Bartelkeite

Bartelkeite is an exceptionally rare mineral, one of scarce natural germanium compounds.^[4] The formula was originally assumed to be PbFeGe₃O₈,^[5] bartelkeite was later shown to be isostructural with a high-pressure form of the mineral lawsonite. Thus, its correct formula is PbFeGe(Ge₂O₇)(OH)₂•H₂O.^[2] Bartelkeite and mathewrogersite are minerals with essential (dominant) lead, iron and germanium.^[6] Both come from Tsumeb, Namibia - a world's "capital" of germanium minerals.^[7]

Bartelkeite
General
Category	Germanate mineral
Formula	PbFeGe(Ge2O7)(OH)2•H2O
IMA symbol	Btk[1]
Strunz classification	9.J0.10 (10 ed) 4/C.08-60 (8 ed)
Dana classification	7.6.2.1
Crystal system	Monoclinic
Crystal class	Spheroidal (2) or prismatic (2/m)
Space group	P21 or P21/m
Unit cell	a = 5.83, b = 13.62, c = 6.31 [Å], β = 127.31° (approximated); Z = 2
Identification
Color	Colorless, white, very pale greenish
Crystal habit	tabular; acicular
Cleavage	{101}, distinct
Mohs scale hardness	4
Luster	Subadamantine
Streak	White
Diaphaneity	Transparent
Density	4.97
Optical properties	Biaxal (-)
Refractive index	nα=1.89, nβ=1.91, nγ=1.91 (approximated)
2V angle	ca. 35o (measured)
References	[2][3][4]

Occurrence and association

Bartelkeite was detected in voids of germanium ore occurring within dolomites. The mineral associates with galena, germanite, reniérite, and tennantite.^[3]

Crystal structure

Bartelkeite is the first analyzed mineral containing both tetrahedrally- and octahedrally-coordinated germanium. It is isostructural with high-pressure form of the silicate lawsonite. In the structure there are:^[2]

• chains of FeO₆ and GeO₆ octahedra, that share edges
• Ge₂O₇ dimers that cross-link the chains
• Pb atoms and water molecules in large cavities of the framework