Cyrilovite
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| Cyrilovite | |
|---|---|
 | |
| General | |
| Category | Phosphate mineral |
| Formula | NaFe3+3(PO4)2(OH)4·2(H2O) |
| IMA symbol | Cyr[1] |
| Strunz classification | 8.DL.10 |
| Dana classification | 42.07.08.01 |
| Crystal system | Tetragonal |
| Crystal class | Trapezohedral (422) H–M symbol: (422) |
| Space group | P41212 |
| Identification | |
| Color | Bright yellow, honey-yellow, orange to brownish yellow, brown |
| Crystal habit | Massive, granular, pseudo cubic, radiating to botryoidal aggregates and crusts |
| Cleavage | None |
| Fracture | Conchoidal |
| Mohs scale hardness | 4 |
| Luster | Vitreous |
| Streak | Yellow |
| Diaphaneity | Translucent |
| Specific gravity | 3.081–3.096 |
| Optical properties | Uniaxial (-) |
| Refractive index | nω = 1.802–1.805, nε = 1.769–1.775 |
| Birefringence | δ = 0.033 |
| Pleochroism | Weak |
| References | [2][3][4][5] |
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.[6]
Cyrilovite is found in granitic pegmatites.[7] It was first discovered in 1953 in a pegmatite at Cyrilov, near Velké Meźiřiči, West Moravia, Czech Republic.[3]
The chemical formula of cyrilovite is NaFe3+3(PO4)2(OH)4·2(H2O).[2] 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.[8] 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.[8] 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.[7] 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.[8]
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.[6] Ferric iron occurs virtually alone in H2O rich minerals such as phosphosiderite, and coupled with Na, K, or Ca in cyrilovite.[7] It is soluble in hot dilute HCl, in hot dilute H2SO4, and, with difficulty, in hot dilute HNO3.[6] In the closed tube, it gives off water and fuses.[6]
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 are 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.[8] Weathering-related cyrilovite, lipscombite, and crandillite-group minerals were formed by percolating meteoric waters under increasing oxygen fugacity.[8]