Original paper

Florkeite, K3 Ca2 Na[Al8 Si8 O32]12H2 O, a new phillipsite-type zeolite from the Bellerberg, East Eifel volcanic area, Germany

Lengauer, Christian L.; Kolitsch, Uwe; Tillmanns, Ekkehart

Abstract

Flörkeite is a new phillipsite-type zeolite. It was found in a Ca-rich xenolith from a quarry at the Bellerberg volcano near Ettringen, East Eifel volcanic area, Germany. The paragenesis includes tobermorite, thaumasite, willhendersonite, gismondine and ellestadite. It forms subparallel, short prismatic, pseudo-monoclinic crystals elongated along [100] and flattened on {010}, with a maximum length of 0.6 mm and a thickness of up to 0.1 mm. Additional forms are {100}, {001} and {201;}. Flörkeite is colourless, transparent, non-fluorescent and has a vitreous lustre and a white streak. It is brittle with irregular fracture, the calculated density is 2.266 gcm-3. Optically, it is biaxial negative with nα = 1.506(2), nβ = 1.514(2), nγ = 1.518(2), 2V = 71.0(5)° at 589.3 nm and 297 K. No pleochroism is observed. Orientation of the indicatrix is X ∧ c = 43°, Y ∧ b = 40° and Z ∧ a = 8°. Electron microprobe analyses yielded (wt%): Na2 O 2.32, K2 O 10.29, MgO 0.04, CaO 8.43, Al2 O3 30.20, SiO2 35.29, sum 86.57. A H2 O content of 15.57 wt% was derived from the singlecrystal structure refinement. The empirical formula calculated on the basis of Al + Si = 16 pfu is K2.96 Ca2.04 Na1.02 Mg0.01 [Al8.03 Si7.97 O31.97]· 11.72H2 O and the ideal formula can be given as K3 Ca2 Na[Al8 Si8 O32]·12H2O. Flörkeite is triclinic, space-group P1 (no. 2), the unit-cell parameters in phillipsite-type setting are a = 19.965(1), b = 14.274(1), c = 8.704(1) Å, α = 88.37(1), β = 125.08(1), γ = 89.57(1)°, V = 2028.2(3)Å 3, Z = 2 and the a : b : c ratio is 1.399 : 1 : 0.610. The five strongest lines in the powder X-ray diffraction pattern are (d in Å/Iobs): 3.235 / 100, 3.162 / 80, 3.135 / 80, 2.736 / 60, 4.956 / 45. The single-crystal structure refinement (R 1 = 4.39 %) confirmed that the structure of flörkeite is the triclinically distorted analogue of phillipsite with a complete (Al, Si) ordering at the T -sites of the framework (mean T (Al)-O: 1.731-1.743 Å and T (Si)-O: 1.615-1.631 Å), which reduces the topochemical symmetry to B2/b. The K+ cations are placed near the centre of 8-rings of the oto units, the Ca2+ and Na+ cations are located within the phi units. The doubling of the a -parameter and the reduction of the real symmetry to P1 (is caused by an ordering of the extra-framework cations and H2 O molecules within the 8-ring channel system along [100], basically Ca- -Ca, Na-Ca-Na and H2 O-K-H2 O. The (Al, Si) ordering is attributed to low-thermal genetic conditions within the Ca-rich xenolithic host, also indicated by the paragenesis with willhendersonite. Holotype material of flörkeite has been deposited at the mineral collection of the Naturhistorisches Museum Wien, Austria. The mineral is named in honour of the German mineralogist and crystallographer Otto Wilhelm Flörke (born 1926).

Keywords

flörkeitephillipsite-(k)new mineralzeolitebellerbergeifeltripotassium dicalcium sodium tecto-octoalumooctosilicate dodecahydrate(alsi) ordering