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The mineralogical status of "cavolinite" from Vesuvius, Italy, and crystallochemical data on the davyne subgroup

Joelle Binon, Elena Bonaccorsi, Heinz-Juergen Bernhardt and Andre-Mathieu Fransolet
The mineralogical status of "cavolinite" from Vesuvius, Italy, and crystallochemical data on the davyne subgroup
European Journal of Mineralogy (June 2004) 16 (3): 511-520

Abstract

Since its description in 1825 as a new mineral species from the Monte Somma caldera, Vesuvius, "cavolinite" has been a source for debate and confusion. Now, "cavolinite" is simply considered as an obsolete name for nepheline, (Na (sub 3) K)[Al (sub 4) Si (sub 4) O (sub 16) ]. In order to shed some light on the status of this mineral, 12 specimens labelled as "cavolinite" were revisited by X-ray diffraction, micro-Raman spectroscopy, and electron microprobe chemical analyses. First, the X-ray powder method shows that all the specimens belong to the davyne subgroup of the cancrinite group. However, among the investigated samples, two populations of "cavolinite" are distinguishable. "Cavolinite" crystals lining small vugs exhibit a prismatic habit, have a chemical composition with about 3.3 Cl per formula unit, and are poor in sulphate (0.01 to 0.08 SO (sub 4) (super 2-) pfu). Moreover, they show superstructure reflections doubling the parameter a of the hexagonal unit cell (a = 25.8 and c = 5.4 Aa). These crystals correspond to quadridavyne, [(Na, K) (sub 6) Cl (sub 2) ](Ca (sub 2) Cl (sub 2) )[Si (sub 6) Al (sub 6) O (sub 24) ]. "Cavolinite" filling up large geodes of the second population occurs as entangled platelets or fibrous masses. Their chemical compositions reveal about 2.3 Cl pfu and are richer in sulphate (0.4 to 0.7 SO (sub 4) (super 2-) pfu). As no superstructure reflections were observed along the a axis, the cell parameters (a = 12.75 and c = 5.35 Aa) correspond to those of davyne [(Na, K) (sub 6) (SO (sub 4) ) (sub 0.5-1) Cl (sub 1-0) (CO (sub 3) ) (sub 0-1) ](Ca (sub 2) Cl (sub 2) )[Si (sub 6) Al (sub 6) O (sub 24) ]. The two populations are also qualitatively distinguishable by micro-Raman spectroscopy, by comparing the relative intensity of the vibration peaks of SO (sub 4) at about 990 cm (super -1) . Moreover, the presence of CO (sub 3) groups is clearly detected in a few samples. The vibration peak for CO (sub 3) (at about 1050 cm (super -1) ) is particularly intense in one of the samples, in agreement with the results of the crystal structure refinement, which points to about 0.9 CO (sub 3) (super 2-) groups pfu. Owing to 160 point analyses performed on the whole collection of samples by electron microprobe, a plot of the alkali contents pfu, Na versus K, shows a complete solid solution between two channel contents, Na (sub 3.6) K (sub 2) and Na (sub 5.4) K (sub 0.4) . The two populations can be recognized here again. All the "cavolinite" samples identified as quadridavyne show a broader range of the Na<-->K substitution than those identified as davyne in which the replacement of Na by Ca in the channels can be more significant.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 16
Serial Issue: 3
Title: The mineralogical status of "cavolinite" from Vesuvius, Italy, and crystallochemical data on the davyne subgroup
Affiliation: Universite de Liege, Departement de Geologie, Liege, Belgium
Pages: 511-520
Published: 200406
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 29
Accession Number: 2004-061254
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables
N40°49'00" - N40°49'00", E14°25'60" - E14°25'60"
Secondary Affiliation: Universita di Pisa, ITA, ItalyRuhr-Universitaet Bochum, DEU, Federal Republic of Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200418
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