Pink-orange crystals of a composition within the ferri-obertiite
compositional space were found in vesicles in a pale beige silicate vein
found from a basalt quarry at Mount Rothenberg, Eifel district, Germany.
Associated minerals are potassic feldspar, alpha quartz paramorphic
afterbeta quartz, eifelite (the second occurrence after the Caspar quarry at
Bellerberg volcano, Eifel region), tridymite, rutile, roedderite and other
amphiboles. The ideal formula of ferri-obertiite is
ANaBNa2
C(Mg3Fe3+Ti)TSi8O22
WO2; the empirical formula derived for the holotype
specimen from Mount Rothenberg from the results of electron-microprobe
analysis and single-crystal structure refinement is
A(Na0.76K0.22)∑0.98
B(Na1.61Ca0.35Mn0.04
2+)∑2.00
C(Mg3.58Mn0.11
2+Fe0.62
3+Ti0.66
4+Cr0.01
3+Zn0.01Ni0.01)∑5.00
T(Si7.82Ti0.12
4+Al0.06)∑8.00O22
W[O1.26F0.55(OH)0.19]∑2.00.
The unit-cell dimensions are a = 9.7901(7),
b = 17.9354(13), c = 5.2892(4)Å, β=
104.142(2)°, V = 900.58 (11) Å3. The space group
is C2/m, Z = 2.
Ferri-obertiite is biaxial (+), with α = 1.664, β = 1.680, γ = 1.722, all
±0.002 and 2V (meas.) = 66.4(3)o, 2V (calc.) = 64.7o.The strongest eight
reflections in the powder X-ray pattern [d values (in Å),
I, (hkl)] are: 2.704, 100, (151);
3.116, 76, (310); 3.388, 72, (131); 8.931, 72, (110); 2.529, 67, (202);
2.583, 39, (061); 2.160, 38, (261); 3.260, 37, (240). Both the mineral and
thename have been approved by the Commission on New Minerals, Nomenclature
and Classification of the International Mineralogical Association (IMA
2015-079); the rock specimen has been deposited at the Museo di Mineralogia,
Dipartimento di Scienze della Terra e dell'Ambiente, Universitàdegli Studi
di Pavia, under the code 2015-02.