Arseniosiderite is a rare arsenate mineral formed by the oxidation of other arsenic-containing minerals, such as scorodite or arsenopyrite. It occurs in association with beudantite, carminite, dussertite, pharmacolite, pitticite, adamite and erythrite. The name arseniosiderite reflects two major elements of the mineral, arsenic and iron (Greek sideros means iron).^[4]

Arseniosiderite
Arseniosiderite, size: 6.4×6.2×5.6 mm
General
Category	Arsenate mineral
Formula (repeating unit)	Ca2Fe3+3(AsO4)3O2·3H2O
IMA symbol	Assd[1]
Strunz classification	8.DH.30 08
Dana classification	42.09.02.03
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	A2/a
Unit cell	a = 17.76, b = 19.53 c = 11.3 [Å], Z = 12
Identification
Formula mass	766.50 g/mol
Color	Bronze brown; yellow to black
Crystal habit	Fibrous
Cleavage	{100} good
Mohs scale hardness	4.5
Luster	Submetallic
Streak	Ochraceous
Diaphaneity	Translucent to opaque
Specific gravity	3.5–3.9, average = 3.7
Optical properties	Biaxial (−)
Refractive index	nα = 1.815, nβ = 1.898, nγ = 1.898
Birefringence	δ = 0.083
Other characteristics	Non-fluorescent
References	[2][3][4]