ALBERT

Beschreibung: Kummerite, ideally Mn 2+ Fe 3+ Al(PO 4 ) 2 (OH) 2 ·8H 2 O, is a new secondary phosphate mineral belonging to the laueite group, from the Hagendorf-Süd pegmatite, Hagendorf, Oberpfalz, Bavaria, Germany. Kummerite occurs as sprays or rounded aggregates of very thin, typically deformed, amber yellow laths. Cleavage is good parallel to {010}. The mineral is associated closely with green Zn- and Al-bearing beraunite needles. Other associated minerals are jahnsite-(CaMnMn) and Al-bearing frondelite. The calculated density of kummerite is 2.34 g cm –3 . It is optically biaxial (–), α = 1.565(5), β = 1.600(5) and = 1.630(5), with weak dispersion. Pleochroism is weak, with amber yellow tones. Electron microprobe analyses (average of 13 grains) with H 2 O and FeO/Fe 2 O 3 calculated on structural grounds and normalized to 100%, gave Fe 2 O 3 17.2, FeO 4.8, MnO 5.4, MgO 2.2, ZnO 0.5, Al 2 O 3 9.8, P 2 O 5 27.6, H 2 O 32.5, total 100 wt.%. The empirical formula, based on 3 metal apfu is (Mn 2+ 0.37 Mg 0.27 Zn 0.03 Fe 2+ 0.33 ) 1.00 (Fe 3+ 1.06 Al 0.94 ) 2.00 PO 4 ) 1.91 (OH) 2.27 (H 2 O) 7.73 . Kummerite is triclinic, , with the unit-cell parameters of a = 5.316(1) Å, b = 10.620(3) Å , c = 7.118(1) Å, α = 107.33(3)°, β = 111.22(3)°, = 72.22(2)° and V = 348.4(2) Å 3 . The strongest lines in the powder X-ray diffraction pattern are [ d obs in Å(I) ( hkl )] 9.885 (100) (010); 6.476 (20) (001); 4.942 (30) (020); 3.988 (9) ( ); 3.116 (18) ( ); 2.873 (11) ( ). Kummerite is isostructural with laueite, but differs in having Al and Fe 3+ ordered into alternate octahedral sites in the 7.1 Å trans -connected octahedral chains.