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

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, (Na₃K)[A1₄Si₄O₁₆]. 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 C1 per formula unit, and are poor in sulphate (0.01 to 0.08 SO₄²- pfu). Moreover, they show superstructure reflections doubling the parameter a of the hexagonal unit cell (a = 25.8 and c = 5.4 Å). These crystals correspond to quadridavyne, [(Na,K)₆Cl₂](Ca₂)[Si₆Al₆O₂₄]. “Cavolinite” filling up large geodes of the second population occurs as entangled platelets or fibrous masses. Their chemical compositions reveal about 2.3 C1 pfu and are richer in sulphate (0.4 to 0.7 SO₄²⁻ pfu). As no superstructure reflections were observed along the a axis, the cell parameters (a = 12.75 and c = 5.35 Å) correspond to those of davyne [(Na,K)₆(SO₄)_0.5–1CI_1–0 (CO₃)_0–1](Ca₂Cl₂)[Si₆Al₆O₂₄]. The two populations are also qualitatively distinguishable by micro-Raman spectroscopy, by comparing the relative intensity of the vibration peaks of SO₄ at about 990 cm⁻¹. Moreover, the presence of CO₃ groups is clearly detected in a few samples. The vibration peak for CO₃ (at about 1050 cm⁻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 CO₃²⁻ 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_3.6K₂ and Na_5.4K_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.