APOPHYLLITE  , a

mineral often classed with the zeolites, since it behaves like these when heated before the blowpipe and has the same mode of occurrence; it differs, however, from' the zeolites proper in containing no aluminium . It is a hydrous potassium and calcium silicate, H7KCa4(Si02)s+44H20 . A small amount of fluorine is often present, and it is one of the few minerals in which ammonium has been detected . The temperature at which the water is expelled is higher than is usually the case with zeolites; none is given off below 2000, and only about half at 250°; this is slowly reabsorbed again from moist air, and is therefore regarded as water of crystallization, the remainder being water of constitution . When heated before the blowpipe, the mineral exfoliates, owing to loss of water, and on this account was named apophyllite by R . J. flatly in 1806, from the Greek afro, from, and OilMov, a leaf . Apophyllite always occurs as distinct crystals, which belong to the tetragdilal system . The form is either a square prism terminated by the basal planes (fig . 2), or an acute pyramid (fig . I) . A promi- nent feature of the mineral is its perfect basal cleavage, on which the lustre is markedly pearly, present- ing, in white crystals, some- what the appearance of the eye of a fish after The optical characters of the mineral are of special interest, and have been much studied . The sign of the double refraction may be either positive or negative, and some crystals are divided into optically biaxial sectors .

The variety known as leucocyclite shows, when examined in convergent polarized

light, a peculiar interference figure, the rings being alternately white and violet-black and not coloured as in a normal figure seen in white light . Apophyllite is a mineral of secondary origin, commonly occurring, in association with other zeolites, in amygdaloidal cavities in basalt and melaphyre . Magnificent groups of greenish and colourless tabular crystals, the crystals several inches across, were found, with flesh-red stilbite, in the Deccan traps of the Western Ghats, near Bombay, during the construction of the Great Indian Peninsular railway . Groups of crystals of a beautiful pink colour have been found in the silver veins of Andreasberg in the Harz and of Guanaxuato in Mexico . Crystals of recent formation have been detected in the Roman remains at the hot springs of Plombieres in France . (L . J .