32 Allen, etc. — Diopside and its Relations 



In several of the larger crystals of the <x-MgSi0 3 a character- 

 istic arrangement of inclusions and lines of growth was observed 

 and strongly resembled the hourglass structure of certain 

 pyroxenes, the hourglass portions of each crystal showing 

 abnormal interference phenomena, due either to incipient 

 changes into the/3-form or to peculiar intergrowths or possibly, 

 but not probably, to strain phenomena. 



The a-form is readily distinguished from the /3-form by its 

 lack of poly synthetic twinning, parallel extinction and stronger 

 birefringence ; from olivine and enstatite by its cleavage and 

 the position of the optic axial plane relative to the cleavage. 



It is of interest to note that the <x-MgSi0 3 , which is unstable 

 below 1365° and cannot be obtained except under very special 

 conditions, has not been observed in nature, thus establishing, 

 as in the case of pseudo-wollastonite, a high temperature limit 

 for the formation of certain minerals. 



Diopside. — This third compound of the series is an excellent 

 crystallizer and can be formed in a number of different ways 

 and at different temperatures. The best crystals were obtained 

 by heating glass of the composition CaMgSi 2 6 in a flux of 

 CaCl 2 in an atmosphere of dry HC1 at 1000° for one week. The 

 crystals varied in size up to 2 mm in diameter, were water-clear 

 and of simple crystallographic habit. Three crystals were 

 measured on the goniometer with reducing attachment. The 

 reflexion signals obtained were not of the best and the values 

 of Table VII are the averages of the observed angles. A 

 number of other crystals were selected and mounted on the 

 goniometer, but the reflexion signals from their faces were 

 often multiple and unsatisfactory, and not suited to improve 

 the results already obtained from the three measured crystals. 











Table VII 



[. 















Artificial 



Diopside 



Natural 



Diopside 



No. 



Letter 



Miller 



Symbol 



<?> 



P 







P 



1 



b 



010 





00 



90 01 



00 



90 00 



2 



m 



110 





43 30 



90 "0 



43 33 



90 00 



3 



s 



11 1 



-1 



24 51 



33 + 



25 07 



33 04 



4 



X 



221 



-2 



35 12 



55 2<> 



35 22 



55 19 



5 



X(?) 



331 



-3 



39 17 



6tf 17 



38 19 



66 04 



For the sake of comparison, the angular values <j> and p for 

 the same forms on natural diopside (Goldschmidt, Winkelta- 

 bellen, p. 283) are included in this table. From these angles 

 the average value of ft = "539; q = "568; e — *276, and \x == 

 73° 59'; or a : b : e = 1-096 : 1 : '591. For natural diopside, 

 j> o = 0-5390 ; q = 0-5670 ; e -- 0-2731 ; fi =-- 74° -09 ; ; or 



