The Influence of analytical error upon the interpretation of chemical variations in clay minerals

Warren, Edward; Ransom, Barbara

doi:10.1180/claymin.1992.027.2.05

Cited by 28 publications

(20 citation statements)

References 36 publications

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“…and X~A' corresponding to those for stoichiometric pyrophyllite (PYRO) and muscovite (MUSC) are indicated in the upper corners of the diagram. The size of the symbols corresponds to the minimum uncertainty in the amount of tetrahedral A1 or octahedral R 3 § calculated for each structural formula (Warren and Ransom, 1992).…”

Section: Site-charge Correlations and Compositional End Membersmentioning

confidence: 99%

“…3 The purpose of the present communication is to determine accurate compositional limits of solid solubility in smectite and illite by critically analyzing compositional data reported in the literature in order to generate sets of thermodynamic components that can be used to describe the chemical interaction ofillite and smectite with their mineralogic and aquatic environment. Many smectite and illite analyses have been published over the past 50 years (Ross and Hendricks, 1945;Kerr et aL, 1950;Earley et aL, 1953;Yoder and Eugster, 1955;Foster, 1951Foster, , 1953Foster, , 1954 3 As discussed by Warren and Ransom (1992), compositional ambiguities in Figure I also arise in part from plotting the compositions of clay minerals that are not perfectly dioctahedral on coordinates which require them to be so. .…”

Section: Introductionmentioning

confidence: 99%

“…However, most of the resulting structural formulas were not generated from chemical analyses of single crystals, but rather from bulk compositions of clay samples that appear from XRD analyses to be composed solely ofsmectite or illite. It has been shown recently (Warren and Ransom, 1992) that XRD analysis alone cannot be used to characterize unambiguously the mineralogy of clay samples. For example, standard XRD analyses of powdered samples in the 35-40 micron size fraction cannot detect less than ~0.5 wt.…”

Section: Introductionmentioning

confidence: 99%

“…For example, it can be seen in Figure 1 that extensive compositional overlap occurs among minerals designated in the literature as smectites, illites, and what are commonly called mixed-layered I/S clays. Although this overlap has been cited as evidence that smectite and illite form a mutual solid solution, it is not necessarily indicative of solid solution between them (Ransom and Helgeson, 1989;Warren and Ransom, 1992). 3 The purpose of the present communication is to determine accurate compositional limits of solid solubility in smectite and illite by critically analyzing compositional data reported in the literature in order to generate sets of thermodynamic components that can be used to describe the chemical interaction ofillite and smectite with their mineralogic and aquatic environment.…”

Section: Introductionmentioning

confidence: 99%

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Compositional End Members and Thermodynamic Components of Illite and Dioctahedral Aluminous Smectite Solid Solutions

Ransom

1993

Clays and clay miner.

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Abstract--Consideration of XRD, TEM, AEM, and analytical data reported in the literature indicates that dioctahedral aluminous smectite and illite form two separate solid solutions that differ chemically from one another primarily by the extent of A1 substitution for Si, the amount of interlayer K, and the presence of interlayer H20. The data indicate that limited dioetahedral-trioctahedral and dioctahedralvacancy compositional variations occur in both minerals. Excluding interlayer H20 and based on a half unit cell [i.e., Oto(OH)2], natural dioctahedral smectite and illite solid solutions fall within the compositional limits represented by Ao.3R~*Si40,o(OH)2-AR2+ R3+ Si40,o(OH)2-Ao2sR~3 R3178Alo.25Si3.7,Ojo(OH)2 for smectites and A~.sR3)`~d~5Sis.~'~(~H)2-A~5R~+5R3.~s~.'Si~9~'~(~H)2-~.9R2+R~9Si3.'~(~H)2 for illites, where A represents either monovalent cations or divalent cations expressed as their monovalent equivalent (e.g., Ca 2 § R-" + stands for the divalent cations Mg 2 § and Fe 2 § and R 3 + refers to the trivalent cations AP § and Fe -3 § Taking account of these compositional limits, smectite and illite solid solutions can be described in terms of nine and six thermodynamic components, respectively, all of which are consistent with both the law of definite proportions and the concept of a unit cell. Thermodynamic components that can be used to describe natural smectite solid solutions in terms of a half unit cell [i.e., Ojo(OH)2] can be expressed as NaA13Si30~o(OH)2, NaA13Si30~o(OH)2.4.5H20, A12Si40~o(OH)2 ,Cao.~A13Si3Oto(OH)2. Of these, NaAI3Si30~o(OH)2 " 4.5H20 provides explicitly for the presence of interlayer H20 in the mineral. Thermodynamic components representing illite solid solutions in natural systems can be written for a half unit cell as KA13Si30,o(OH)2, KMg3AISi30,o(OH),, KFe3AISi3OIo(OH)2 , AIzSi40~o(OH)~, KFe2AISi~O~o(OH)_,, and K3AISi40~o(OH)2. The calculations and observations summarized below indicate that neither smectite nor illite occur in nature as stoichiometric phases and that the two minerals do not form a mutual solid solution corresponding to mixed-layered illite/smectite.

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Section: Site-charge Correlations and Compositional End Membersmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Compositional End Members and Thermodynamic Components of Illite and Dioctahedral Aluminous Smectite Solid Solutions

Ransom

1993

Clays and clay miner.

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“…However, chemical analysis of phyllosilicates in low-grade rocks is fraught with difficulty because of the small grain sizes, mixed layering, and complex intergrowths that exist at scales beyond the resolution limits of the electron microprobe and conventional analytical techniques (Peacor 1992;Warren and Ransom 1992). Major contamination of an analysis is usually easily detected.…”

Section: Introductionmentioning

confidence: 99%

Chlorite Geothermometry?—Contamination and Apparent Octahedral Vacancies

Jiang

Peacor

Buseck

1994

Clays and clay miner.

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Abstract--The large contents of octahedral vacancies in published formulae of chlorite from hydrothermal systems and clastic sequences are shown to be largely caused by inclusion of other minerals. Verification is provided by analytical electron microscope (AEM) analyses of chlorite in pelitic rocks from the Gasp6 Peninsula, Quebec and the Gulf Coast, Texas. The Gasp6 chlorite occurs as discrete crystals locally coexisting with corrensite, and the Gulf Coast chlorite is free of mixed layers other than local serpentinelike 7-~ layers. Unlike most electron microprobe analyses (EMPA), but like other AEM analyses, the reported chlorite formulae do not have significant octahedral vacancies, are not Si-rich and (Fe + Mg)-poor relative to classic metamorphic chlorite, and have nearly equal amounts oftetrahedral and octahedral AI. The studied chlorites and those in metabasites and clastic rocks that could be positively identified as containing no or minimal mixed layering or submicroscopic intergrowths have little or no Ca or alkalis. In contrast, EMPA of chlorite reported for other clastic sequences show variable amounts ofNa + K + 2Ca that exhibit a poorly defined positive correlation with the proportion of octahedral vacancies. The EMPA of chlorite from the Salton Sea and Los Azufres geothermal fields that were suggested to contain temperature-dependent amounts of tetrahedral A1 (and thus used as "'chlorite geothermometers") show compositional characteristics similar to those reported for several saponite-chlorite transition series in metabasites.Continuous increases in octahedral occupancy and tetrahedral Al with increasing metamorphic grade are attributed to decreases in abundance of mixed layers or fine-grained intergrown minerals that commonly occur as a result of increasing crystal size and homogeneity in prograde sequences. Use of"chlorite geothermometry'" based on the proportion of apparent octahedral vacancies or tetrahedral A1 is therefore unwarranted and leads to inaccurate temperature estimates.

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