APOPHYLLITE

This article appears in Volume V02, Page 195 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: APO-ARN

APOPHYLLITE , a

mineral

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

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

APPEARANCE (from Lat. apparere, to appear)

of
the eye of a fish after
The optical characters of the mineral are of

special

SPECIAL

interest

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

GREAT

Indian Peninsular railway. Groups of crystals of a beautiful

pink

PINK

colour have been found in the silver veins of Andreasberg in the Harz and of Guanaxuato in

Mexico

. Crystals of

recent

RECENT

formation have been detected in the Roman remains at the hot springs of Plombieres in France. (L. J. S.)

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