480 MINERALOGY 



rocks. Their formation takes place at comparatively low tempera- 

 tures and some even at temperatures near zero, as phillipsite has 

 been found in the volcanic mud at the bottom of the Pacific Ocean. 



Zeolites are formed synthetically by heating their constituents in 

 the correct proportions, in each case, in a sealed tube with water, 

 to temperatures between 170 and 250. In a similar way they are 

 often separated from the percolating waters of many hot springs. 

 Their water content is held with different degrees of firmness; 

 some even lose their water to dry air and reabsorb it without any 

 material physical change. This water is therefore said to take no 

 part in the formation of the crystalline molecule. Zeolites which 

 have had their water driven out at a temperature below redness 

 will not only reabsorb the water from moist air, but will take up in 

 the same way alcohol, ammonia, silicon chloride, and coloring matter. 

 If the zeolite is fused, it loses this property, as the molecule is changed 

 in nature, for from fused natrolite on cooling nepheline is formed ; 

 from apophyllite, hexagonal calcium metasilicate ; from chabazite, 

 anorthite ; from heulandite pyroxene is produced. 



The zeolites are so near alike in their occurrence and appearance 

 that unless well crystallized it is difficult to distinguish them, and 

 often a quantitative determination of silica and water is necessary 

 for the identification of species. They all boil when heated in the 

 blowpipe flame, from which the group takes its name. 



They are usually associated with datolite, prehnite, pectolite, 

 quartz, calcite and pyrite, or chalcopyrite. 



APOPHYLLITE 



Apophyllite. Hi4K 2 Ca 8 (SiO3)i 6 . 9 H 2 O ; a hydrated metasilicate 

 of calcium and potassium ; Tetragonal ; Type, Ditetragonal Equa- 

 torial; c = 1.2515; 001 A 101 = 51 22'; 001 A 111 = 60 32'; 

 Common forms, a (100), c(001), m (110), p(lll); Twinning 

 plane, 111 rare ; Cleavage, basal perfect, m less so ; Brittle ; Frac- 

 ture, uneven; H. = 4.5-5; G. = 2.3-2.4; white, gray, or pale 

 shades of yellow, green, or red; Streak, white; Transparent to 

 translucent; Luster, vitreous, on the base pearly ; co = 1.534; = 

 1.536; <o = .002; Optically ( ) often showing anomalies. 



B.B. Exfoliates and fuses to a white blebby enamel. Colors 

 the flame violet (potassium) or shows potassium through the blue 

 glass. In the closed tube yields water. Decomposed with HC1 

 without gelatinization. Often shows fluorine. 



