Vernadite (MnO2·nH2O) is a mineral with a poorly ordered structure. Its synthetic analogue is designated δ-MnO2. Birnessite and vernadite are independent mineral species and cannot be described further under the same name. They have similar hexagonal unit-cell parameters, a0, but different c0 parameters. Rancieite has a structure similar to that of birnessite. Calcium bearing, 14-Å birnessite occurring in nature was first described by the authors. In addition to the todorokite having the parameters a0 = 9.75 Å, b0 = 2.84 Å, and c0 = 9.59 Å, other species of natural todorokite are known having a0 parameters that are multiples of 4.88 Å equal to 14.6 and 24.40 Å, the b0 and c0 parameters being the same.

Investigations were conducted to determine the hydrothermal transformations of synthetic birnessite exchanged with different metal ions. Autoclaving in a Teflon-lined stainless steel pressure vessel at 155°C for 24 hr of Mg-, Ca-, La-, and Co-saturated birnessite yielded manganese minerals having 10-Å X-ray powder diffraction (XRD) spacings. The autoclaved Mg-birnessite yielded a mineral identical to natural todorokite in its infrared (IR) spectrum and XRD patterns. High-resolution transmission electron microscopy (HRTEM) provided images having 10-, 12.5-, 15-, and 20-Å wide fringes indicating heterogeneous channel widths in the crystallographic a direction, and IR spectroscopy produced bands at 757, 635, 552, 515, 460, and 435 cm-1, confirming the product obtained by autoclaving Mg-birnessite to be todorokite. Prolonged autoclaving of Mg-birnessite yielded manganite (γ-MnOOH) as a by-product; manganite did not form when the autoclaving time was shortened to 8 hr. Also, when Ca-saturated samples were autoclaved, the product gave d-values of 10 Å, but the XRD lines were broad and heterogeneity of the channel sizes was evident from HRTEM observations. The Ca-derivative had an IR spectrum similar to that of natural todorokite. Images showing 10-Å lattice fringes were observed by HRTEM for the Ni-saturated sample, which also produced an XRD pattern similar to that of the Mg-saturated sample. Co- and Lasaturated samples did not form todorokite, although HRTEM of La-saturated samples indicated some 10-Å lattice fringes that were unstable in the electron beam. Birnessite saturated with Na, K, NH4, Cs, Ba, or Mn(II) gave products having 7-Å spacings upon autoclaving.

The hot spring water discharging from a flank of an active volcano is precipitating unique monomineralic manganese deposits over volcanic terrain. The major and trace element chemistry, XRD mineralogy, DTA, and SEM observations indicate that the deposits consist of 10 Å phyllomanganate (buserite) accommodating inter-layer Ca and Mg with negligible amounts of detrital minerals. Other metallic elements can be accommodated by buserite, but concentrations are negligible ranging less than 10 ppm to 500 ppm. Abundance and pattern of REE (less than 100 ppm in total) are similar to those from hydrothermal manganese deposits. The buserite is enriched in Ca and Mg but depleted in Na in comparison with those in the spring water. The distribution coefficients for Ca, Mg and Na between the buserite and the host water were calculated assuming an ion-exchange equilibrium in the Yuno-Taki Fails, which proved applicable to other manganese deposits from surficial environments on land and oceans.

Todorokite is a common Mn oxide (with a tunnel structure) in the Earth surface environment, and can be obtained by hydrothermal treatment or refluxing process from precursor buserite with a layered structure. Several chemical reaction conditions for the phase transformation from Na-buserite to todorokite at atmospheric pressure were investigated, including temperature, pH, crystallinity of precursor Na-buserite, the amount of the interlayer Mg2+ of the Mg-buserite and clay minerals. The results showed that the conversion rate and crystallinity of todorokite decreased with falling temperature, and Mg-buserite could not be completely transformed to todorokite at lower temperatures (40°C). The poorly crystalline Na-buserite could be converted into todorokite more easily than highly crystalline Na-buserite. Todorokite can be prepared at pH 5–9, but the rate of conversion and crystallinity of todorokite did vary with pH in the order: neutral ≈ alkali > acidic. The conversion rate of todorokite decreased with decreasing interlayer Mg2+ content of the Mg-buserite. The presence of montmorillonite or goethite slowed the formation reaction of todorokite in the refluxing process, and the reaction time was prolonged when the amounts of those minerals were increased.

Freshwater ferromanganese nodules (FFN) from Green Bay, Lake Michigan have been investigated by X-ray powder diffraction (XRD), micro X-ray fluorescence (XRF), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy (STEM). The samples can be divided into three types: Mn-rich nodules, Fe-Mn nodules, and Fe-rich nodules. The manganese-bearing phases are todorokite, birnessite, and buserite. The iron-bearing phases are feroxyhyte, goethite, 2-line ferrihydrite, and proto-goethite (intermediate phase between feroxyhyte and goethite). The XRD patterns from nodule cross sections suggest the transformation of birnessite to todorokite. The TEM-EDS spectra show that todorokite is associated with Ba, Co, Ni, and Zn; birnessite is associated with Ca and Na; and buserite is associated with Ca. The todorokite has an average chemical formula of Ba0.28p2+(Zn0.14p+2Co0.05p3+Ni0.02p2+) (Mn4.99p4+Mn0.82p3+Fe0.12p3+Co0.05p3+Ni0.02p2+) O12·nH2O. Barium is the main cation in the structural tunnels of the todorokite. The average chemical formula of birnessite and Ca-buserite are: Na0.14p+Ca0.19p2+(Mn1.48p4+Mn0.52p3+0.52) O4·nH2O and Ca0.27p2+(Mn1.46p4+Mn0.54p3+)O4·nH2O, respectively. Most nodules have a concretionary structure of alternating Fe/Mn layers, commonly with a core of reddish feldspar containing phyllosilicates and hematite micro-crystals. Other cores consist of goethite, cristobalite, tridymite, hercynite, hematite-bearing quartz, coal, and chlorite-bearing rock fragments. The hexagonal or hexagonal-like structures of hematite micro-crystals and clay minerals in the cores may serve as heterogeneous nucleation sites for the Mn-oxides and Fe-(oxyhydr)oxides. The alternating Fe/Mn layers in FFNs might be caused by oscillatory redox condition fluctuations at the sediment-water interface due to changes in water level.

The occurrence of halloysite and/or kaolinite in clay-rich, vein-like zones in saprolites in Hong Kong has provided the opportunity to examine the conditions determining the formation of one kaolin mineral or the other and also the nature of their particles. Clay-rich zones within tuffaceous or granitic saprolites from six different hillside sites have been examined in replicate samples by optical and scanning electron microscopy, X-ray diffraction, and thermal analysis. Kaolin minerals, sometimes together with Mn oxides and Fe oxides/oxyhydroxides, have formed within discontinuities within the altered host rocks. The fabrics of kaolin infills generally indicated several generations of kaolin formation and that shear and deformation have commonly occurred within the infills. The infills were either light or dark in color. Light-colored infills often comprised pure, or nearly pure, halloysite or kaolinite. Dark Mn- and Fe-rich infills all contained kaolinite, while including some halloysite. The very halloysitic, light-colored infills occurred in saprolites in both granite and tuff as long tubular shapes in parallel bunches. The light-colored, very kaolinitic infills occurred in tuff only, in large platy or near-platy shapes within vermiform packets. In dark-colored infills, early kaolin mineral crystallization was limited by impurities from the breakdown of primary minerals leaving dissolved and re-precipitated compounds of Mn and Fe within the infill. Kaolin minerals in infill at all the sites except one are considered to have formed as a result of weathering. The exception comprises white infills in tuff that are composed of extremely small, closely packed particles, suggesting formation by hydrothermal action. Generally, the kaolin minerals have formed by neogenesis out of solution in the discontinuities. Drying, with the formation of Mn and/or Fe oxides/oxyhydroxides, had occurred several times, indicating enhanced drainage. Where drying had occurred, kaolinite had formed. Where indications of drying in infills were absent, halloysite was predominant.

Todorokite is a common Mn oxide mineral in terrestrial and ocean-floor environments, and it is commonly synthesized from layered Na-buserite. Pyrophosphate, which is known to form strong complexes with Mn(III) at a pH range of 1–8, was added to a suspension of Na-buserite in order to sequester the available Mn(III) in Na-buserite. No Mn(III)-pyrophosphate complex was formed in solution at pH 10, and the treated Na-buserites were converted completely to todorokite. Significant transformation reductions were observed when Na-buserite was treated with pyrophosphate solution at pH 7. The presence of Mn(III) within the MnO6 octahedral sheets of Na-buserite is critical for the transformation from layered buserite to tunnel-structured todorokite at atmospheric pressure. At lower pH, two effects are combined to reduce the amount of Mn(III) in the layers: (1) the complexing power of pyrophosphate is increased; and (2) the transformation from Na-buserite to H-birnessite, which is concomitant with the migration of Mn(III) from layers to the interlayer, and the partial disproportionation of Mn(III). The results showed that Mn(III) played a key role in the transformation of layered Na-buserite to tunnel-structured todorokite at atmospheric pressure.

Todorokite is a common manganese oxide mineral, with a tunnel structure, found in Earth surface environments, and is easily synthesized from layered birnessite. The aim of the current study was to prepare birnessites with different average manganese oxidation states (AOS) by controlling the ${\text{MnO}}_4^{-}{\text{/Mn}}^{2+}$\$\end{document} ratio in concentrated NaOH or KOH. A series of (Na,K)-birnessites, Na-birnessites, and K-birnessites with different AOS was synthesized successfully in strongly alkaline media. The (Na,K)-birnessites and Na-birnessites prepared in NaOH clearly contained both large (500–1000 nm) and small (40–400 nm), plate-shaped crystallites. The K-birnessites prepared in KOH media consisted mostly of irregular (100–200 nm), plate-shaped crystallites. The degree of transformation of birnessite to todorokite at atmospheric pressure decreased as the AOS values of (Na,K)-birnessites and Na-birnessites increased from 3.51 to 3.80. No todorokite was present when a Na-birnessite with an AOS value of 3.87 was used as the precursor. Pyrophosphate, which is known to form strong complexes with Mn3+ at a pH range of 1–8, was added to a suspension of (Na,K)-birnessites in order to sequester the available Mn3+ in (Na,K)-birnessites. Removal of Mn3+ from birnessite MnO6 layers by pyrophosphate restricted transformation to todorokite — no (Na,K)-birnessite transformed to todorokite after pyrophosphate treatment. The interlayer K+ initially within (Na,K)-birnessites could not be completely ion-exchanged with Mg2+ to form todorokite at atmospheric pressure. No todorokite was forthcoming from K-birnessites even from those with small AOS values (3.50).

Accurate identification of iron and manganese oxides in soils presents great difficulty because, not only are they always in admixture, but the poor crystallinity of the species present leads to difficulties in interpretation of diffraction patterns. A method involving the combined use of electron microscopy, selected area electron diffraction and electron probe microanalysis on the same crystals gives unambiguous identification. Its application is described in relation to the various iron and manganese oxides found in a set of soils from all parts of the world. The occurrence of hausmannite in soils has been definitely established.