30 Allen, etc. — Diopside and its delations 



The optical data of the monoclinic Mg-pyroxene form 

 given in the above article can now be supplemented by more 

 accurate figures in several instances ; and at the same time 

 several errors in the crystallographic data can be rectified. 

 The /3-MgSi0 3 is monoclinic and the observed form and 

 measured angles are listed in Table V above.* 



The reflexion signals from many of the faces were multiple 

 and wide variations in the angles occur. The best average 

 ratios from these values are about 



p = -58 q = -60 e = -046 fx = 87°26 ; or 

 a : b : c= 1*03 : 1 : 0-60 ; /3 = 87°26' ; 



values which are closely similar to those for enstatite but less 

 so for diopside. The variations are not such, however, as to 

 preclude isomorphic relations between the two. The plane of 

 the optic axes lies normal to the plane of symmetry and not in 

 the plane of symmetry as in most pyroxenesf ; the bisectrix c 

 is inclined to the vertical axis c, 21*8°. The optic axial angle 

 was measured on the universal stage and by means of the two 

 screw micrometer ocular. Care was taken to select favorable 

 sections and the probable error in each case was not large. 

 The average of all good determinations by both methods is 

 2V = 53'5°±1°, or 2E = 96°. The overlapping of the twin- 

 ning lamellae often causes abnormal variations in this optic 

 axial angle and it proved a difficult matter to find suitable 

 sections. Etch figures on the cleavage face were also obtained 

 and will be considered later, together with the etch figures of 

 the other members of this series. 



The a-MgSiOg, to which reference has already been made, 

 crystallizes readily and is obtained by quenching the crystal- 

 lized melt from temperatures above 1365° to prevent its 

 inversion to the /3-form. Once obtained it can be held for 

 apparently an indefinite period at ordinary temperatures with- 

 out inversion to the more stable /3-form. The crystals are 

 orthorhombic in symmetry and in certain positions bear strong 

 resemblance to the characteristic crystal habit of olivine. On 

 the surface of a melt held at 1510° and then chilled rapidly, 

 several crystals, water-clear and sharply bound crystallographi- 

 cally, occurred, and three of them (1 X '5 X '2 mm ) were measured 



* Compare with Table II on page 393, vol. xxii, 1906, this Journal, in which 

 several letters were unfortunately transposed, although the relations are 

 correctly represented in the gnomonic projection plat on the same page. 



f The relations of the different pyroxenes, particularly of the magnesium 

 iron group, have been recently discussed in an interesting paper by W. Wahl 

 (Die Enstatit-augite, Tschermak's Miner. Petrogr. Mittheil., xxvi, 1-131, 

 1907), who proposes the name clino-enstatite for the /?-MgSi0 3 or magnesium 

 pyroxene. The suggestion is a good one and may well be adopted, the two 

 latter terms being long and cumbersome. 



