Glauconite average

Glauconite is also a mica mineral with a large part of the octahedral A1 replaced by Fe(III). An average composition (14) is... 

Figure 2. Projection on planes Na—K—Mg and Ca—K—Mg of the composition of possible equilibrium phases and of marine sediment minus carbonate. CH = chlorite, IL — illite (hydromica), = montmorillonite, PH = phillipsite, SED = sediments [according to Goldschmidt (7)], CA = average calcareous, SI = average siliceous, AR = average argillaceous sediments (from Ref. 22), OW = ocean water, GL = glauconite,...

The converse is true of the Mg ion. It is more abundant in the octahedral sheets of the low-temperature 2 1 dioctahedral minerals, attaining an average value of 3.55% in the montmorillonites and even higher values in glauconite and celadonite. Mg in the octahedral position increases the size of the octahedral sheet and decreases structural strain. 

A number of people have compiled data on the range and average composition of glauconite (Hendricks and Ross, 1941 Smulikowski, 1954 Borchert and Braun,1963). For the present review 69 analyses and 82 structural formulas from the literature were selected. 

Even though the three calculations have a number of analyses in common, the similarity is more than one might expect. The slightly lower interlayer ion content in the present average formula is probably due to the inclusion of more lMd glauconites... 

The cation population of the octahedral sheet of glauconite includes a wider range of values than is found for most other 2 1 clays, with possible exception of the beidellites and nontronites. The average for the 82 samples is 2.054 and 77% of the values are larger than 2.00. The values range from 1.94 to 2.31. Some of these high... 

Fig.6. Variations of average interlayer cation total with average octahedral total for sixty-six analyzed glauconites and celadonites. Data divided into eight approximately equal groups. (After Tyler and Bailey, 1961.)...

Divalent iron is considerably more abundant in glauconite than in illite and montmorillonite although the Mg content of glauconite is similar to that of mont-morillonite. Octahedral Fe3+ is five times more abundant in glauconite than in illite and montmorillonite, and octahedral A1 is less than one-third as abundant. The total number of trivalent ions in the octahedral position averages 1.45 as compared to 1.76 for montmorillonite and 1.68 for illite. The distribution of total trivalent ions in the octahedral sheet of glauconites is approximately normal (Fig.5). Reported values ranged from 1.15 to 1.89 however, as with the A1 2 1 clays, there is a deficiency of values less than 1.30. 

In glauconites, octahedral A1 ranges from 0.00 to 1.28 and Fe3+ from 0.35 to 1.77 (11 out of 82 samples have more octahedral A1 than Fe3+). Within these limits, there is a continuous isomorphous series between A1 and Fe3+. The composition of the basic unit cell of many glauconites is within the range of composition of the nontronite clays however, on the average, glauconites have more Fe2 + and Mg and less tetrahedral A1 than the average nontronite. The most variable components within the layer are the Al3+ and Fe3+ ions in the octahedral sheet. Fe3+ increases as A1 decreases. 

Table XXI, after Smulikowski (1954), shows the average composition of glauconites divided on the basis of age and lithology. Smulikowski concluded ...

Analysis No. 1 represents the only truly tetrasilic celadonite. The average value for Si in the 17 analyses is 3.83. Except for Nos. 17-19 variations are between 3.73 and 4.00. Octahedral occupancy is close to 2 for all celadonites, with an average of 2.05. Interlayer cations often number substantially less than 1.0 and average 0.81 with a low value of 0.38 (No. 14). This, of course, is typical also for glauconites and hydrous micas. ... 

Present data indicate that Fe3+-rich low-charge clays increase their layer charge by increasing the Mg and Fe2+ content of the octahedral sheet at the expense of Fe3 + more so than of Al. The average Al content of glauconite and celadonite is similar to that of nontronite, but the Fe3+ values are lower. With increased octahedral charge there is an increase anion-anion repulsion and the octahedral sheet increases relatively more in the c direction than the 6 direction, which also favors the large cations. Thus, relatively less tetrahedral Al is required to afford the sheet size differential to allow sufficient tetrahedral rotation to lock the K into place. 

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