ELISHA MiTCHElvIv SCIENTIFIC SOCIETY. 41 



it is present as a phosphate, either in combination with 

 the cerium or as an isomorphous mixture, thus: 



IV III 1 I IV 



Ce.Ce.COsPO). and R.ThCOsPO)^; 

 and that it is altered to the silicate by siliceous waters. 



PHYSICAL PROPERTIES. 



The crystalloo-raphic form of monazite is monoclinic, 

 and the commonly occurring- planes are ortho- and cli- 

 no-pinacoids and domes, the unit prism and the unit 

 pyramid. The basal pinacoid is rare, having* been 

 observed only on crystals from the Urals and from Al- 

 exander County, N. C. 



The usual crystal habit is tabular, parallel to the 

 ortho-pinacoid; also short columnar, and sometimes 

 elong-ated parallel to the prism. Twins are not com- 

 mon, the twinning* plane being* usually parallel to the 

 ortho-pinacoid. 



These crystals vary in size from the microscopic nee- 

 dles of cryptolite, which have a thickness of 0.00015 to 

 0.00062 inch, to the abnormally large crystals of Ame- 

 lia County, Va., 5 inches in leng-th. The more g-eneral 

 variation lies between ^^ and 1 inch. 



The cleavag-e of monazite is most perfectly devel- 

 oped parallel to the basal pinacoid, it is also distinct, 

 as a rule, parallel to the ortho-pinacoid; and some- 

 times visible parallel to the clino-pinacoid. The mine- 

 ral is brittle, w^th a conchoidal to uneven fracture; the 

 hardness is 5 to S.S\ specific g-ravity 4.64 to 5.3; lustre 

 resinous to waxy; the crystal faces are splendent in 

 fresh, pure specimens, dull in weathered, impure spec- 

 imens; the color is honey yellow, yellowish brown, am- 

 ber brown, reddish brown, brown or greenish yellow; 

 the purest specimens are transparent, becoming- trans- 

 lucent, and even opaque in the impure varieties. 



