Abstract: This paper presents a literature survey of compositions of the fibrous zeolites mesolite, natrolite, thomsonite and their derivatives such as pseudomesolite, high-Na mesolite, tetranatrolite, paranatrolite, ranite, and gonnardite, and evaluates them in the light of new electron probe analyses and X-ray powder data for gonnardites and associated minerals from Aci Castello, Gignat, Hills Port, Kladno, and Lamo. The analyses are plotted on the basis of bivalent vs. trivalent cations per 80 oxygen cell and a new chemical classification is tentatively proposed. It is concluded that ranite is definitely not synonymous with gonnardite and until species status is confirmed it is useful to retain this term as a Ca- and Al-rich disordered variety of natrolite. It is further concluded that natrolite and tetranatrolite contain up to 2 Ca, ranite 2–4 Ca, gonnardite 4–6 Ca and thomsonite 6–8 Ca atoms with corresponding limits on the Al atoms. Compositions are governed by NaSi = CaAl and to some extent by Na2 = Ca type replacements and the Al-content generally varies sympathetically with Ca-content. The plot reveals that most high-Na mesolites are ranites, a number of gonnardites are ranites and one or two are tetranatrolites. The compositional field of gonnardite crosses that of mesolite (and pseudomesolite), but these minerals can be easily distinguished optically and by their powder patterns. The unit cell volumes increase in the order tetranatrolite, ranite, gonnardite and paranatrolite, therefore if the 1040 (or 1460) line can be identified in the powder patterns one can distinguish between these minerals. New infrared spectra of gonnardite, ranite and tetranatrolite are compared with each other and with published spectra, and differences are noted. DSC results for gonnardite and ranite are compared and appear to be diagnostic.