THE FELSPARS. 



while aii^ite sinks. By careful dilutioii (lUiirlz and felspars can be 

 then separated, and even Ortlioclase, from the Triclinic ones. This is 

 also proposed by Goldschmidt* as a very convenient and very accurate 

 means of obtainin<,' the sp. gr. of the constituent minerals of a rock 

 even when only one or two j^rains can be detached, for if the dilution 

 be carried so far that the fragment remains suspanded anywhere 

 indifferently in the fluid the sp. gr. of the solid and that of fluid must 

 be e<]ual, and ma\' be easily determined in the case of the latter by 

 means of the sp. gr. bottle. 



The optical method of determiuin what is the particular felspar 

 in a rock is founded on the fact that the position of the optic axes 

 with respect to those of form varies in the different species. Of 

 course Ortlioclase is distinguished from the Plagioclases by its mono- 

 clinic symmetry — i.e., iu a section of a rock some crystal sections will 

 probably be found iu which " extinction " happens when the com- 

 position line of the twin structure is parallel to the principal plane of 

 one of the Nicols. It is, however, the discrimination of the Triclinic 

 specifics which is so difficult, and in many cases quite impossible. We 

 have to pick out in the section those cr3'stal sections which extinguish 

 at ejual angles ou both sides the Nicol plaue, and then measure this 

 angle in as many cases as we cau find. Now symmetrical extiuctious 

 only show that the plane of section has accidentally passed through the 

 crystal perpendicular to the twinning plaue, and therefore the 

 extinction angle may vary within wide limits, and it is only by noticing 

 the maximum angle that we cau form any conclusion whatever; f e.fj., 

 in trying to determine a felspar a short time ago I found an extinction 

 of 10° on one side and 17^° on the other — i.e., 33i° from one to the other. 

 Now if I had only been able to find this one tolerably symmetrical 

 extinction I could not have told which felspar it was. It would 

 probably not have been Albite, seeing that the angle in that case 

 should not have exceeded 31^", but I could have gone no further. 

 When, however, I found further angles of 44°, 48°, 53J°, 54°, 56°, 58°, 

 66°, 71°, 73°, there was enough to assure me of the presence of a very 

 basic Livie felspar, and the observation of one grain showing the zonal 

 structure to be presently mentioned with an extinction angle of 48° iu 

 some parts and of 82° in others, made the presence of both Labradorite and 

 Anorthite almost a certainty. Iu this case a large number of observa- 

 tions could be made, and therefore such a degree of probability 

 produced that I was not at all surprised tj find the observations quite 

 confirmed by Szabo's flame reactions, some of the grains which I 

 was able to detach being much more fusible than others, and all 



* Neues Jahrbuch I., 18S1. Beilage Band, p. 179. 



t The followius table of the ansle between the extiuctiou positions of two 

 Twin Lamellffi in the various Felspars may be of service. The section is 

 supposed perpendicular to the plaue of twin' composition, aud the auglo given 

 is the maximum observable for each species: — Ortlioclase, 0"; Microcline, 3G°; 

 Albite, 314- ; OliKoclase, 37-; Labradorite, Glr'; Auorthite, 74*-aud upwards. 

 Iu sections parallel to the Basal cleavage : — Microcliue, 30 — 32- : Albite, 7^ — S-" 

 Oligoclase, -2,°— 3^ ; Labradorite, 10-^—14*- ; Auorthite, 57-'— 74^. 



