GJERDINGENITE-Fe FROM NORWAY, A NEW MINERAL SPECIES IN THE LABUNTSOVITE GROUP: DESCRIPTION, CRYSTAL STRUCTURE AND TWINNING

Abstract

Gjerdingenite-Fe occurs as yellow prisms or laths up to 1 mm long in miarolitic cavities of a sodic granite (“ekerite”) at Gjerdingselva, Lunner, Oppland, Oslo Region, Norway. The simplified formula is K 2 [(H 2 O) 2 (Fe,Mn)][(Nb,Ti) 4 (Si 4 O 12 ) 2 (OH,O) 4 ]·4H 2 O, i.e. , the mineral is an analogue of those with root name kuzmenkoite, which have Ti > Nb. The mineral is monoclinic, C 2/ m , with a 14.529(2), b 13.943(2), c 7.837(2) A, β 117.61(1)°, V 1406.8(9) A 3 , Z = 2. The crystal structure was refined to R = 0.054 on the basis of 1677 observed reflections; twinning on {001} simulates an orthorhombic F -centered lattice. The partitioning of cations and H 2 O molecules in the sites B , B ′, C (occupying channels of the zeolite-like structure) is discussed; for the refined crystal, the following crystal-chemical formula is established by comparison of chemical and structure-refinement data: {[K 1.20 Na 0.72 (H 2 O) 2.08 ] ∑4 [K 2.08 (H 2 O) 1.92 ] ∑4 }[(H 2 O) 3.40 K 0.56 Ca 0.04 ] ∑4 (Fe 0.95 Mn 0.75 □ 0.30 ) ∑2 (Nb 4.30 Ti 3.45 Fe 0.20 Zr 0.05 ) ∑8 (Si 15.90 Al 0.10 ) ∑16 O 48 [(OH) 4.16 O 3.84 ] ∑8 ·4H 2 O ( Z = 1). The strongest six reflections of the powder pattern [ d obs in A( I )( hkl )] are: 6.92(80)(020,001), 6.42(50)(200,201), 4.94(70)(021), 3.225(100)(421,400,402), 3.114(80)(041,022), and 2.512(50)(441,401,403). The mineral is optically biaxial (+) with α 1.6676(2), β 1.7001(4), γ 1.794(1); 2 V meas = 58.5(2.3)°, 2 V calc = 63.71(1)°. The Mohs hardness is about 5; D meas = 2.82(2), D calc = 2.830 g/cm 3 . The pseudo-orthorhombic crystals are flattened on {001} and elongate along [010]. Five crystal forms were observed: {100}, {010} (subordinate), {001}, {201}, and {021}.