Conventions in Reporting Chemical Analyses

Chemical analyses of minerals are communicated through a set of standardized conventions that allow results from different instruments and laboratories to be compared directly. Understanding these conventions is essential for reading published analyses correctly and for detecting potential errors or missing components.

Major Elements: Weight Percent Oxides

Major element compositions are reported as weight percent (wt%) of each constituent. For minerals in which oxygen is the principal anion — the great majority of minerals — the convention is to report each metal element not as the element itself but as the weight percent of its corresponding oxide (e.g., SiO₂, Al₂O₃, FeO, Fe₂O₃). In this procedure, the amount of oxygen is not directly measured. Instead, the weight percent of each cation is measured and the oxygen required by the stoichiometry of the relevant oxide is calculated and added in. For simple anion minerals such as arsenopyrite, the report uses elemental percentages directly — for example, 34.30% Fe, 46.01% As, and 19.69% S. The reported oxide forms do not represent actual oxide species present in the mineral’s crystal structure; they are simply a mathematical convention. ^[1]

Reporting Water: H₂O⁺ and H₂O⁻

Water in a mineral sample may appear in two fundamentally different forms and must be reported separately. H₂O⁻ represents actual molecular water adsorbed onto the surfaces of mineral grains or trapped in small voids in the sample. This water is not part of the crystal structure and is detected by measuring the weight loss when the sample is heated to 100–110°C. H₂O⁺ represents hydrogen that is genuinely part of a mineral’s crystal structure — as in hydroxyl (OH⁻) groups in micas or amphiboles — though not necessarily present in the form of water molecules. Water analyses are not normally very reliable, and H₂O⁺ data are typically available only from wet chemical methods. ^[1]

The F and Cl Oxygen Correction

When fluorine or chlorine is present in a mineral — as in fluorapatite, phlogopite, or hornblende — a correction must be applied to the oxygen total. The standard oxide-reporting procedure assumes all anions are oxygen, but F⁻ and Cl⁻ displace oxygen on a one-for-two basis: for every two F⁻ or Cl⁻ actually present, one O²⁻ must be subtracted. The correction term (labeled O ≡ F,Cl in the analysis table) is calculated as:

O ≡ F,Cl = 0.226 × wt% Cl + 0.421 × wt% F

This value is subtracted from the raw oxide sum to give the corrected total. In a typical biotite analysis (Table 9.2), for example, the raw sum reaches 100.38%; subtracting the O ≡ F,Cl correction of 0.46% brings the corrected total to 99.92%. ^[1]

Analytical Totals

Analytical totals rarely sum to exactly 100%. A total below 100% may indicate that one or more elements were not detected or not included in the analysis. A total at or above 100% does not guarantee completeness — compensating errors (one element measured too high and another too low) can produce an apparently satisfactory sum. Shared reference standards — carefully analyzed mineral or rock samples distributed among laboratories — are used to calibrate instruments and ensure that analyses from different facilities are directly comparable. ^[1]

Trace Elements

Trace element concentrations are reported in parts per million (ppm) by mass, or occasionally as ppm of the oxide. Trace elements either substitute in small amounts for a major cation in the crystal structure, or they occupy interstitial sites not normally occupied in the ideal structure — in either case they represent structural defects. Rare earth element (REE) data are routinely normalized by dividing the measured concentration of each REE by its abundance in an average chondrite meteorite. Chondrites are primitive meteorites whose composition is inferred to represent the bulk chemistry of the solar system. Chondrite normalization removes the saw-tooth Oddo–Harkins effect (even-numbered elements are more abundant than odd-numbered neighbors) and allows REE patterns from different samples and localities to be compared directly on the same plot. ^[1]

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