On the mineral nomenclatures: the dominant-valency rule

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Marco E. Ciriotti
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On the mineral nomenclatures: the dominant-valency rule

Messaggio da Marco E. Ciriotti » mar 14 ago, 2018 11:21

▪ Bosi, F. (2018): On the mineral nomenclatures: the dominant-valency rule. IMA2018 Abstract book. The ever-growing mineral diversity in the Earth and the Solar System: new minerals, including nano- and biominerals, and related nomenclature/classification issues, 354.

The development of nomenclature to identify and classify minerals is a fundamental step to understand the processes that govern mineral diversity. Apparently, two approaches could be used to distinguish mineral species: (1) the dominant-valency approach (e.g., Hatert and Burke 2008), which identifies mineral species by determining the most abundant proportion of root-charge arrangement; (2) the dominant-end-member approach, which identifies species by determining the most abundant proportion of end-member component. However, mineral species identified by the dominant valence may not necessarily correspond to the dominant end-member. In general, the dominant-valency rule should be preferred because it alone can lead to unambiguous mineral identification.
As the dominant-valency and the dominant-constituent rules are recommended by the IMA-CNMNC, it is worthwhile to analyze their basic aspects, here identified as follows:
–– - (i) the principle of the formula electroneutrality;
–– - (ii) the definition of end-member formula (Hawthorne 2002) that includes heterovalent-pair of ions at one site;
–– - (iii) the concept of charge-constraint at the crystallographic sites which leads to severe restrictions on the atomic arrangements.
However, the dominance-valency rule has a problem. It is based on the dominance of a chemical constituent of the dominant valency state at a given crystallographic site, where the term constituent refers to individual ions, molecular group, vacancy or group of ions with the same valency state. Unsuccessful applications of the dominance-valency rule, resulting in charge-unbalanced end-member formula, can occur if specific heterovalent-pairs of ions (or ion-vacancy) are not regarded in the same manner as a “single constituent”. In order to avoid the construction of unrealistic end-member formula, heterovalent-pair of ions can be required by charge-constraints, dictated by mineral composition.
The consequences of these findings are reflected in mineral series related by coupled substitutions. It is well-know that mineral series characterized by coupled heterovalent substitutions at two crystallographic sites are typically identified first by the dominant-valency rule and then by the dominant-constituent rule (e.g., Hatert and Burke 2008). The reason of this hierarchical procedure can be ascribed to the change of the site total charges involved in the substitution. On the other hand, one would expect to identify mineral series using directly the dominant-constituent rule if the site total charge is preserved in the chemical substitution. This is the case of mineral series characterized by coupled heterovalent substitutions at one crystallographic site, in which heterovalent-pairs of ions can be considered as single constituent for classification purposes. Some intermediate garnet and tourmaline compositions nicely show this issue.
Marco E. Ciriotti

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