Properties of the Feldspars. 121 



remaining crystal fragments were relatively very small com- 

 pared with the separating lanes of glass, but the orientation 

 of the tiny particles still remained perfectly undisturbed. 



The evidence contained in this series of slides shows plainly 

 that we have here an unfamiliar condition — a case of a crys- 

 talline compound persisting for a long time above its melting 

 temperature for a given pressure. Albite or orthoclase glass sin- 

 ters tightly at 800°. At the temperature when melting began, 

 therefore (below 1200°), the charge consisted of crystal frag- 

 ments of microscopic size imbedded in a large vitreous mass of 

 the same composition and known temperature. These fragments 

 melted so slowly over the 50° included between the first slide 

 (Plate I, 2) and the last (Plate I, 5), with the rate of heating 

 slow (1° in 2 min.), and the upper temperature -continued for 

 30 minutes, as to leave considerable portions unmeltecl at the 

 close. Furthermore the extreme viscosity, of which further 

 evidence will be given directly, and the absence of any dis- 

 turbance in the orientation of the particles indicating flow, 

 assured us that the lanes of glass represented actual melting 

 and not an inflow of glass from without. Finally, the per- 

 fectly homogeneous character of the glass and the unchanged 

 appearance of the crystals as heating progressed, gave no hint 

 of any chemical decomposition. 



In the hope of obtaining a point of value for comparison 

 with the melting points of the other feldspars, some time and 

 patience were expended in trying to locate the lowest tempera- 

 ture at which certain evidence of melting appeared. We did 

 not extend any single trial beyond a single day, so that our 

 results can not pretend to establish the lowest point at which 

 albite melts. Such an effort with a natural specimen known 

 to contain impurities would yield nothing of value. Mitchell 

 County albite showed signs of melting after four hours at 

 1100°. Under a high power the crystal edges appeared 

 weathered or toothed — strongly resembling the incipient melt- 

 ing of the ice on a frosted window pane. These extremely 

 fine teeth could be followed through the slide on exposed 

 edges. At 1125° (Plate I, 6, X300) a four-hours heating gave 

 unmistakable glass in tiny pockets and lanes. 



The above experiments with the Cloudland albite were com- 

 pleted before we obtained the Amelia County material, but the 

 latter proved to be so much nearer to the type of pure soda 

 feldspar that nearly all the experiments were repeated with it 

 except that the crystal blocks were imbedded in powdered 

 crystals. We did not develop any new fact, however ; the 

 effects noted above reappeared in the same order except per- 

 haps that melting went on a little faster in the Amelia County 

 specimen. As much melting was found after one-half hour at 



