Rankin and Wright — Ternary System CaO-Al 2 2 -Si0. 2 . 7 



(3) Calcium orthosilicate, VCaO.SiO, (CaO 65*0, Si0 2 35'0). 

 — This compound occurs in three forms, a, ft, 7, and possibly a 

 fourth, £', which is monotropic. The high temperature a form 

 melts at 2130° ± 20° ; at 1420° ± 2° it undergoes a reversible 

 change to the ft form, which in turn passes reversibly to the 

 7 form at 675°. The inversion ft to 7 is accompanied by an 

 increase in volume of about 10 per cent which shatters the 

 charge into fine dust ; the phenomenon, frequently observed 

 in the burning of portland cement, is well known as dusting. 



a %CaO.SiO^ occurs in irregular grains so intricately inter- 

 woven and twinned that a satisfactory measurement of their 

 optical properties is exceedingly difficult. They show occasion- 

 ally prismatic development with good cleavage parallel with 

 the prism axis. Twinning is a characteristic feature and is 

 often extremely complicated ; different sets of twinning 

 lamellae occur intersecting at various angles and, together, 

 form an interpenetrating mass not unlike microcline or leucite. 

 Occasionally in a thin particle only one set of polysynthetic 

 twinning lamellae is present, and resembles then a plagioclase 

 section twinned after the albite law. The crystal system is 

 either monoclinic or triclinia Refractive indices : a Na = 1715 

 =b 0-002, ft^ = 1-720 dz 0-003, 7 Na = 1-737 =b 0'002. Bire- 

 fringence fairly strong, about 0*023 ; optic axial angle large ; 

 plane of optic axes nearly parallel with the elongation of the 

 crystals ; optical character positive. Extinction angles in 

 twinned flakes were observed as high as c : a' = 18° though 

 smaller angles are more common. Hardness between 5 and 6. 



ft 2CaO.Si0 3 appears in irregular rounded grains, often 

 prismatic in shape with cleavage parallel to the prism axis. 

 The crystal system is apparently orthorhombic, though the 

 presence of twinning lamellae with low extinction angles up to 

 c : a' = 10° in certain of the grains which may be of the a 

 form would indicate a still lower symmetry. Refractive in- 

 dices : a Na = 1-717 db 0-002 ; 7 Na = 1*735 ± ; 002 ; birefringence 

 medium, the interference colors in the iinely-powdered grains 

 rarely exceeding blue of the second order ; optic axial angle 

 large ; optical character positive. Plane of optic axes parallel 

 with the elongation. The optical properties of the a. and ft 

 forms are unfortunately so similar that it is often exceedingly 

 difficult to distinguish between them. The intricate twinning 

 of the a form does not appear in the ft form, and is apparently 

 the best criterion to use. The birefringence of the ft form 

 appears also to be slightly weaker than that of the a form. 



<y2CaO.SiO i is obtainable from the inversion of the ft form 

 on cooling only as a fine powder, by reason of the "dusting" 

 noted above. It is colorless, prismatic in habit, with perfect 

 cleavage along the prism axis. Occasionally indications of 



