Na Ca2 Si4 O10 F
Agrellite [Na Ca2 Si4 O10 F] was named after Stuart Agrell, English mineralogist 1. It is triclinic with space group P1 and Z = 4. Due to the presence of pseudo-C-center, the sodium polyhedra and the silicate tetrahedra occur in pairs, whose configurations are nearly identical. The crystal structure of agrellite consists of two different NaO8 polyhedra which are distorted cubes, two each of CaO5F octahedra and CaO6F2 polyhedra, and two different [Si8O20] double chains. These double chains are hollow tubes, formed by the polymerisation of two vlasovite-type [Si4O11] single chains, consisting of corner-sharing four-membered rings. The silicate tubes, whose diameter is defined by a basket-shaped six-membered ring, run parallel to the c-axis and are hexagonally close-packed in the (001) plane. The sodium atoms occurring in cavities cross-linking these tubes to form sodium silicate layers parallel to the (010) plane; these layers alternate with the calcium polyhedral layers along the b-axis to form a three-dimensional framework. The average Si-O bond length is 1.619 Å; the non-bridging Si-O bond lengths (average 1.579 Å) are significantly shorter than the bridging ones (average 1.632 Å). The average Na-O bond length within the NaO8 polyhedra is 2.667 Å. Within the CaO5F octahedra and CaO6F2 polyhedra the average Ca-O bond lengths are 2.374 and 2.595 Å, and the average Ca-F bond lengths are 2.197 and 2.430 Å 2.
Bands observed in the IR spectrum of agrellite are assigned to the SiO4 group. Hence Si-O-Si and O-SiO asymmetric vibrational mode vas produces bands at 1145, 1090, 1033, 1012, 967 and 860 cm-1. While vs O-SiO symmetric vibrational mode has only one band, found at 786 cm-1, vs SiOSi symmetric vibrational mode produces bands around 698, 677, 653 and 613 cm-1. The rest of the bands, observed at 538, 497, 485, 460, 436 and 420 cm-1, are assigned to the vs Si-O symmetric vibrational mode 3,4.
No Raman spectrum of agrellite has been published so far.
1. The Photo-Atlas of Minerals., The Gem & Mineral Council, Los Angeles County Museum of Natural History, 1998.
2. Ghose, S., & Wan, C. n. (1979). Agrellite, Na(Ca,RE)2Si4O10F: a layer structure with silicate tubes. Am. Mineral., 64(5-6), 563-572.
3. Gittins, J., Bown, M. G., & Sturman, D. (1976). Agrellite, a new rock-forming mineral in regionally metamorphosed agpaitic alkalic rocks. Can. Mineral., 14(Pt. 2), 120-126.
4. Konev, A. A., Vorob'ev, E. I., Sapozhnikov, A. N., Paradina, L. F., Lapides, I. L., & Ushchapovskaya, Z. F. (1987). Strontium-containing agrellite from the Murunskoe alkaline massif. Mineral. Zh., 9(3), 73-79.
SELECTED REFERENCES ON SPECTROSCOPY OF
AGRELLITE:
1. Gittins, J., Bown, M. G., & Sturman, D. (1976). Agrellite, a new rock-forming mineral in regionally metamorphosed agpaitic alkalic rocks. Can. Mineral., 14(Pt. 2), 120-126.
2. Konev, A. A., Vorob'ev, E. I., Sapozhnikov, A. N., Paradina, L. F., Lapides, I. L., & Ushchapovskaya, Z. F. (1987). Strontium-containing agrellite from the Murunskoe alkaline massif. Mineral. Zh., 9(3), 73-79.
Original
spectra shown for this mineral can be obtained on request from J.T. Kloprogge
(E-mail t.kloprogge@qut.edu.au), or
R.L. Frost (E-mail r.frost@qut.edu.au).
Postal
address:
Centre
for Instrumental and Developmental Chemistry, Queensland University of
Technology, GPO Box 2434, Brisbane, Qld 4001, Australia.
Fax +61
7 3864 1804
For more information see our general website at: http://www.sci.qut.edu.au/sci_schps.html
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