PETROLOGY AT GOOSE CREEK SHANNON. 



Analysis of albitic pegmatite. 



25 



Constituent. 



SiO,.. 



AlA- 



FeoOs 



FeO. 



MnO- 



MgO- 



CaO- 



NaaO- 



K2O-. 



TiOs. 



P2O5- 



H,0-. 



Per cent. 



68. 74 



13.24 



1.22 



1. 38 

 . 05 



2. 02 

 5.90 

 5. 76 



. 36 



1. 44 



. 59 



,46 



101. 16 



Ratios. 



I. 145 

 . 129 

 .008 

 .019 



. 051 

 . 105 

 . 094 

 . 004 

 .018 

 .004 



Norm. 



Quartz 23. 101 



Orthoclase 2.22 



Albite 49.25 



Anorthite 8. 62J 



Apatite 1. 34 



Magnetite 1. 86 



Ilmenite 1. 67 



Titanite 1.37 



Wollastonite .35 



Diopside 11. 02J 



Salic 83. 19 



/ Femic 17. 61 



100. 80 



This norm falls into class III, order 3, rang 2, subrang 5 of the 

 quantitative classification. The extreme difference between the 

 composition of this rock and that of the normal pegmatite is well 

 shown by their respective norms. 



The field relations of the albitic pegmatites to the normal plagio- 

 clasic diabase pegmatite deserve further study as the work in the 

 quarry progresses from year to year. In many cases the two types 

 of rock seem intimately mixed, probably as the result of Assuring 

 of the main rock body during consolidation. In many of the masses 

 the albitic rock, rich in micropegmatite, seems to be located more or 

 less centrally within a larger surrounding mass of the plagioclase rock, 

 which is where they should normally occur if the hypothesis of their 

 origin here favored is correct. 



An interesting example of transition from one type to the other is 

 furnished by the specimen illustrated in plate 8. At the bottom of 

 the plate, the base of the feathery aggregate of augite, which presum- 

 ably grew outward from the wall of the chamber in which this mass 

 consolidated, the rock contains feldspar which seems to be largely 

 plagioclase. A short distance upward in the specimen the feldspar, 

 both in the larger crystals and in the interstitial micropegmatite, 

 becomes largely albite, and the borders of the augite blades are largely 

 altered to diopside. The rock contains small miarolitic cavities and 

 considerable amounts of chalcopyrite which has in part replaced feld- 

 spar of micropegmatite, giving quartz-chalcopyrite micrographic inter- 

 growths. The specimen as a whole seems pseudomorphous after 

 normal diabase pegmatite, the albite and diopside appearing to have 

 been developed by substitution of material from a residual liquid 

 located centrally in the pegmatite mass, although the possibility that 

 this reaction was hydrothermal rather than magmatic is by no means 

 precluded, nor is it certain that the albite of the rock is not a 

 product of original magmatic consolidation. 



