180 



Calcium-chloro-aluminate 1 ): [3CaO,Al z Oa,CaCl 2 fiH<f)]+4H 2 0, is 

 monoclinic, with /? = 87 13', but pseudo- 

 hexagonal, because the sface(301) and .(3 10) 

 include an angle of about 60 with the 

 plane of symmetry (010). Twinning occurs 

 by three sets of lamellae, intersecting at 

 120, and with (110) as twinning-plane ; the 

 crystals appear as thin hexagonal plates 

 parallel to the apparent basal face (0001). 



Fig. 139. 



At a temperature of 36 C. they become 



Scoiezite; section parallel really rhombohedral and uniaxial. 



to (001} (Lacroix). ScoUzite: CaAl^Si 3 O w + 3 H 2 is monocli- 

 nic, with an axial ratio a: b:c = 0,0764: i: 0,3434, and /3 8<fi8'. 

 The prism-angle (110): (UO) is 8837|'. The space-lattice is, there- 

 fore, pseudo-tetra- 

 gonal. Twins occur 

 with (100) as twin- 

 ning-plane; a sec- 

 tion parallel to 

 (001) shows the 

 division in fields 

 and strips, as 

 drawn in fig. 130. 

 (Lacroix). 



How perfectly 

 the shape of such 

 crystals may ap- 

 proximate to real 

 hexagonal or trigo- 

 nal symmetry, may 

 also been seen 

 from the stereogra- 

 phical projection 

 of a Rontgenpat- 

 tern of cerussite, 



parallel to (001} . and obtained by us in our series of investigations 

 on the symmetry of the Ront gen-patterns of isomorphously related 



Fig. 140. 



Stereographical Projection of the Rontgen-pattern 

 of Cerussite; plate parallel to 



!) G. Friedel, Bull, de la Soc. Miner., 20, 122, (1897); O. Miigge, Neues 

 Jahrb. f. Miner., Beil. End., 14, 264, (1901). 



