Abstract:
The new mineral has been found in hydrothermal assemblage in the Hilairitovoye hyperagpaitic pegmatite at Kirovsky apatite mine, Kukisvumchorr Mt., Khibiny alkaline massif, Kola Peninsula, Russia. It is associated with microcline, sodalite, cancrisilite, aegirine, calcite, natrolite, fiuorite, narsarsukite, labuntsovite-Mn, man-gan-neptunite, donnayite, etc. It occurs as coarse rhomb-like lammellar and tabular crystals up to 0.1 x0.2x0.4 mm in size, combined in worm-like and fan-like aggregates up to 1 mm. The mineral is transparent, dark-orange to bright-orange, streak yellowish, luster vitreous. Cleavage (001) perfect mica-like, fracture laminated; scales are flexible but not elastic. Mohs' hardness 3-3.5. £>(meas.) 2.60, Z>(calc.) 2.65 g/cm3. Optically biaxial (-), a = 1.534, p = 1.562, у = 1.563, 2K(meas.) = 10°. Pleochroism is strong: X- yellowish to colorless, Y- brown, Z- deep brown. Orientation: X=c. Chemical composition (electron probe, H20 by Penfield method), wt.%: Na20 4.55, K20 10.16, CaO0.ll <http:>, MgO0.18, MnO 24.88, FeO0.68, ZnO0.15, A1203 0.20, Si02 50.87, ТЮ2 0.17, F 0.23, H20 7.73, -О = F2 -0.10, total 99.81. The empirical formula calculated on (Si, Al)12(0,OH,F)36 is: K3.o4(Na2.o7Cao.o3)i2.io(Mn4.95Feo.i3Mgo.o6Tio.o3Zn0.o3)i5.2o(Siii.94Alo.o6)Ei2027.57(OH)g.26F0.i7-1.92H20. The simplified formula (Z= 2) is: K3Na2Mn5Sii2(0,OH)36-2H20. Monoclinic, P2,/m or P2X. The unit cell dimensions are: a = 12.55, b = 5.721, с = 26.86 A, P = 114.04°, V= 1761_A3. The strongest reflections of the X-ray powder diagram (d,k-ЦЩ) are:. 12.28-100[002L 4.31-81[114], 3.555-62[301^ 212^3.063- 52[008, 316];.2.840-9_0[312^ 021, 309], 2.634-88[219, 1.0.10,. 124L 2.366-76[226, 3.1.10, 323], 2.1_09-54[42-33, 42-44, 519, 414], 1.669-64[2.2.13, 3.2ЛЗ, 623, 6.1.13], 1.614-56[5.0.16, 137, 333, 711]. The IR spectrum is given. Middendorfite is phyllosilicate related to bannisterite, parsettensite and members of the ganophyllite group and the stilpnomelane group. It is named in the memory of A. F. von Middendorf (1815- 1894), an outstanding German-Russian scientist-naturalist who made first mineralogical studies on the Khibiny massif. The type specimen is deposited in Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow.