Mineralogical nomenclature in solid-solution series follows a system that has been called the 50% rule, more correctly the 100%/n rule or the dominant-constituent rule, in which the constituents are atoms (cations or anions), molecular groups, or vacancies. Recently developed systems of nomenclature for the arrojadite and epidote groups have shown that a group of atoms with the same valency state must also be considered as a single constituent to avoid the creation of impossible end-member formulae. The extension with this dominant-valency rule is imposed by all cases of coupled heterovalent-homovalent substitutions. End members with a valency-imposed double site-occupancy may result from single-site heterovalent substitutions and from coupled heterovalent substitutions at two sites where there is a disparity in the number of these two sites.

A simplified definition of a mineral group is given on the basis of structural and compositional aspects. Then a hierarchical scheme for group nomenclature and mineral classification is introduced and applied to recent nomenclature proposals. A new procedure has been put in place in order to facilitate the future proposal and naming of new mineral groups within the IMA-CNMNC framework.

Mineral species should be identified by an end-member formula and by using the dominant-valency rule as recommended by the IMA–CNMNC. However, the dominant-end-member approach has also been used in the literature. These two approaches generally converge, but for some intermediate compositions, significant differences between the dominant-valency rule and the dominant end-member approach can be observed. As demonstrated for garnet-supergroup minerals, for example, the end-member approach is ambiguous, as end-member proportions strongly depend on the calculation sequence. For this reason, the IMA–CNMNC strongly recommends the use of the dominant-valency rule for mineral nomenclature, because it alone may lead to unambiguous mineral identification. Although the simple application of the dominant-valency rule is successful for the identification of many mineral compositions, sometimes it leads to unbalanced end-member formulae, due to the occurrence of a coupled heterovalent substitution at two sites along with a heterovalent substitution at a single site. In these cases, it may be useful to use the site-total-charge approach to identify the dominant root-charge arrangement on which to apply the dominant-constituent rule. The dominant-valency rule and the site-total-charge approach may be considered two procedures complementary to each other for mineral identification. Their critical point is to find the most appropriate root-charge and atomic arrangements consistent with the overriding condition dictated by the end-member formula. These procedures were approved by the IMA−CNMNC in May 2019.

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In an effort to better understand vivianite formation processes, four Lake Baikal sediment cores spanning two to four interglacial stages in the northern, central and southern basins and under various biogeochemical environments are scrutinized. The vivianite-rich layers were detected by anomalous P-enrichments in bulk geochemistry and visually by observations on X-radiographs. The millimetric concretions of vivianite were isolated by sieving and analysed by X-ray diffraction, scanning electron microscope (SEM), microprobe, infrared spectroscopy, inductively coupled plasma atomic emission spectrometry and mass spectrometry (ICP-AES, ICP-MS). All the vivianites display similar morphological, mineralogical and geochemical signature, suggesting a common diagenetic origin. Their geochemical signature is sensitive to secondary alteration where vivianite concretions are gradually transformed from the rim to the center into an amorphous santabarbaraite phase with a decreasing Mn content. We analysed the spatial and temporal distribution of the concretions in order to determine the primary parameters controlling the vivianite formation, e.g., lithology, sedimentation rates, and porewater chemistry. We conclude that vivianite formation in Lake Baikal is mainly controlled by porewater chemistry and sedimentation rates, and it is not a proxy for lacustrine paleoproductivity. Vivianite accumulation is not restricted to areas of slow sedimentation rates (e.g., Academician and Continent ridges). At the site of relatively fast sedimentation rate, i.e., the Posolsky Bank near the Selenga Delta, vivianite production may be more or less related to the Selenga River inputs. It could be also indirectly related to the past intensive methane escapes from the sediments. While reflecting an early diagenetic signal, the source of P and Fe porewater for vivianites genesis is still unclear.