Structural glauconite

Other resolubilized trace metals precipitate as replacement ions in existing solids such as fecal pellets and bone. Examples of these fiassilized materials include barite, phosphorite, and glauconite. These precipitates contain small amounts of a variety of trace metals as well as other elements. As a result, their chemical composition is variable and their structure is usually amorphous, making it difficult to assign them an empirical formifia. 

The main method used to distinguish the relative quantities of neoformed illite is by the polymorph or structure of the material. Using the criteria that 2M and 3T polymorphs of dioctahedral potassic mica are high temperature forms (Velde, 1965a), the determination of the relative quantities of lMd, and 1M vs. 2M, 3T polymorphs permits a semi-quantitative estimation of the proportion of neo-formed or low temperature illite present in a specimen. A method commonly used is a determination of relative intensities of X-ray diffraction peaks of non-oriented mica (Velde and Hower, 1963 Maxwell and Hower, 1967). Usually only 2M and lMd polymorphs are present in illite specimens which simplifies the problem. The 1M polymorph is typical of ferric illites and celadonite-glauconites, the more tetrasilicic types. 

It is interesting to note that the 1M polymorph represents an ordered form while lMd structures are disordered (Guven and Burnham, 1967) and that the typical sequence in the process of glauconitization is lMd to 1M (Burst, 1958). Illite remains, for the most part, disordered even in Paleozoic sedimentary rocks (Velde and Hower, 1963). This would suggest that the glauconite structure, being more symmetric, might be more stable than illite, a point which will be discussed when experimental studies are considered. 

Figure 17. Proposed phase relations where K is a mobile component and Al, Fe are immobile components at about 20°C and several atmosphere water pressure for aluminous and ferric-ferrous mica-smectite minerals. Symbols are as follows I illite G = non-expanding glauconite Ox = iron oxide Kaol = kaolinlte Mo montmorillonite smectite N nontronitic smectite MLAL aluminous illite-smectite interlayered minerals Mlpe = iron-rich glauconite mica-smectite interlayered mineral. Dashed lines 1, 2, and 3 indicate the path three different starting materials might take during the process of glauconitization. The process involves increase of potassium content and the attainment of an iron-rich octahedral layer in a mica structure.

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. 

The phenomenon of increased hardness occurs principally in minerals of sheet and chain structures, which link together through the cations (silicates and aluminosilicates, as well as hydrated sheet minerals, such as glauconite, melilite and gypsum—M ranging from 0 to about 1.25), and also in minerals of skeletal structures (borates, phosphates, sulphates, nitrates, carbonates, such as calcite, dolomite and others—Ah from 0 to about 1.15). For this reason, the hardness analysis of minerals with weak bonds demands consideration of the fact that just as the basic crystallo-chemical factors, so is hardness influenced by the form of domains (component parts of structures) in all anisodesmic minerals of chain, sheet or skeletal structure. Depending on the form of domain (and also according... 

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. 

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.

Since this review was originally completed, Foster (1969) published a review in which similar conclusions are drawn about the glauconites and celadonites. The lack of correlation between iron and potassium content in glauconite is substantiated in her paper. Foster considered the process of glauconitization to be of two separate, unrelated processes, incorporation of iron into the crystal structure and fixation of potassium in interlayer positions, with incorporation of iron and development of negative layer charge preceding complete fixation of potassium . 

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). 

The Fe2+ content of celadonites (continental origin) and glauconites (marine origin) is identical suggesting that its abundance is not controlled by environmental conditions. Structural control is more likely. Apparently, layer strain is less and the structure more stable when there are 0.20 large Fe2+ ions in the octahedral sheet. 

Cimbalnikova, A., 1971. Chemical variability and structural heterogeneity of glauconites. Am. Mineralogist, 56 1385-1392. 

Bailey SW (1975) Cation ordering and psendosymmetiy in layer sihcates. Am Mineral 60 175-187 Bailey SW (1984a) Classification and structures of the micas. Rev Mineral 13 1-12 Bailey SW (1984b) Crystal chemistry of the true micas. Rev Mineral 13 13-60 Bailey SW (1984c) Review of cation ordering in micas. Clays Clay Minerals 32 81-92 Bailey SW (1986) Report of the AlPEA Nomenclature Committee (llhte, Glauconite and Volkonskoite). AIPEA Newsl 22 1-3... 

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