SECT. 3] 



PELAGIC SEDIMENTS 



697 



majority of grains had a composition An < 30. As is true of quartz, most of the 

 fine-grained feldspar is probably wind -transported from continental elastics. 



Authigenic orthoclase has been observed in a few instances; Mellis (1952, 

 1959) described a sample from the west Atlantic in which orthoclase had re- 

 placed the original plagioclase in altered andesite boulders. This sample indi- 

 cates highly alkaline conditions during crystallization, as it is known from 

 controlled synthesis experiments that orthoclase grows only above a pH of 

 about 9 at low temperatures; directly below this limit, zeolites form the stable 

 phases. 



Rex (1958) attempted to establish the regional variability in the total amount 

 of feldspar in reference to the known amount of quartz in the Pacific. The great 

 variability of X-ray diffraction from given crystallographic planes in the feld- 

 spar series, as well as interference from other minerals in the range of inter- 



Feldspar/quartz 

 relative units 



10 r 



Ti/AI and Cr/AI 

 arbitrary units 



200 300 



Miles NNE of Oahu 



Fig. 26. Relative ratio feldspar/quartz in the North Pacific off Hawaii, showing the rapidly 

 vanishing influence of the basaltic province. This is also reflected by the decrease 

 in the relative concentration of titanium and chromium, mainly occurring in pyroxene 

 crystallites. (From Rex, 1958.) 



planar spacings used, limits the interpretation of these data. Nevertheless, some 

 general relationships appear well established, such as an increase in the relative 

 amount of feldspar around the Hawaiian Islands (Fig. 26) indicating the con- 

 tribution from the basaltic province. 



Olivine is unstable on the deep-ocean floor and hence is not observed in quan- 

 tities in pelagic sediments. However, in association with cosmic nickel-iron 

 spherules, olivine and pyroxene chondrules have been observed which strongly 

 resemble those in chondritic meteorites, and are probably of extra-terrestrial 

 origin (see further page 674 and Fig. 10b). 



Pyroxene minerals are ubiquitous and are useful indicators of the origin of 

 pyroclastics. The absence of hypersthene-enstatite among the frequently occur- 

 ring pjTOxene grains in South Pacific sediments indicates an origin within the 



