SHAM EHOROCISI: . ae 65 
quartz, being less resistant to mechanical abrasion, but coarser than the 
clay substance. The term feldspar used in this connection is not strictly 
accurate, since there are a number of feldspathic minerals having various 
compositions. Yet since one kind of feldspar, the orthoclase, predomi- 
nates so largely, the error for all practical purposes is not great enough 
to affect the technical results. In speaking of feldspathic matter we, 
therefore, mean orthoclase. In some clays, oligoclase is more abundant 
than the orthoclase, but anorthite is as a rule insignificant in amount. 
The following table of chemical compositions will show the differ- 
ence in the constitution of four feldspars: 
Percentage Composition: |Percentage 
Chemical Ratio of 
NIENING. ilar, 7 =| Sites te 
Silica. |Alumina.| Potash. | Soda. | Cleum) Alumnia. 
Ort noGlasas | 1 0) A On GIO s oll CoO Used PIG cesoo |aogome BoO2 8 il 
Albite..... Wa AO, GOs | GS.70 | 19.8 looses. Tele ae eters 3.55 1 
Oligoclase..) Na,O CaO 4.5 SiO.) 63.00 DAY. Oats reat 10.0 30) 3.68 3 i 
AMO OT; || CHO) AOn BHOnool 43020 | O50 I occoce | acces ADI | Wolk 3 I 
The Function of Feldspar in Cement.—Feldspar, like kaolinite, 
is a silicate, it being a double silicate of alumina and alkali. The silica, 
being in the combined state, is readily available for the purposes of 
cement-making, and hence we find that on heating it easily combines 
with the lime. The difference between clay substance and feldspar in this 
respect is not great. Clay substance is more readily decomposed by lime 
on heating, but this is to some extent due to the extreme fineness of 
its particles and their shape, which makes them especially ready to 
come into intimate contact with the lime particles. feldspar is hence an 
admirable source of silica. The alkali, being a powerful flux, causes the 
clay containing feldspar to become less refractory. While clay sub- 
stance is an ideal refractory material, fusing at a temperature above the 
melting point of platinum, approximately at 1830° C., feldspar melts at 
about 1290° C. Hence any content of feldspar in clay is bound to reduce 
the melting point of the latter unless it has already been lowered below 
the melting point of feldspar by means of other fluxes, like iron, lime, 
magnesia, free alkalies, etc. 
We can therefore say that feldspar in clay which is used for making 
cement reduces the temperature of combination with lime, first on account 
of its initial greater fusibility; second, because of its readily available 
silica. We can say, hence, that it is a desirable constituent of clay, and 
the more so because it brings in a comparatively small amount of alumina, 
one part of silica introducing only 0.284 part of alumina. 
5—S. G. Bull. 38. 
