ANALCITE  , a commonly occurring

mineral of the zeolite group . It crystallizes in the cubic system, the common form being the icositetrahedron (211), either alone (fig . I) or in combination with the cube (too); sometimes the faces of the cube predominate in size, and its corners are each replaced by three small triangular faces representing the icositetrahedron (fig . 2) . Although cubic in form, analcite usually shows feeble double refraction, and is thus optically anomalous . This feature of analcite has been much studied, Sir David Brewster in 1826 being the earliest investigator . Crystals of analcite are often perfectly colourless and transparent with a brilliant glassy lustre, but some are opaque and white or pinkish-white . The hardness of the mineral is 5 to 52, and its specific gravity is 2.25 . Chemically, analcite is a hydrated sodium and aluminium silicate, NaAlSi2Os+H2O; small amounts of the sodium being sometimes replaced by calcium or by potassium . The water of crystallization is readily expelled by heat, with modification of the optical characters of the crystals . Before the blowpipe the mineral readily fuses with intumescence to a colourless glass . It is decomposed by acids with separation of gelatinous silica .

Analcite usually occurs, associated with other zeolitic minerals, lining amygdaloidal cavities in basic volcanic rocks such as

basalt and melaphyre, and especially in such as have undergone alteration by weathering; the Tertiary basalts of the north of Ireland frequently contain cavities lined with small brilliant crystals of analcite . Larger crystals of the same kind are found in the basalt of the Cyclopean Islands (Scogli de' Ciclopi or Faraglioni) N.E. of Catania, Sicily . Large opaque crystals of the pinkish-white colour are found in cavities in melaphyre at the Seisser Alpe near Schlern in southern Tirol . In all such cases the mineral is clearly of secondary origin, but of late years another mode of occurrence has been recognized, analcite having been found as a primary constituent of certain igneous rocks such as monchiquite and some basalts . The irregular grains, of which it has the form, had previously been mistaken for glass . Owing to the fact that analcite often crystallizes in cubes, it was long known as cubic zeolite or as cuboite . The name now in use was proposed in 1797 in the form analcime, by R . J . Hairy, in allusion to the weak (avaXtas) electrification of the mineral produced by friction . Euthallite is a compact, greenish analcite, produced by the alteration of elaeolite at various localities in the Langesund-fjord in southern Norway . Eudnophite, from the same region, was originally described as an orthorhombic mineral dimorphous with analcite, but has since been found to be identical with it . Cluthalite, from the Clyde (Clutha) valley, is an altered form of the mineral .

(L . J .