Day and Shepherd — Lime-Silica Series of Minerals. 295 



rapidly, this change could be checked, in part at least, for a 

 considerable period of time. The a-form belongs either to 

 the monoclinic or triclinic crystal system, probably the former, 

 and shows prismatic development, with good cleavage after a 

 face in that zone. The crystals are transparent and colorless 

 and occur as fine aggregates so intricately intergrown that the 

 precise determination of their optic properties is practically 

 impossible. Twinning is a characteristic feature of this form r 

 and is often extremely complicated. Different sets of the 

 twinning lamellae cut each other at various angles and the 

 sections then resemble plates of microcline or leucite. Extinc- 

 tion angles measured along the prism axis were noted as high 

 as c : a = 18°, although smaller angles appear more frequently. 

 The hardness is between 5 and 6. The refractive indices were 

 measured by the method of refractive liquids: a = l*714±'003 ; 

 £ = l-720±-004 ; 7 = l*737±-003. Birefringence about -02. 

 The optical character is positive; 2 V = 81°; 2E> 180°. This 

 optic axial angle was measured on a section nearly perpendicu- 

 lar to an optic axis by the graphical method recently described 

 by Becke.* The method is founded on the difference in cur- 

 vature of the dark hyperbolic bar which passes through the 

 optic axis, for different axial angles in the diagonal position. 

 In place of the revolving drawing table of Becke, a different 

 type of instrument which can be clamped directly to the 

 microscope was used in this laboratory with satisfactory results. 

 This method is only an approximate one and the figures 

 obtained by its use may vary considerably from the actual 

 values, a condition recognized by Becke himself in the origi- 

 nal paper. The plane of the optic axes is about parallel to 

 the direction of elongation of the crystals. 



(b) The crystallites of the fiform are stable between 675° 

 and 1410°; they are also prismatic in shape and show good 

 cleavage parallel to the prism axis. They are colorless and 

 transparent, and are orthorhombic in crystal system. Their 

 hardness and density could not be determined directly, as this 

 form, at low temperatures, inverts rapidly to the 7-form and 

 can furthermore be obtained only in powdered condition. 

 The least ellipsoidal axis c is parallel to the c crystallographic 

 axis and the plane of optic axes lies in the direction of elonga- 

 tion of the crystals. The optic axial angle is very large. 

 The refractive indices, a = 1*722±'003 and 7 = l-733db'003, 

 were determined by the method of refractive liquids. The 

 birefringence is not strong, about *01. The /3-form is distin- 

 guished from the preceding a-form most readily by the 

 absence of twinning and by its parallel extinction. The pro- 

 * F. Becke, Tscherm. Min. u. pet. Mitth. xxiv, 1905. 



