320 Penfield — Crystallography of C cesium- Mercury llalicl.es. 



Two cleavages were observed, one perfect parallel to the 

 clinopinacoid, a second less perfect parallel to the base. In 

 polarized light clinopinacoid tables give an extinction, inclined 

 about 23° to the vertical axis in the acute angle ,3. Basal 

 plates show in conveigeut light an optical axis not far removed 

 from the' center of the field. The plane of the optical axes is 

 the clinopinacoid. 



These crystals furnish an excellent illustration of inclined 

 faced hemihedrism, as recently developed by Prof. Geo. H. 

 Williams,* who has shown that it is of frequent occurrence 

 on pyroxene. 



Type 1 : 2. 



Monoclinie 

 Orthorhombic" 



CsHgoCU, 



CsHg,ClBr 4 



CsHg. 2 Br 5 



CsHg 2 l5. not measured 



a 

 1-6099 

 0-586 

 0-590 



b: c 



1: 1-3289, f3 = 78" 



1: 



1: 1-15 



54' 



CsHg 2 Cl 6 was made in slender lath-shaped crystals, over 



10 mra long in the 

 28 - 29 - direction of the 



symmetry axis 

 but not over |- mm 

 in diameter. 

 Fig. 2 8 repre- 

 sents a simple, and 29 a twin crystal, with the orthopinacoid 

 as twinning plane. The forms and angles are as follows : 



a, 100, %•% 

 i, 010, i-i 



c, 001, O 

 m, 110, I 



d, 011. l-i 

 p, 111, -1 



Two orthodomes were also identified, 101. and 201, but they 

 were very small and yielded only approximate measurements. 



Measured. 

 a*c. 100 a 001 = *78° 54' 

 a a m, 100 a 110 = *57 40 

 c ~d, 001 a 011 =*52 31 



Measured. Calculated, 



c a m, 001 a 110 = 84° 5' 84 & 5' 



TOa#, 110 aIII = 31 12 31 8 



a ~p, 100 a 111 = 58 4 58 3 



1) a>, 010 a 111 =47 19 47 19 



30 



*S\ 



The plane of the optical axes is at right angles to the sym- 

 metry plane and the obtuse bisectrix is nearly normal 

 to the base. 



Both CsHg 2 ClBr 4 and CsHgBr 5 were made in 

 rectangular tablets, fig. 30, which were not over 

 1-J mm - in greatest diameter and were very thin. Twins 

 were common, with the unit prism as twinning 

 plane, and the plates often penetrated at angles of 

 about 60° and 120°, reminding one of little cerussite 

 twins. 



This Journal, xxxviii, p. 115, 1889. 



