174 



in 



CaBa(C0 3 );cerussiie: PbCO 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 ditrigonal 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 orthorhom- 

 bic 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 

 (/* A?^)> w ^h the twinningplanes (110), and less 

 often (130). They are optically biaxial, but their 

 appearance is completely ditrigonal 1 ). 



Another beautiful example of a pseudo-hexagonal 

 substance is potassium-sulphate. The prism-angle (110) : (110) is 

 here 5936'; repeated twins occur with (130) as twinning-plane, 

 and the simulative effect is sometimes so great, that the crystals 

 have wholly the aspect of true hexagonal bipyramids. 



Calcium-chloro-aluminate 2 ) : [iCaO,Al 2 3 ,CaCl 2 ,6H 2 0] + 4H 2 0, 

 is monoclinic, with /3 = 8713', but pseudo-hexagonal, because 

 the faces (310) and (3lO) 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). At a temperature of 36 C. they become really rhombo- 

 hedral and uniaxial. 



.Fig. 136. 



Cerussite. 



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

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



2) 0. Friedel, Bull, de la Soc. Miner. 20. 122. (1897) ; O. Miigge, Neue 

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



