Day and Shepherd — Lime-Silica Series of Minerals. 301 



The products whose composition extended from the ortho- 

 silicate to pure lime were not homogeneous and contained both 

 end members in variable amounts. If quenched quickly, the 

 melts consisted of line grains of the high refracting calcium 

 oxide and the a-form of the orthosilicate ; if allowed to cool 

 slowly, the a-form inverted ordinarily to the 7-form with 

 attendant shattering or u dusting " of the product. 



Thermal evidence showed that the eutectic for the orthosili- 

 cate and calcium oxide is very close in composition to that 

 of the orthosilicate. Preparations of this composition were 

 examined under the microscope but no definite eutectic struc- 

 ture was observed. In such cases thermal evidence alone 

 must be relied upon to determine the eutectic point, as the 

 optical data are entirely inadequate. 



Summary. — There are only two definite compounds of lime 

 and silica capable of existing in contact with the melt. They 

 are : 



(1). The pseudo-hexagonal metasilicate melting at 1512° and 

 inverting into wollastonite at about 1200°. The metasilicate 

 is able to take up a v small amount of either lime or silica in 

 solid solution. 



(2). The orthosilicate of calcium, which melts at 2080° and 

 possesses three polymorphic forms : 



The a-form, which crystallizes in the monoclinic system, has 

 a density of 3*27 and a hardness of between 5 and 6. 



The /3-modification crystallizes in the orthorhombic system 

 and has a density of 3*2S. 



The 7-modification has a density of 2*97, and also crystal- 

 lizes in the monoclinic system. The disintegration or " dust- 

 ing " of the orthosilicate and of all lime-silica mixtures above 

 51 per cent lime is due to the 10 per cent volume-change 

 accompanying the ft < > 7 inversion. 



The inversion point a to j3 occurs at 1410°, £ to 7 at 675°. 



There are three eutectics in the series : — tridymite + the 

 metasilicate at 37 per cent CaO, 1417° ; the metasilicate ■+■ 

 orthosilicate at 54 per cent of lime, 1430° ; and orthosilicate -f- 

 lime at 67-J- per cent of lime, 2015°. 



The orthosilicate is readily attacked by water, which dis- 

 solves out the lime in large quantities. This is probably the 

 reason why it is not found as a natural mineral. 



The density of fused CaO is 3'32 ; its hardness 3-K It 

 fuses in the electric arc but its fusion temperature is not accu- 

 rately measurable. Lime crystallizes in the isometric system 

 and possesses no polymorphic forms. 



Silica begins to melt at about 1600° to an extremely viscous 

 liquid, so that an exact melting point cannot be determined. 

 It has been shown that for all temperatures above 1000° pure 



