Eakle.] 



I'd lacheite. 



From these values the elements and axial ratio for the monoclinic 

 crystals become 



p = 0.0(197 ; g = 0.3556 ; e = 0.4563 ; fi = 62° 51'. 



a : 6 : c = 0.6554 : 1 : 0.3996 ; /J = 117° 9l 



Forms. — The forms occurring on the crystals are as follows: 



c = = ) 001 1 &=0oo=|010j d=— 20=|2"0lj 



m= oo = jnoj rt = ooO=jiooj p = — 1 =jTllj 



?=oo2=jl20| »=01 = joilj s=— 12 = jT2lj 



t = oof = j 450 j o = 02 = J 021 1 



The base \00l\, unit prism { 110? and clino-prism \120\ are the 

 predominating forms. The prism faces are invariably vertically 

 striated. The pinacoid { 010 j is usually narrow, but always per- 

 fect, without striatums. The other forms occur rarely aud are 

 always very narrow, sometimes mere line faces, on the edges 

 between the main forms. Figure 1, PI. 20, shows the habit and 

 usual combination; Fig. 2 shows, in addition to the main forms, 

 some of the rarer ones, in their relative development. Fig. 3 is 

 an orthographic projection of the forms on the basal plane. 



Calculated Table. — The following table gives the calculations 

 for the eleven forms, arranged to correspond to Goldschmidt's 

 Winkeltabellen : 



a 



= 0.6654 





lg a = 9.816.")()0 



lg ao = 0.214881 



lgpo = 



9.785116 



ao = 1.6411 



mi 



= 0.6097 



c 



= 0.3996 





lg c = 9.601625 



lg bo = 0.398375 



lg 90 = 



9.550962 



b = 2.5025 



90 



= 0.3556 



180 - 





\ 9.949300 

 lg sm /X 1 



lg, = ) 

 lgcos/tj 



9.659271 



lg JE9 = 



80 



= 0.23415J 



h = 0.8898 



e = 



= 0.4563 



Number. 



u 



© 



■S 



U 

 9 



% 





P 





% 



1 



V 



x' 



(Prism) 



X 



y 



y' 



d' 

 = tg p 



1 



c 







001 



90° 00' 27° 09' 



27° 09' 



0° 00' 



27° 09' 



0°00' 



0.5128 









0.5128 



2 



b 



Ooo 



010 



00 ;90 00 



00 



90 00 



00 



90 00 







00 



00 



3 



a 



oo0 



100 



90 00 



i i 





90 00 



00 



90 00 



00 



00 











4 



m 



GO 



110 



59 45 









90 00 



59 45 



30 15 



1.7147 



CD 





5 



I 



cc2 



120 



40 36- 



i L 







< t 



40 36- 



49 23- 



0.8573 









6 



t 



oof 



450 



53 54- 



i i 





I i 





53 54- 



36 05- 



1.3716 







i I 



7 



d 



—20 



■ 



01 



90 00 



40 37 



40 37 



00 



40 37 



00 



0.8576 









0.8576 



8 



n 



01 



Oil 



52 04 



33 02 



27 09 



21 47 



25 28 



19 34- 



0.5128 0.3996- 



0.6501 



9 







02 



021 



32 41 



43 31 





38 38 



21 50 



35 25 





0.7992 



0.9496 



10 



P 



— 1 



Til 



23 20 



23 31 



9 47 



21 47 



9 06 



21 30 



0.1724 0.3996 



0.4352 



11 



s 



—12 



121 



12 10 



39 16 





38 38 



7 40 



38 13- 





0.7992 



0.8177 



