystals; hence only the calculated density from the unit cell parameters and empirical formula was obtained, giving the value 3.81 g/cm³ .^[5]

Optical properties

Metarachite is biaxial negative, it exhibits its 2V is calculated at 52°.^[7][8] Metarauchite is non-pleochroic and displays no observable dispersion and a Gladstone-Dale index^[9] of 0.03.^[8]

Chemical properties

Metarauchite is a Ni- and As-dominant member of the group of hydrated uranyl arsenates and phosphates with the autunite-type sheet containing divalent cations in their inter-layer. It is only isostructural with synthetic metakirchheimerite. Metarauchite displays prominent substitution for Mg with Co, but magnesium is more commonly substituted with Ni in species found at the Schneeberg locality. The empirical formula for metarauchite is (Ni0_0.82Co_0.12Mg_0.02)_Σ0.96(UO₂)_2.01[(AsO₄)_1.88(PO₄)_0.03(SiO₄)_0.02]_Σ1.93•8.21H₂O^[5] (on the basis of 20 O,OH atoms). Simplified, the formula is Ni(UO₂)₂(AsO₂)₂•8H₂O.^[10]

Chemical composition (Jáchymov)

Oxide	wt%	Range
NiO	6.05	5.58–7.10
CoO	0.91	0.7–1.13
MgO	0.09	0.00–0.24
UO3	56.72	55.65–60.97
As2O5	21.31	19.87–23.22
P2O5	0.22	17.14–17.69
SiO2	0.09	3.43–5.18
H2O	14.61	1.37–1.43
Total	100%	-

Chemical composition (Schneeberg)

Oxide	wt%	Range
NiO	5.48	4.81–6.86
CoO	0.81	0.57–1.09
ZnO	0.14	0.00–0.42
MgO	0.50	0.17–0.88
UO3	58.42	59.64–64.49
As2O5	18.34	14.03–23.61
P2O5	0.22	0.64–3.83
SiO2	0.09	0.00–0.42
H2O	14.19	-
Total	100%

Crystallography

Metarauchite is in the triclinic crystal system, with a space group of P1 and is classed as a pinacoidal. The cell parameters are a = 7.194(2) Å, b = 9.759(5) Å, c = 13.231(7) Å, α = 75.53(3)°, β = 84.01(3)°, γ = 81.59(3)°, which shows an a:b:c ratio of 0.737:1:1.356. The unit cell volume is 887.53 ų (Calculated from Unit Cell). Metarauchite crystals show characteristic multiply twinned along {011}.^[8]

Powder X-ray diffraction

d-spacing intensity^[8]

8.625	(100)
5.078	(25)
5.044	(29)
4.277	(29)
3.568	(50)
3.492	(28)
3.424	(26)
2.990	(21)

See also

List of minerals