Properties and possible structures of hydronium trihydrate HgO ~ and hexahydrate H~O + are considered. From the 2330 minerals listed in Strunz's mineralogical tables nineteen were selected that appear to contain H+.4H~O or H+.7H~O, and the possibility of discrete HgO + or HI~O + groupings existing in their crystal lattice is discussed. From a crystallochemical point of view the most probable examples of hydronium hydrate compounds are layer lattice minerals like H-montmorillonite, H-vermiculite, troegerite, H-meta-autunite, sabugalite, and hewettite. T HE existence of the individual H~O + pyramidal complex, called the hydronium ion, or sometimes the hydroxonium or oxonium 1 ion, in crystalline substances is a well-established fact. The best examined examples of hydronium compounds are 'hydrates' of strong inorganic acids. These compounds contain oxygens associated with three nearly equidistant protons forming H30+ ions, as has been proven directly by means of nuclear magnetic resonance (Richards and Smith, 1951), Raman (Taylor and Vidale, 1956), infra-red spectroscopy (Bethell and Sheppard, 1953), X-ray diffraction (Luzzati, 1951), and neutron scattering (Janik, 1964). 2 Several authors have also indirectly demonstrated the occurrence of the hydronium ion in minerals (see, for instance, Shishkin, 1951 ; Halla and van Tassel, 1956 ; Smith, Gruner, and Lips-comb, 1957; Kubisz, 1961a, b, and 1964; White and Burns, 1963; Brophy and Sheridan, 1965; Ross and Evans, 1965). 1 The author prefers the name 'hydronium', which is consistent with the nomenclature introduced in the Encyclopedia of Chemical Technology (New York, 1952) where the ending 'onium' is used for cations that are coordination complexes of nonmetallic atoms (e.g., R30+, t~S+, RAN+ , where R = H or an organic group). The name 'hydroxo' is used to designate OH ligands in coordination compounds, while 'oxouium' compounds are coordination compounds, commonly of certain oxygen-containing organic substances. For other references concerning more recent works on this subject see, for instance, Savoie and Gigu6re (1964).
