468 MINERALOGY 



to 3 with the imperfect orthopinacoidal cleavage cracks. Basal 

 cleavage fragments show one optic axis, while the acute bisectrix 

 is nearly perpendicular to 100. Optically ( ). 



Epidote appears as a secondary mineral in igneous rocks and in 

 schists, where it is derived from the alteration of feldspars and 

 pyroxene or amphibole, usually associated with chlorite. It is also 

 the product of metamorphism, and is found in contact zones, as 

 well as in granular limestones where it is associated with vesu- 

 vianite, garnets, hematite, and pyroxenes. It rarely occurs as a 

 primary mineral of igneous rocks. In the United States epidote 

 is a common mineral at various localities along the Atlantic slope 

 in the New England, Middle, and Southern states. Its synthesis, 

 like zoisite, is uncertain. 



ALLANITE 



Allanite. Ca^Al . Ce . Fe) 2 (A10H) (Si0 4 ) 3 ; Composition vari- 

 able ; Monoclinic ; Type, Digonal Equatorial ; & : b : c = 1.5509 : 

 1 : 1.7691; p = 64 39' = 100 A 001 ; 100 A 110 = 54 34'; 001 A 101 

 = 63 24'; 001 A 001 = 58 3' ; Common forms, c(001), a (100), m 

 (110), d(lll), n(lll); Twinning plane, 100; Cleavage, 100 and 

 001 in traces ; Brittle ; Fracture, uneven ; H. = 5.5-6 ; G. = 

 3.5-4.2; Color, pitch brown to black or yellowish; Streak, pale 

 gray or greenish ; Luster, pitchy to dull ; Opaque to subtranslu- 

 cent. 



B.B. Fuses easily with intumescence. Becomes magnetic 

 in R. F. After strong ignition in the closed tube yields water. 

 Gelatinizes with HC1 ; the solution freed of silica yields reactions 

 for cerium, page 571. 



General description. Either tabular in habit parallel to 100, 

 or acicular parallel to the orthoaxis ; also granular or massive. 



The elongated variety has been described under the name of 

 orthite ; it contains much water, even as high as 17 per cent., while 

 the true allanite contains only one or two per cent. 



Allanite is an epidote in which some of the iron is replaced by the 

 rare elements, cerium, lanthanum, didymium, yttrium, or erbium ; 

 the amount of each varies with the locality ; the total of them all 

 is about 20 per cent. 



Allanite occurs as an accessory mineral in igneous rocks, more 

 often in those rich in silica, as granites and pegmatites ; also in 

 schists and crystalline limestones. 



