Common forms are--the pinakoid _c_ {111}, prisms _a_ {101}, and _m_ {211}, and pyramids _r_ {100} and _e_ {110}. _c_ ∧ _r_ = 31° 22´. Habit tabular parallel to _c_, rhombohedral with _e_ prominent, or prismatic with _a_ prominent. Cleavage ∥ _c_ very good, ∥ _a_ difficult. The colour is brown or red to brownish- or bluish-red. Brittle. Hardness 5 to 5¹⁄₂; sp. gr. 2·92 for eudialyte, 3·0 to 3·1 for eucolyte. The double refraction is strong, being positive for eudialyte, negative for the Norwegian variety, eucolyte. From careful microscopic examination, Ramsay has found that zones of positive and negative birefringence, as well as isotropic (singly-refracting) zones can occur on the same crystal, and he suggests that the mineral is really composed of two isomorphous compounds forming mixtures. In view of the continuous variation of optical properties in an isomorphous series like the felspars, such an explanation seems doubtful. The optical behaviour of minerals is very often anomalous, and the phenomena in this case are probably due to repeated twinning, with some alteration in the double refraction, or to the lamellar intergrowth of two varieties having slightly different optical properties. On heating, the mineral evolves moisture and readily fuses. It is easily attacked even by dilute acids, being named by Strohmeyer (1819) on account of this property. The dilute hydrochloric acid solution reddens turmeric paper--a test for the presence of zirconium. It is found in Greenland, usually embedded in felspar, in Norway, in Lapland and in Arkansas, being generally associated with minerals rich in alkalies, _e.g._ ægirine, ælæolite, nepheline, sodalite, arfvedsonite, etc. ~Beckelite.~--This is a mineral similar in composition to eudialyte, though not so complex, and of more recent discovery.[54] It is a silicate of ceria earths and lime, in which zirconia replaces silica; the oxygen ratio (_i.e._ ratio of oxygen in basic oxides to oxygen in acid oxides) is 3 : 1, and the formula Ca₃R´´´₄(Si,Zr)₃O₁₅, where R = rare earth metals, chiefly of the cerium group. It is thus a salt of an acid H₁₈Si₃O₁₅ [= 3H₆SiO₅ = 3(3H₂O,SiO₂)] with zirconium and silicon vicarious. [54] _Abstr. Chem. Soc._ 1905, ~88~, ii, 177. The crystals appear to belong to the cubic system, occurring in cuboid grains, and in octahedra and dodecahedra. It is brown, and isotropic, with cubic cleavage. Sp. gr. = 4·15. It is soluble in hot hydrochloric acid, even after ignition; the solution gives the turmeric test for zirconium. It was found in a dyke in an ælæolite syenite, near the Sea of Azov. * * * * * The following minerals (see list) are also to be placed in the class of mixed silicates: _Arfvedsonite_ and _cataplejite_, complex zircono-silicates. _Hiortdahlite_ (Guarinite) and _Lavenite_, zircono-silicates with fluorine. _Caryocerite_, _Melanocerite_ and _Steenstrupine_, complex fluosilicates.