Zircon is one of the least easily altered minerals; by the prolonged action of chalybeate and other waters, during many geological ages, however, it gradually changes, losing silica and gaining lime, oxides of iron, and water. Some of these altered varieties have received special names, as, _e.g._ Auerbachite, Malacone, Cyrtolite, and Alvite; but none of them is of special interest. ~Naegite.~[52]--This rare mineral is a silicate closely related to zircon, but of rather more complex composition. It may be represented as silicate of zirconium, ZrSiO₄ (zirconia = 55·3, silica = 20·6 per cent.), with rare earths (chiefly yttria, 9·1 per cent.), uranium (UO₃ = 3 per cent.), and thorium (ThO₂ = 5·0 per cent.), partly as silicates, partly as columbates and tantalates ((Cb,Ta)₂O₅ = 7·7 per cent.).[53] [52] _Beiträge zur Mineralogie von Japan_, 1906, ~2~, 23. [53] An earlier analysis (_Abstr. Chem. Soc._ 1905, ~88~, [ii.], 177) gave over 20 per cent. of uranous oxide, UO₂; the greater part of this appears to have been zirconia, ZiO₂. It is tetragonal, usually occurring in globular aggregates of crystals. The measurable angles are extremely close to those of zircon, and it is probable that naegite is isomorphous with the series mentioned above under Thorite. The hardness is 7¹⁄₂, the sp. gr. 4·091. The colour is dark green or brown, becoming dull by weathering. The double refraction is extremely weak. So far it has only been found in the ‘placer’ tin deposits or ‘gravel tin’ of Japan. * * * * * The following minerals (see list) are also to be included in this sub-class: _Alvite_ (Anderbergite or Cyrtolite), _Auerbachite_, _Malacone_, _Oerstedite_ and _Tachyaphaltite_, altered varieties of zircon. _Calciothorite_, _Eucrasite_ and _Freyalite_, altered varieties of Thorite. _Pilbarite_, _Thorogummite_ and _Yttrogummite_, hydrated silicates of thorium with uranium and other metals. (_c_) COMPLEX SILICATES ~Eudialyte~ (Eucolyte).--This is a complex silicate of alkalies, lime, ferrous oxide, rare earths, etc., containing chlorine and a high proportion (up to 17 per cent.) of zirconia. The empirical formula is given by Dana as Na₁₃(Ca,Fe)₆Cl(Si,Zr)₂₀O₅₂. Brögger gives the simpler metasilicate formula R´₄R´´₃Zr(SiO₃)₇, where R = (Na,K,H), R´´ = (Ca,Fe,Mn,CeOH), and Zr(OCl) may partly function as an acid in place of SiO₂. The true formula, however, is quite uncertain, as the zirconia may function either as an acidic or basic oxide. The fact that a mineral of such exceedingly complex composition occurs in perfectly well-defined crystals indicates the intricate nature of the problems to be solved in mineral chemistry. The crystals are rhombohedral, _a_ : _c_ = 1 : 2·1116.