Glauconite structural formulas

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. 

TABLE XVI Statistical data on structural formulas of eighty-two glauconites ... 

Table XVII contains a selection of glauconite analyses and structural formulas. No effort was made to determine the free iron oxide in most of these samples. Bentor and Kastner (1965) found 2.09% and 3.85% free iron oxide in two samples of the seven they analyzed. Apatite is a common constituent of many glauconites and, as a result, fairly high P2O5 values are often reported.

Average compositions and structural formulas of selected glauconite suites (after Smulikowski, 1954)... 

Parry and Reeves (1966) reported a lustrine glauconitic mica that has a structural formula intermediate between glauconite and illite. Porrenga (1968) also reported a clay mineral of an intermediate nature (Table XXIII). 

It can be surmised that even though X-ray data indicated only expandable material, there must be significant interlayering with illite or other non-expandable mica-like phases such as glauconite-celadonite in order to give such a high structural charge imbalance. If not, one wonders why illites, with a similar chemical formula, are not expandable as well. 

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