Chiavennite | |CaMn(H2O)2| [Be2Si5O13(OH)2] | ||
Ferrochiavennite | |Ca1-2Fe(H2O)2| [Be2Si5O13(OH)2] | ||
Morphology: | |||
Spherical aggregates of platy, spear-shaped crystals. | |||
Physical properties: | |||
Cleavage: {100}, {010}, and {001} good to perfect. Hardness: ~3. D (chiavennite) = 2.56 - 2.64 g/cm3. D (ferrochiavennite) = 2.67 g/cm3. Luster: Vitreous to pearly. Streak: White to pale ocher. |
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Optical properties: | |||
Color: Pale orange-yellow, colorless to yellow or orange-yellow in thin section. |
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Crystallography: | |||
Unit cell data: Ferrochiavennite
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Name: | |||
Chiavennite was described and named by Bondi et al. (1983) for the locality at Tanno, Chiavenna, Rhetic Alps of Italy. The mineral was later classified as a zeolite by Coombs et al. (1997). It is a rare mineral occurring in two known localities in Italy and the Oslo area of Norway, although more recently a boron-bearing chiavennite has been reported from Utö, Stockholm, Sweden (Langhof and Holstram, 1994). |
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Crystal structure: | |||
The structure of both chiavennite and ferrochiavennite consists of an interrupted framework of SiO4 and BeO4 tetrahedra (Tazzoli et al., 1995). Be and one Si tetrahedral site (Si1) form a chain of singly connected 4-rings parallel to the a-axis (see figure). The Be tetrahedra link to zigzag chains of Si3 tetrahedra, which in turn are linked to another zigzag chain of Si2 tetrahedra. Both zigzag chains are parallel to the c-axis (-CHI). The Be tetrahedra each have a hydroxyl on one apex. The framework contains nine-ring channels parallel to the c-axis. Ca cations (orange) and H2O molecules (blue) are located within the nine-ring channels, where each cation is coordinated with four framework oxygens, two hydroxyls, and two H2O molecules. Mn cations (red) are located between Be-Si1 4-ring chains, where they are coordinated with four framework oxygens and two hydroxyls. The chemical analyses suggest that some Al may be substituting for Si. The Si1-O bond lengths average 1.627, indicating that the Si1 site may be the one containing the Al. (The Si-O bond length for tetrahedra with no Al is commonly about 1.605.) |
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Chemical composition: | |||
Analyses of chiavennite are available from two localities, Chiavenna, Italy (Bondi et al., 1983) and Longangen, Norway (Raade et al., 1983). If the cell contents are calculated by the method used for other zeolites, the results give a large +E%. A positive error indicates too many cations are assigned to the framework, such as the Al, or some non-framework cations are missing. | |||
Occurrences: | |||
Chiavennite occurs in altered alpine pegmatite dikes cutting the Chiavenna mafic and ultramafic complex in the Rhetic Alps of Italy. Crusts of chiavennite and bavenite on pre-existing beryl indicates late stage reaction with pegmatitic fluids. Bondi et al. (1983) suggest that the Be came from beryl, of course, but Ca and Mn may have been derived from the mafic and ultramafic wall rocks. Ferrochiavennite is described by Grice et al. (2013) from two syenite pegmatite localites in Norway: Blåfjell, Langangen, Telemark, and the AS Granit larvikite quarry, Tveladen, Vestfold. |
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References: | |||
Bondi, M., Griffin, W.L., Mattioli, V., and Mottana, A. (1983) Chiavennite, CaMnBe2Si5O13(OH)2∙2H2O, a new mineral from Chiavenna (Italy). Am. Mineral. 68, 623-627. Grice, J.D. Kristansen, R., Friis, H., Rowe, R., Poirier, G.G., Selbekk., R.S., Cooper, M.A., and Larsen, O.A. (2013) Ferrochiavennite, a new beryllium silicate zeolite from syenite pegmatites in the Larvyk pluton complex, Oslo region, southern Norway. Can. Mineral. 51, 285-296. |