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The crystal structure of tobermorite 14 Å (plombierite) was solved by means of the application of the order-disorder (OD) theory and was refined through synchrotron radiation diffraction data. Two polytypes were detected within one very small crystal from Crestmore, together with possibly disordered sequences of layers, giving diffuse streaks along c à . Only one of the two polytypes could be refined: it has B11b space group symmetry and cell parameters a 5 6.735(2) Å , b 5 7.425(2) Å , c 5 27.987(5) Å , c 5 123.25(1)1. The refinement converged to R 5 0.152 for 1291 reflections with F o 44s(F o ). The characteristic reflections of the other polytype, F2dd space group, a % 11.2 Å , b % 7.3 Å , c % 56 Å , were recognized but they were too weak and diffuse to be used in a structure refinement. The structure of tobermorite 14 Å is built up of complex layers, formed by sheets of sevenfold coordinated calcium cations, flanked on both sides by wollastonite-like chains. The space between two complex layers contains additional calcium cations and H 2 O molecules; their distribution, as well as the system of hydrogen bonds, are presented and discussed. The crystal chemical formula indicated by the structural results is Ca 5 Si 6 O 16 (OH) 2 Á 7H 2 O. J ournal

Brief outline 2 1.2 Hierarchical classification of structures 3 1.3 Short recapitulation of non-modular categories of similarity 5 1.4 Elements of modular description. Types of homologous/ polysomatic series 9 1.5 Polysomatism 35 1.6 Chemical composition series 41 1.7 Variable-fit homologous series and series with a combined character 42 1.8 Merotype and plesiotype series 57 1.9 Ordered derivatives of solid solutions as modular structures 110 1.10 Principles of prediction of modular structures 113 2 OD structures 127

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The name ‘tobermorites’ includes a number of calcium silicate hydrate (C-S-H) phases differing in their hydration state and sub-cell symmetry. Based on their basal spacing, closely related to the degree of hydration, 14, 11 and 9 Å compounds have been described. In this paper a new nomenclature scheme for these mineral species is reported. The tobermorite supergroup is defined. It is formed by the tobermorite group and the unclassified minerals plombièrite, clinotobermorite and riversideite. Plombièrite (‘14 Å tobermorite’) is redefined as a crystalline mineral having chemical composition Ca5Si6O16(OH)2·7H2O. Its type locality is Crestmore, Riverside County, California, USA. The tobermorite group consists of species having a basal spacing of ~11 Å and an orthorhombic sub-cell symmetry. Its general formula is Ca4+x(AlySi6–y)O15+2x–y·5H2O. Its endmember compositions correspond to tobermorite Ca5Si6O17·5H2O (x = 1 and y = 0) and the new species kenotobermorite, Ca4Si6O15(OH)2·5H2O (x = 0 and y = 0). The type locality of kenotobermorite is the N'Chwaning II mine, Kalahari Manganese Field, South Africa. Within the tobermorite group, tobermorite and kenotobermorite form a complete solid solution. Al-rich samples do not warrant a new name, because Al can only achieve a maximum content of 1/6 of the tetrahedral sites (y = 1). Clinotobermorite, Ca5Si6O17·5H2O, is a dimorph of tobermorite having a monoclinic sub-cell symmetry. Finally, the compound with a ~9 Å basal spacing is known as riversideite. Its natural occurrence is not demonstrated unequivocally and its status should be considered as “questionable”. The chemical composition of its synthetic counterpart, obtained through partial dehydration of tobermorite, is Ca5Si6O16(OH)2. All these mineral species present an order-disorder character and several polytypes are known. This report has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification

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