Middendorfite, a new mineral species, has been found in a hydrothermal assemblage in Hilairite hyperperalkaline pegmatite
at the Kirovsky Mine, Mount Kukisvumchorr apatite deposit, Khibiny alkaline pluton, Kola Peninsula, Russia. Microcline, sodalite,
cancrisilite, aegirine, calcite, natrolite, fluorite, narsarsukite, labuntsovite-Mn, mangan-neptunite, and donnayite are associated
minerals. Middendorfite occurs as rhombshaped lamellar and tabular crystals up to 0.1 × 0.2 × 0.4 mm in size, which are combined
in worm-and fanlike segregations up to 1 mm in size. The color is dark to bright orange, with a yellowish streak and vitreous
luster. The mineral is transparent. The cleavage (001) is perfect, micalike; the fracture is scaly; flakes are flexible but
not elastic. The Mohs hardness is 3 to 3.5. Density is 2.60 g/cm
3 (meas.) and 2.65 g/cm
3 (calc.). Middendorfite is biaxial (−), α = 1.534, β = 1.562, and γ = 1.563; 2
V (meas.) = 10°. The mineral is pleochroic strongly from yellowish to colorless on
X through brown on
Y and to deep brown on
Z. Optical orientation:
X =
c. The chemical composition (electron microprobe, H
2O determined with Penfield method) is as follows (wt %): 4.55 Na
2O, 10.16 K
2O, 0.11 CaO, 0.18 MgO, 24.88 MnO, 0.68 FeO, 0.15 ZnO, 0.20 Al
2O
3, 50.87 SiO
2, 0.17 TiO
2, 0.23 F, 7.73 H
2O; −O=F
2−0.10, total is 99.81. The empirical formula calculated on the basis of (Si,Al)
12(O,OH,F)
36 is K
3.04(Na
2.07Ca
0.03)
Σ2.10(Mn
4.95Fe
0.13Mg
0.06Ti
0.03Zn
0.03)
Σ5.20(Si
11.94Al
0.06)
Σ12O
27.57(OH)
8.26F
0.17 · 1.92H
2O. The simplified formula is K
3Na
2Mn
5Si
12(O,OH)
36 · 2H
2O. Middenforite is monoclinic, space group:
P2
1/
m or
P2
1. The unit cell dimensions are
a = 12.55,
b = 5.721,
c = 26.86 Å; β = 114.04°,
V = 1761 Å
3,
Z = 2. The strongest lines in the X-ray powder pattern [
d, Å, (
I)(
hkl)] are: 12.28(100)(002), 4.31(81)(11
[`4]\overline 4
), 3.555(62)(301, 212), 3.063(52)(008, 31
[`6]\overline 6
), 2.840(90)(312, 021, 30
[`9]\overline 9
), 2.634(88)(21
[`9]\overline 9
, 1.0.
[`1]\overline 1
0, 12
[`4]\overline 4
), 2.366(76)(22
[`6]\overline 6
, 3.1.
[`1]\overline 1
0, 32
[`3]\overline 3
), 2.109(54)(42–33, 42–44, 51
[`9]\overline 9
, 414), 1.669(64)(2.2.
[`1]\overline 1
3, 3.2.
[`1]\overline 1
3, 62
[`3]\overline 3
, 6.1.
[`1]\overline 1
3), 1.614(56)(5.0.
[`1]\overline 1
6, 137, 333, 71
[`1]\overline 1
). The infrared spectrum is given. Middendorfite is a phyllosilicate related to bannisterite, parsenttensite, and the minerals
of the ganophyllite and stilpnomelane groups. The new mineral is named in memory of A.F. von Middendorff (1815–1894), an outstanding
scientist, who carried out the first mineralogical investigations in the Khibiny pluton. The type material of middenforite
has been deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow.
Original Russian Text © I.V. Pekov, N.V. Chukanov, V.T. Dubinchuk, A.E. Zadov, 2006, published in Zapiski Rossiiskogo Mineralogicheskogo
Obshchestva, 2006, No. 3, pp. 42–52.
Considered by the Commission on New Minerals and Mineral Names, Russian Mineralogical Society, June 22, 2005. Approved by
the Commission on New Minerals and Mineral Names, International Mineralogical Association, September 5, 2005 (proposal 2005-028).