The hydrothermal synthesis of kaolinite was examined in the Al2O3-SiO2-H2O system to study inhibitory effects of additional ions on the formation of kaolinite. Syntheses were carried out with amorphous starting materials and salt solutions of various concentrations in Teflon pressure vessels at 220°C for 5 days. The reaction products were characterized by XRD, IR, DTA-TG, NMR and TEM. In all of the runs using solutions with cation concentrations less than 0.001 M, no significant effect on the formation of kaolinite was observed. The inhibitory effect of the univalent cations Li+, Na+ or K+ was less than that of divalent cations such as Mg2+ or Ca2+. The addition of trivalent Fe3+ or excess Al3+ ions interfered with the formation of kaolinite significantly. Sulfate and acetate solutions interfered with the formation of kaolinite more than chlorides and nitrates. No crystalline product was obtained using a 1.0 M basic solution of carbonate or hydroxide. The addition of the lithium ion to the system affected the crystallization of kaolinite only slightly. The use of 0.1 M LiCl and LiNO3 solutions for the syntheses improved crystallization of kaolinite along the [001] direction.

A comparative study is reported in which kaolinite has been hydrothermally synthesized at several pH conditions. The syntheses were carried out at 220 °C for 3 to 10 d with distilled water or acidic solutions using a mixture of silica-gel derived from alkoxide and gibbsite with a Si/Al ratio of 1:1 as the starting material. Use of acidic solution for the synthesis promotes the dissolution of the starting materials and leads to kaolinitization at an earlier stage of the reaction. However, the rate of kaolinitization is found to be rather slow, in comparison to the reaction with distilled water. The synthetic kaolinite was characterized by X-ray powder diffraction pattern. Kaolinite synthesized with distilled water was poorly grown for direction of the stacking. For example, crystallite size along the c*-axis = 155 Å, whereas kaolinite synthesized with acidic solution gave a higher crystallite size along the c*-axis, such as 253 Å in the case of the synthesis with 0.1 N HCl. Hinckley index of the synthetic kaolinite was varied from 0.35 to 0.80 by the acidity of the reaction. Different kaolinitization processes are implied by differences observed in the rate of kaolinitization, which has an influence on the nature of the stacking faults of the kaolinite.

The crystal structure of synthetic deuterated kaolinite was refined by Rietveld analysis using time-of-flight (TOF) neutron powder diffraction data. For non-hydrogen atoms, Cl symmetry was assumed. Starting models were tested in which only the direction of O-D vectors was varied. The constraints were introduced to all Al-O, Si-O and O-D bonds. The refinement adopting the former gives Pl(C1), a = 5.169(1) Å, b = 8.960(2) Å, c = 7.410(2) Å, α = 91.26(2)°, ß = 104.99(2)°, γ = 89.93(1)°, Rwp = 3.17%, R1 = 5.78% and S = 1.34 with constraints of l(Al-O)= 1.93 ± 0.05 Å, l(Si-O)= 1.62 ± 0.03 Å and l(D-O) = 0.95 ± 0.15 Å. The inner O-D vector points toward the tetrahedral sheet. All inner-surface O-D groups form H bonding with basal O atoms in the next kaolinite layers. The results agreed with those obtained from natural kaolinite.

Kaolinite was hydrothermally synthesized from two kinds of silica-alumina gels to examine the effect of the structure of the starting material. Two kinds of gels were prepared by precipitation at different pH conditions (pH = 9.6 and 4.2) from solutions containing water glass and aluminum sulfate. Na ions in the gels were removed with a resin before the hydrothermal treatment, but a slight amount of sulfate ions was still present in the gels. The nuclear magnetic resonance spectra of the starting gels suggested that the gel prepared at pH 9.6 consists of networks with alternating SiO4- and A1O4-tetrahedra (partially AlO6-octahedra), whereas the gel prepared at pH 4.2 consists of a sheet structure related to allophane. After the hydrothermal treatment at 220°C for 9 days, kaolinite particles with spherical shape were obtained from the former gel, and platy kaolinite was crystallized from the latter one. The difference in morphology of synthetic kaolinite was attributable to the structures of the starting gels, and the pH values in the hydrothermal reactions were not very significant to the morphology.

2H NMR spectra of synthetic deuterated kaolinite have been collected in the temperature range from 150 K to 350 K. Hydroxyl groups show a Pake doublet pattern with an asymmetry factor of 0. They are almost fixed spatially, and undergo a wobbling motion with increasing temperature. The quadrupole coupling constant is 273 ± 3 kHz at 150 K, which indicates that interlayer hydrogen bonding is relatively weak.

The ancylite supergroup has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, with the general crystal chemical formula (M3+xM2+2–x)(CO3)2[(OH)x⋅(2–x)H2O] (1 ≤ x ≤ 2, Z = 2). The ancylite supergroup can be divided into two groups defined by different proportions of the M cation and hydroxyl anion and/or water molecule: the ancylite group is defined for 1 ≤ x ≤ 1.5; the kozoite group is defined for 1.5 < x ≤ 2. The ancylite supergroup minerals are orthorhombic with space group Pmcn, or monoclinic with space group Pm11, and have a crystal structure with species-defining trivalent and divalent M cations (M = La3+, Ce3+, Nd3+, Ca2+, Sr2+ and Pb2+) which centre ten-vertex polyhedra formed by oxygen atoms at three independent O sites. Two vertices of the triangular (CO3)2– anion are oxygen atoms, whereas the third one, O(3), is statistically filled with (OH)– groups and H2O molecules. The triangular faces of three oxygen atoms of MO10 coordination polyhedra join the chains of this ten-vertex polyhedron, which is extended along the c axis. The (CO3) triangles connect chains in three dimensions. To date, eight valid mineral species with M2+ = Sr2+, Ca2+ and Pb2+ belong to the ancylite group [ancylite-(La), ancylite-(Ce), calcioancylite-(La), calcioancylite-(Ce), calcioancylite-(Nd), gysinite-(La), gysinite-(Ce) and gysinite-(Nd)]. Two hydroxyl carbonates with only rare earth elements as species-defining cations, kozoite-(La) and kozoite-(Nd) are members of the kozoite group.

Polytypism in gümbelite and its relationship to the fibrous or ribbon-like morphology exhibited by this Mg-rich illite were investigated by powder X-ray diffraction (XRD), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). Comparison between the XRD pattern from oriented fibers using a conventional powder diffractometer and a randomly oriented pattern using a Gandolfi camera suggested that 2M2 is dominant but other polytypes belonging to subfamily A also exist, and that the fiber axis of gümbelite is parallel to <110> in 2M2, <110> in 2M1, and <100> in 1M. The EBSD analyses confirmed these crystallographic directions directly from individual crystals. Electron diffraction and high-resolution TEM showed that twinning and intergrowths of various polytypes including both subfamilies are common in a single crystal and that the two types of rotations [2n60° and (2n+1)60°] between adjacent layers are often randomly mixed at the monolayer level. The data suggest that high densities of twinning and intergrowths account for the origin of the fibrous morphology along <110> for 2M1 and 2M2 polytypes. Volume restriction in a confined vein space may also play a role.

In this communication we present a brief response to Hawthorne (2023) who, in a paper in volume 87, doi.org/10.1180/mgm.2023.8 (this journal), claims evidence for violations of the electroneutrality principle in mineral formulae derived through IMA–CNMNC procedures: i.e. the dominant-constituent rule, the valency-imposed double site-occupancy, the dominant-valency rule, and the site-total-charge approach (STC).

His statement is not correct as the STC method is based on the end-member definition; thus, it cannot violate the requirements of an end-member, particularly the laws of conservation of electric charge. The STC was developed to address the shortcomings in the previous IMA–CNMNC procedures.

The real question is: which method to use to define an end-member formula? Currently, there are two approaches: (1) STC, which first identifies the dominant end-member charge arrangement and then leads to the dominant end-member composition; (2) the dominant end-member approach.

New guidelines for the nomenclature of polymorphs and polysomes have been approved by the the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA–CNMNC). Several cases can be distinguished. (i) Polymorphs with different crystal systems are distinguished by the prefixes cubo- (cubic), hexa- (hexagonal), tetra- (tetragonal), trigo- (trigonal), ortho- (orthorhombic), clino- (monoclinic) and anortho- (triclinic). (ii) Polymorphs with different crystal systems but showing a pseudosymmetry should show the prefix ‘pseudo-’. (iii) Polymorphs with the same crystal system but different space groups are distinguished by the prefix ‘para-’. If three or more polymorphs show the same crystal system but different space groups, the space group notation may be added as a suffix, though such a nomenclature should be avoided if possible. (iv) Polymorphs with the same space group are distinguished by the prefix ‘para-’. (v) Minerals with polymorph suffixes but with different chemical compositions cannot be considered as true polymorphs, so we recommend using the prefix ‘meta-’, which indicates a close but significantly different chemical composition. (vi) Polysomatic symbols should be placed as a suffix, which indicates the number and types of modules that alternate in the structure, such as in the högbomite supergroup, or as prefixes as in the sartorite homologous series. These recommendations have to be applied for future new mineral proposals, when the authors decide to use structural prefixes or suffixes, however modifications of historical and well-established names have to pass through the CNMNC for approval. In order to be consistent with the new guidelines, 25 mineral names are now modified: domeykite-β becomes trigodomeykite; fergusonite-(Y)-β becomes clinofergusonite-(Y); fergusonite-(Ce)-β becomes clinofergusonite-(Ce); fergusonite-(Nd)-β becomes clinofergusonite-(Nd); ice-VII becomes cubo-ice; roselite-β becomes anorthoroselite; sulphur-β becomes clinosulphur; mertieite-II becomes mertieite; mertieite-I becomes pseudomertieite; uranophane-α becomes uranophane; uranophane-β becomes parauranophane; gersdorffite-P213 becomes gersdorffite; gersdorffite-Pa3 becomes paragersdorffite; gersdorffite-Pca21 becomes orthogersdorffite; betalomonosovite becomes paralomonosovite; lammerite-β becomes paralammerite; nováčekite-I becomes hydronováčekite; nováčekite-II becomes nováčekite; halloysite-7Å becomes halloysite; halloysite-10Å becomes hydrohalloysite; metauranocircite-I becomes metauranocircite; taimyrite-I becomes taimyrite; uranocircite-II becomes uranocircite; andorite IV becomes quatrandorite; and andorite VI becomes senandorite.