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Zaccagnaite-3R, a new Zn-Al hydrotalcite polytype

Posted by Marco E. Ciriotti

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Marco E. Ciriotti

Zaccagnaite-3R, a new Zn-Al hydrotalcite polytype
March 27, 2012 05:04AM

Registered: 7 years ago
Posts: 4,441

Reference:
▪ Lozano, R.P., Rossi, C., La Iglesia, A., Matesanz, E. (2012): Zaccagnaite-3R, a new Zn-Al hydrotalcite polytype from El Soplao cave (Cantabria, Spain). American Mineralogist, 97, 513-523.

Abstract:
We have recently discovered significant amounts of zaccagnaite, a natural Zn-Al-CO3 hydrotalcite in the El Soplao cave (north Spain). The El Soplao zaccagnaite is speleothemic, i.e., formed in the cave, and therefore it represents a new cave mineral. The origin of El Soplao zaccagnaite is related to the diagenesis of Zn- and Al-rich ferromanganese speleo-stromatolites, where it occurs as a pore-filling cement that likely precipitated at low temperature (≤ ~11 °C). In some stromatolite layers, the abundance of zaccagnaite crystals is large enough to enable their physical separation. This has allowed us to obtain its X-ray powder-diffraction pattern, infrared spectrum, and differential thermal/thermogravimetric profiles.
The cell parameters of the El Soplao zaccagnaite, refined from X-ray powder diffraction data are: a = 3.06616(1) and c = 22.6164(1) Å [α = β = 90°, γ = 120°; V = 184.139(1) Å3; Z = 3], consistent with a new trigonal polytype of zaccagnaite: zaccagnaite-3R. Besides, the El Soplao zaccagnaite shows some features previously unknown in natural hydrotalcites, such as octahedral-like morphologies and fluorescence zoning. Electron microprobe analyses revealed that the El Soplao zaccagnaite-3R has an unusual chemistry for natural hydrotalcites, as it is significantly more rich in Al (Zn2+/Al3+ = 1.6) than the hexagonal (2H) polytype (Zn2+/Al3+ = 2.0). The simplified chemical formula deduced from electron microprobe analysis is (Zn0.6Al0.4)(OH)2(CO3)0.2·0.5H2O, where C and water were calculated by stoichiometry. The carbon content calculated by stoichiometry (2.2 wt%) is in good agreement with that measured with the electron microprobe on gold-coated samples (2.5 wt%). The presence of interlayer water and CO3 groups was confirmed by thermogravimetric analysis coupled to mass spectroscopy, and by the analysis of the infrared spectrum.

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Marco E. Ciriotti Wrote:
-------------------------------------------------------
> Reference:
> ▪ Lozano, R.P., Rossi, C., La Iglesia, A.,
> Matesanz, E. (2012): Zaccagnaite-3R, a new Zn-Al
> hydrotalcite polytype from El Soplao cave
> (Cantabria, Spain). American Mineralogist, 97,
> 513-523.
> 
> Abstract:
> We have recently discovered significant amounts of
> zaccagnaite, a natural Zn-Al-CO3 hydrotalcite in
> the El Soplao cave (north Spain). The El Soplao
> zaccagnaite is speleothemic, i.e., formed in the
> cave, and therefore it represents a new cave
> mineral. The origin of El Soplao zaccagnaite is
> related to the diagenesis of Zn- and Al-rich
> ferromanganese speleo-stromatolites, where it
> occurs as a pore-filling cement that likely
> precipitated at low temperature (≤ ~11 °C). In
> some stromatolite layers, the abundance of
> zaccagnaite crystals is large enough to enable
> their physical separation. This has allowed us to
> obtain its X-ray powder-diffraction pattern,
> infrared spectrum, and differential
> thermal/thermogravimetric profiles. 
> The cell parameters of the El Soplao zaccagnaite,
> refined from X-ray powder diffraction data are: a
> = 3.06616(1) and c = 22.6164(1) Å [α = β =
> 90°, γ = 120°; V = 184.139(1) Å3; Z = 3],
> consistent with a new trigonal polytype of
> zaccagnaite: zaccagnaite-3R. Besides, the El
> Soplao zaccagnaite shows some features previously
> unknown in natural hydrotalcites, such as
> octahedral-like morphologies and fluorescence
> zoning. Electron microprobe analyses revealed that
> the El Soplao zaccagnaite-3R has an unusual
> chemistry for natural hydrotalcites, as it is
> significantly more rich in Al (Zn2+/Al3+ = 1.6)
> than the hexagonal (2H) polytype (Zn2+/Al3+ =
> 2.0). The simplified chemical formula deduced from
> electron microprobe analysis is
> (Zn0.6Al0.4)(OH)2(CO3)0.2·0.5H2O, where C and
> water were calculated by stoichiometry. The carbon
> content calculated by stoichiometry (2.2 wt%) is
> in good agreement with that measured with the
> electron microprobe on gold-coated samples (2.5
> wt%). The presence of interlayer water and CO3
> groups was confirmed by thermogravimetric analysis
> coupled to mass spectroscopy, and by the analysis
> of the infrared spectrum.

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