The Crystal Structure of Raite from a 3x3x60 um3 Needle, Data from ID13 at ESRF. J.J. Pluth and J.V. Smith, Dept. of Geophysical Sciences, CARS and MRSEC, The University of Chicago, Chicago, IL 60637; D.Y. Pushcharovsky, Dept. of Geology, Moscow State University, Moscow, Russia 119899; A. Bram and C. Reikel, ESRF, BP 220, 38043, Grenoble, France; R.W. Broach, UOP Research Center, Des Plaines IL 60017
Beamline ID13 at ESRF was designed for micro-diffraction and micro-SAXS applications. For this experiment it consisted of an undulator source that was focused by an ellipsoidal mirror and monochromatized by a channel-cut Si(111) crystal to produce a 0.6883Å (18 keV) 30 um beam. A data set was collected for Raite using a kappa single crystal diffractometer and a CCD detector (110mm fluorescent screen lens coupled). The unit cell, space group and the intensities were determined using DENZO. The refined lattice constants of the monoclinic unit cell are: a 15.1(1); b 17.6(1); c 5.290(4)A; [beta] 100.5(2)[ring]; V = 1382.33Å3; space group C2/m. A total of 6599 intensities was obtained (2-theta max = 58[ring]) and merged to yield 1164 unique reflections (R =0.05). The software package SHELXTL was used to first solve the structure and then to refine the crystal structure. The structure, including all reflections, refined to a final R = 0.07.
Raite occurs in hyperalkaline rocks from the Kola peninsula, Russia. Considering both cationic and anionic components, its structural formulas can best be presented in the following form:
{Na2(Mn3+)3[Si8O20](OH)2} +{(Ti4+)0.25Na[(OH)0.3(H2O)0.7]10} -.
The structure consists of alternating layers of a hexagonal chicken wire pattern of 6-membered SiO4 rings. Tetrahedral apices of a chain of Si 6-rings, parallel to the c, alternate in pointing up and down. Two 6-ring Si layers are connected by edge sharing octahedral bands of Na and Mn3+ also parallel to c. The band consists of the alternation of finite Mn-Mn and Na-Mn-Na chains. As a consequence of the misfit between octahedral and tetrahedral elements, regions of the Si-O layers are arched and form one dimensional channels bounded by 12 Si tetrahedra and 2 Na octahedra. The channels along the short c-axis in raite are filled by isolated Na(OH,H2O)6 octahedra. The distorted octahedrally coordinated Ti4+ also resides in the channel and provide the weak linkage of these isolated Na octahedra and the mixed octahedral tetrahedral framework. Raite is structurally related to intersilite, palygorskite, sepiolite and amphibole.