329 



The descriptions by Jack ( ? ) and Ransome ^ of the albite 

 present in albitite rocks, described by them, strongly suggest 

 the presence of chequer albite in these rocks. 



(b) Quartz Albitite.— D l J° = 2'640. This forms.the most 

 abundant type, and it is with this rock that the albite is 

 associated. Here the quartz is present as blue opalescent 

 grains as in the main granite. Microscopically, the minerals 

 present are quartz, albite, and as accessories apatite, zircon, 

 and rutile. The latter mineral is present in idiomorphic 

 prismatic crystals and, also, as geniculate twins (twinning 

 plane [101]), giving a sagenite network. 



The albite has a refractive index less than Canada Balsam. 

 In sections perpendicular to the albite lamellae, the 

 symmetrical extinction is 16° and on sections showing as 

 untwinned the maximum extinction is 19° from the (001) 

 cleavage. Further confirmation is provided by the extinction 

 given on (010) for the two parts of a Carlsbad twin. The sign 

 of the birefringence is positive. 



(c) Muscovite Albitite. — This is developed at Barn Bluff. 

 The rock in hand specimens has an altered appearance. 

 Through the felspar can be seen very small veinlets of quartz, 

 and portions of the felspar show alteration with limonitic 

 material. Muscovite is recognizable. Under the microscope, 

 the minerals present are albite, muscovite, and aecessorily, 

 quartz, apatite, zircon, and rutile. 



The albite possesses the same characteristics as in the 

 other occurrences, and the peculiar chequer twinning is 

 observed. 



Some of the mica is associated with quartz in little tufts 

 and rosettes. Both mica and quartz are probably secondary. 

 Some muscovite, however, is undoubtedly primary. The rock 

 has apparently suffered some change, due to the presence of 

 mineralizers, but the results are not as clearly demarcated as 

 in other examples. 



A study of the literature of albite, aplites, or pegmatites 

 indicates that this type of rock is comparatively rare. 



Rocks of this type were first described under the name 

 albitite by Turner ( 9 ) from Plumas Co., Sierra Nevada. These 

 aplites occur as dykes, and consist essentially of albite in 

 granular aggregates. Quartz is occasionally completely absent, 

 but may occur plentifully in the same dyke. Muscovite may 

 or may not be present. Iron ores and apatite are sparingly 

 distributed, and garnet is an occasional accessory. Duparc 



(7)R. L. Jack: Geol. Surv. S. Austr., Bull. No. 3, 1914, p. 16. 

 (8) F. L. Ransome: Journ. Wash. Acad. Sci., vol. i., No. 4, 

 1911, pp. 114-118. 



(9)H. W. Turner: 17th Ann, Rep. U.S.G.S., pp. 728, et seq. 



