FINE-GRAINED NONCALCAREOUS MATERIAL 



127 



ON THE NATURE OF THE FINE-GRAINED NONCALCAREOUS MATERIAL 

 IN PACIFIC DEEP-SEA CLAYS 



The results obtained from this study are of two main 

 sorts: first, those which throw light on the distribution 

 of calcium carbonate in Pacific deep-sea deposits, and 

 which have been discussed in the preceding section; sec- 

 ond, information obtained from chemical, mechanical, 

 and X-ray analyses, and from a study of temperature- 

 dehydration relations, on the nature and origin of the 

 fine-gi-ained noncalcareous material which is the prin- 

 cipal constituent of Pacific deep-sea clays. We may 

 now summarize and coordinate the results of the second 

 kind, especially with regard to the red clays of the north- 

 east Pacific. 



(1) The chemical composition of deep-sea clays 

 from the northeast Pacific is very constant, the extreme 

 values of the silica sesquioxide ratios in the ten samples 

 analyzed deviating by only 3 per cent from the average 

 value of 4.0. The molecular amount of alumina is near- 

 ly four times that of ferric oxide. There are more 

 equivalents of divalent than of monovalent bases; the 

 order of importance being magnesium, potassium, sodi- 

 um, and calcium; the ratio between silica and total bases 

 is close to 7. Owing to the presence of siliceous re- 

 mains of organisms, the ratios between silica and ses- 

 quioxides in the remainder of the noncalcareous samples 

 analyzed are wider than in northeast Pacific clays, but 

 the proportions between the other constituents are of 

 about the same order of magnitude. 



(2) Colloidal fractions separated from these clays 

 contain relatively less silica and more alumina and iron 

 than do the whole samples. The following empirical for- 

 mula roughly agrees with the observed chemical compo- 

 sition of the colloidal fractions (the amount of water not 

 being taken into account): 



(Mg, K2, Na2, Ca)0 • 2(Al,Fe)203 • 6 Si02 



This is close to the formula of the clay mineral beidel- 

 lite, which often is found in bentonites and is similar to 

 the principal clay mineral or minerals of many soil col- 

 loids. The fact that the average silica sesquioxide ratio 

 in the colloidal fractions is somewhat less than 3 indi- 

 cates that more or less kaolinite, halloysite, or musco- 

 vite may be present. 



(3) The colloidal fractions from two northeast Pa- 

 cific red clays were found by Professor W. P. Kelley to 

 have base exchange capacities of about 55 milliequiva- 

 lents per 100 grams, close to those of two California 

 soil colloids which are believed to consist largely of a 

 beidellite-like clay mineral, together with quartz. Mag- 

 nesium apparently is the chief replaceable base. 



(4) Northeast Pacific red clays and a terrigenous 

 clayey mud from off the South American coast were 

 found by Dr. P. G. Nutting to dehydrate on heating in a 

 manner similar to soil colloids containing a beidellite- 

 like mineral. Water is gradually driven off between 

 room temperature and approximately 450°, after which 

 there is a relatively sharp decrease of about 3 per cent 

 in water content between 450° and 600°, followed by a 

 further slight loss at higher temperatures. The curves 

 of these clays differ from curves obtained on bentonites 

 in that the latter lose more water below 100°. 



(5) The X-ray powder diagrams given by colloidal 

 fractions separated from northeast Pacific red clays are 



closely comparable both in position and intensity of dif- 

 fraction lines with those of California soil colloids 

 containing a beidellite-like clay mineral and quartz. The 

 principal lines of quartz are present in every noncalcar- 

 eous sample examined. The greater intensity of quartz 

 lines in the patterns of separates containing coarser par- 

 ticles explains the relatively higher silica sesquioxide 

 ratios of the whole samples, as compared with their col- 

 loidal fractions. Kaolinite, halloysite, or muscovlte are 

 perhaps also present. 



(6) As in chemical composition, most red clays from 

 the northeast Pacific are strikingly alike also in the dis- 

 tribution of particle sizes, having median diameters of 

 about 1.1 microns, coefficients of sorting slightly less 

 than 3, and small negative skewnesses. Particles of 

 sand size, mostly organic skeletal remains, are usually 

 present in amounts less than 1 per cent, whereas the 

 average proportions of silt and clay are about 13 and 87 

 per cent, respectively. 



Several converging lines of evidence thus lead to two 

 conclusions. (1) The red clays of the northeast Pacific, 

 and probably also terrigenous and pelagic noncalcareous 

 clays of certain other areas, contain about 50 per cent of 

 particles less than one micron in diameter. (2) These 

 colloidal particles are almost exactly similar in many 

 ways to those soil colloids which are formed under con- 

 ditions of moderate leaching by neutral or slightly alka- 

 line waters, and consist of beidellite-like clay minerals 

 together with small amounts of quartz. Quartz is a major 

 constituent also of the fractions of larger particle size 

 in these sediments. 



Two explanations are possible for the ultimate ori- 

 gin of the fine-grained materials in red clays from the 

 northeast Pacific. They may have been formed in situ, 

 as Murray believed, from the decomposition of volcanic 

 debris, or they may represent finely divided soil mate- 

 rial which has been carried from the continents in sus- 

 pension in the sea, or by the wind. 



Most bentonites, which probably were formed by the 

 decomposition of acid volcanic ash, contain quartz to- 

 gether with clay minerals of the montmorillonite beidel- 

 lite group. Many residual soils formed in situ are as 

 fine-grained as these red clays. It is possible, therefore, 

 that the red clays may have been formed by decomposi- 

 tion on the surface of the sea floor under conditions 

 which over long periods effectively reproduced those of 

 certain types of subaerial weathering; namely, oxidizing 

 conditions and slow leachin;^ by slightly alkaline waters 

 which contain large amounts of metallic cations. 



On the other hand, the striking similarity between 

 the red clays and soil colloids strongly suggests that at 

 least considerable amounts of these materials may be 

 soils transported from land. 



It is interesting to speculate on the means of trans- 

 portation whereby that part of the fine material which is 

 of continental origin has been carried to its present po- 

 sition. Many analyses, by various workers, of atmos- 

 pheric dust and of fine-grained wind deposits, such as 

 loess, show that these are coarser in texture than deep- 

 sea clays and contain small amounts of clay minerals. 

 It seems reasonable to believe, therefore, that the col- 

 loidal materials have been transported chiefly in suspen- 



