179 



a featherlike striation, as the final indication of the twinning-process 

 which has led to the deceptive form of the crystal; but its mimetic 

 character is immediately revealed by optical investigation: indeed, 

 the optical character, as a direct manifestation of its molecular 

 structure, appears to be that of true biaxial crystals. 



In fig. 137 basal sections are reproduced of some compound twins 

 of aragonite: CaC0 3 ; witherite: BaC0 3 ', barytocalcite or bromlite: 

 (Ca,Ba)(C0 3 ); cerussite : PbC0 3 ; chlorite; potassium-sulphate : K 2 SO^ ; etc. 



Aragonite has a prism-angle of 6348', and repeated twinning 

 occurs with (110) as twinning- plane. The polysynthetic twins, espe- 

 cially when they are built up by fine lamellae, simulate a hexagonal 

 or di trigonal individual, but optical investigation easily proves that 

 only a mimetic hexagonal form of orthorhombic individuals is present. 



Witherite occurs always in the shape of repeated twins which 

 closely simulate hexagonal or ditrigonal individuals. 

 The orthorhombic pseudo-hexagonal mineral has a 

 prism-angle (110): (110) = 6212 / ; the twins are 

 usually very complex, the faces rough and striated. 



The optical properties reveal the lower symmetry 

 very clearly. 



Bromlite (of Bromley Hill, Cumberland), the form 

 of which is very nearly that of witherite, is found in 

 dihexahedral pyramids formed by complex twinning; 

 optical investigation shows that the simulative 

 crystal is a combination of six individuals, as shown 

 in the figure. 



Cerussite has a prism-angle of 6244' ; the ortho- 

 rhombic mineral forms apparently hexagonal twins 

 (fig. 13$), with the twinning planes (110), and less often (130). 

 They are optically biaxial, but their appearance is completely 

 ditrigonal l ). 



Another beautiful example of a pseudo-hexagonal substance is 

 potassium-sulphate. The prism-angle (110) : (ifO) is here 5936'; 

 repeated twins occur with (130) as t winning-plane, and the simula- 

 tive effect is sometimes so great, that the crystals have wholly 

 the aspect of true hexagonal bipyramids. 



in 



Fig. 138. 

 Cerussite. 



*) Cf. also: J. Beckenkamp, Centralbl. f. Miner., (1917), p. 25; O. Miigge, 

 Neues Jahrb., Beil. End., 14, 247, (1901). 



