Cyrilovite

Cyrilovite (NaFe₃³⁺(PO₄)₂(OH)₄•2(H₂O)) is a hydrous sodium iron phosphate mineral. It is isomorphous and isostructural with wardite, the sodium aluminium counterpart.[4]

Cyrilovite is found in granitic pegmatites.[5] It was first discovered in 1953 in a pegmatite at Cyrilov, near Velké Meźiřiči, West Moravia, Czech Republic.[2]

Composition

The chemical formula of cyrilovite is NaFe₃³⁺(PO₄)₂(OH)₄•2(H₂O)).[1] Parent phosphate minerals, fluorapatite and triplite-zwieselite, were transformed by hydrothermal alteration and weathering to give a complex, microcrystalline intergrowth of secondary phosphate minerals that include cyrilovite.[6] The sequence of phosphate transformations ended with the formation of cyrilovite within the fluorapatite fractures and the replacement of fluorapatite by lipscombite and crandallite-group minerals.[6] Fransolet suggest that a part of the leached Na leads to the precipitation of cyrilovite, in the fissures cause by the volume decrease resulting from the transformation of typhylite to heteresoite.[5] Mobilization of alkalis and of relatively immobile elements including aluminium and rare earth elements are subsequently incorporated into precipitating cyrilovite, lipscombite and crandallite-group minerals.[6]

The chemical analysis shows substitution not only of Al for Fe, but also of K and Mn for Na, measured and observed specific gravities are considered to be in good agreement.[4] Ferric iron occurs virtually alone in H₂0 rich minerals such as phosphosiderite, and coupled with Na, K, or Ca in cyrilovite.[5] It is soluble in hot dilute HCl, in hot dilute H₂SO₄, and, with difficulty, in hot dilute HNO₃.[4] In the closed tube, it gives off water and fuses.[4]

Geologic occurrence

The mineral wardite is capable of crystallizing in a similar form to that of cyrilovite because of their closely related chemical compositions. Between wardite’s composition, NaAl₃(PO₄)₂(OH)₄•2(H₂O), and cyrilovite’s composition, NaFe₃(PO₄)₂(OH)₄•2(H₂O), they’re able to form end members of a series of solid solutions. Either of the two minerals can occur in various proportions in a series of solid solutions in the wardite mineral group. Cyrilovite is a rare accessory mineral in some oxidizing phosphate-bearing granite pegmatitles and iron deposits. The sequence of phosphate transformations ended with the formation of cyrilovite within the F-apatite factures and the replacement of F-apatite by lipscombite and crandillite-group minerals.[6] Weathering related cyrilovite, lipscombite, and crandillite-group minerals were formed by percolating meteoric waters under increasing oxygen fugacity.[6]

Structure

The crystal structures of natural wardite and of the isomorphous cyrilovite have the space group P41212, Z=4). Hydrogen atoms were not located, but reasonable positions can be estimated. The cell dimentsions of cyrilovite are: c=19.4, a=7.32 Å.[4]

The individual crystals are usually smaller than 0.1 mm and many of them are intergrown. Crystals are squat and when single tend to lie on the basal pinacoid. The pinacoid {001} and the dipyramid {113} are the dominant forms; all the faces of thse forms tend to be present and equally well developed. The dipyramid {012} is not always present. The direction for the a-axis is at 45° angle to the smallest primitive unit cell.[4]

Physical properties

Cyrilovite is a vitreous translucent mineral that can appear in colors ranging from a bright yellow, honey-yellow, orange to brownish yellow, or brown and it has a hardness of 4. It has a yellow streak. The mineral is classified under the space group P41212 and is tetragonal.[1]

References

1. ^ a b c http://rruff.geo.arizona.edu/doclib/hom/cyrilovite.pdf Handbook of Mineralogy
2. ^ a b http://www.mindat.org/min-1206.html Mindat.org
3. ^ http://webmineral.com/data/Cyrilovite.shtml Webmineral data
4. ^ a b c d e f Lindberg, M.L. (1957) Relationship of the minerals avelinoite, cyrilovite, and wardite. American Mineralogist, 42, 204-213
5. ^ a b c Fransolet, A.M., Cooper, M.A., Cerny, P., Hawthorne, C., Chapman, R. (2000) The tanco pegmatite at Bernic Lake, Southeastern Manitoba. The Canadian Mineralogist, 38, 893-898
6. ^ a b c d e Lottermoser, B., Lu, J. (1997) Petrogenesis of rare-element pegmatites in the Olary Block, South Australia .1. Mineralogy and chemical evolution. Mineralogy & Petrology, 59, 1-19

Breitinger, D.K., et al. (2004) Combined vibrational spectra of natural wardite. Journal of Molecular Structure, 706, 95-99.

Cooper, M., Hawthorne, F.C., Cerny, P. (2000) Refinement of the crystal structure of cyrilovite from cyrilov, western Moravia, Czech Republic. Journal of Czech Geological Society, 45, 95-100.

List of minerals