An end-member natroalunite of composition (K0.04Na0.96)(Al2.98Fe0.02)(SO4)2(OH)6 occurs in saprolitic clays in a laterite profile developed over Deccan Trap basalts at Matanumad in the Kutch region of Gujarath state, India. Pseudocubic natroalunite crystals, ranging from 0.5 to 1 µm in size, have replaced pseudohexagonal, 2-µm-size platelets of kaolinite. The reverse reaction (kaolinization of natroalunite) has also been observed in the laterites and bauxites immediately adjacent to the saprolite. The kaolinite crystallites that replaced natroalunite occur as a mosaic of anhedral platelets that range in size from 0.1 to 0.3 µm. The formation of natroalunite at Matanumad postdates the laterization of the basalts. It is related to the post-diagenetic formation of sulfuric acid-rich solutions by the oxidation of pyrite in the Eocene-Oligocene black shales of the Matanumad basin. The kaolinitic clays at the top of the saprolite zone have been intensely altered by the acidic solutions, leading to a high concentration of natroalunite.

Members of an alunite-natroalunite solid solution series occur in intimate association with hydrated halloysite in deposits within caves of the Guadalupe Mountains, namely Carlsbad Cavern and Lechuguilla Cave. The alunite and natroalunite crystals consist of cube-like rhombs; crystal diameters range from 0.5 to 8 µm. This mineral association is found in sediments within bedrock pockets, solution cavity fills, floor deposits and wall residues. Sulfur stable isotope values (δ34S, CTD) for cave alunite and natroalunite are negative [+0.1 to −28.9 per mill (‰); n = 12 and mean = 16.8‰] and are comparable to the cave gypsum and native sulfur values reported by other investigators. The association of alunite/natroalunite with hydrated halloysite in these cave deposits suggests that the cave-forming waters contained significant concentrations of sulfuric acid. Formation of these minerals is related to the excavation of the carbonate rocks that formed Carlsbad Cavern, Lechuguilla Cave and other caves of the Guadalupe Mountains. The sulfuric acid-bearing waters, when exposed to clay-rich sediments, converted clay minerals and quartz to alunite/natroalunite and hydrated halloysite.

The Biga Peninsula of NW Turkey is host to many kaolin and halloysite deposits with mineralization occurring at the intersections of fault zones in contact with Late Eocene-Miocene calc-alkaline volcanic rocks. Distinguishing between the relative overprinting of hypogene by supergene processes in these deposits is a challenge and important because they affect the physical-chemical properties of minerals and their potential for industrial applications. This study examines the Sarıbeyli-Sığırlı and Bodurlar kaolin deposits in NW Turkey, which were formed from similar volcanics as evidenced by 40Ar/39Ar. Late Eocene (34.2 ± 0.20 Ma) to Early Oligocene (32.7 ± 0.17 Ma) ages for both primary volcanic rocks and alunites are consistent with surrounding rocks in the Çanakkale region. Criteria used to distinguish hypogene alteration from supergene alteration processes come from X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Raman spectroscopies, thermal gravimetric analysis (TGA), scanning and transmission electron microscopy (SEM, TEM), and elemental analyses. Isotopic δ18O depletion and δD enrichment of the Sarıbeyli-Sığırlı deposit suggests that it was more influenced by magmatic waters than was the Bodurlar deposit. The Bodurlar deposit contains a paucity of dickite compared to the Sarıbeyli-Sığırlı deposit, which is evidenced by lower TGA endotherms, higher ratios of XRD intensities for reflections at 1.316 Å and 1.307 Å, distinctive FTIR absorbance bands at 3620 cm− 1 and 3652 cm−1, and relative Raman intensities of the γ1 and γ5 vibrational modes.

A genetic model is proposed whereby these deposits are mainly formed through an acid-sulfate hydrothermal alteration, in what appears to be a volcanic-hydrothermal system. The extent of hydrothermal alteration was controlled by fault density and the initial texture of the volcanic rocks. These steam-heated environments included sulfide-enriched vapors and groundwater mixed to varying degrees in the vadose zone. The Sarıbeyli-Sığrlı and Bodurlar deposits, respectively, contain mineral assemblages that reflect both hypogene (kaolinite, alunite, dickite) and supergene (kaolinite, halloysite, jarosite) processes. These observations offer a basis for comparing and discriminating the relative influence of these two important alteration processes responsible for the formation of kaolin deposits in NW Turkey and around the world.

Hydrothermal solutions related to magmatic intrusions that occurred during the Oligo-Miocene resulted in advanced multi-stage alterations and, to varying degrees, kaolinization over a wide area of the Biga Peninsula. The most important formations in these kaolin deposits occurred along the NE–SW-trending Çan-Etili-Bayramiç fault zone. The Bahadırlı, Duman, and Çaltıkara quarries are well preserved kaolin deposits throughout these fault zones located within the Çan Volcanics. Mineralogical, geochemical, and isotopic analyses were performed to identify the environments of formation of kaolins and the origin of the hydrothermal solution that led to the formation of these deposits.

The mineralogical assemblages of each of the quarries differ from each other. The Bahadırlı kaolin quarry comprises kaolinite + alunite ± quartz ± smectite ± plagioclase ± K-feldspar. The Çaltıkara kaolin deposits consist of kaolinite + quartz + alunite ± iron-oxide and ore minerals. The Duman kaolin quarry was considered in two different slopes as a hanging wall block and a footwall block, and the blocks contain kaolinite + quartz + plagioclase ± smectite and kaolinite ± alunite ± smectite ± quartz ± plagioclase ± K-Feldspar ± gypsum, respectively. During petrographic investigations, it was observed that kaolinization occurred generally in K-feldspar and plagioclase phenocrysts and partially in the matrix. Mineralogical and micromorphological investigations revealed that the kaolin group includes the dioctahedral minerals kaolinite and halloysite. According to the trace element contents of the kaolinites, the Çaltıkara and Bahadırlı deposits had a hypogene origin, while the Duman deposit had a supergene origin. δ18O isotopic values of kaolinites ranged from +10.3 to +18.3‰. δ34S isotopic values ranged from –17.2 to +20.2‰ of alunite, galena, and pyrite minerals, which indicate different formation environments for the kaolin quarries. 40Ar/39Ar dating of alunite revealed that the timing of acid-sulfate alteration was compatible with the magmatic intrusions. All these data revealed the mineralogical, chemical, and isotopic differences caused by the effect of different hydrothermal processes in three kaolin deposits located on the same fault zone, close to each other and similar in age.

Altered basaltic tephra from Haleakala, Maui, are characterized using multiple techniques in order to identify the minerals formed under a variety of conditions and to understand the soil formation processes here. We collected samples that are representative of typical bulk weathered material in the crater, as well as solfataric alteration in a hydrothermal environment. For this study X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy are coupled with spectroscopic techniques including Mössbauer, visible-infrared reflectance, transmission infrared, and thermal infrared emission spectroscopies to analyze these samples. The unaltered tephra are composed of feldspar, glass, pyroxene and olivine. Observed alteration products include Fe oxides, phyllosilicates and sulfates, as well as SAED amorphous Al-Si-bearing material. These samples are potential analogs for altered volcanic material on Mars as the pedogenic influences and contact with plants and animals are minimal. Results from this study may help to determine spectral signatures of these samples that could be used for identification on Mars of the minerals observed here.

Halloysite in El-Gideda iron mine occurs as very soft, light and white-to-pinkish white pockets and lenses ranging in diameter from 50 cm to 1 m within the iron ore. Highly hydrated halloysite is the main constituent of these pockets beside some kaolinite and alunite. The diffraction pattern of the clay fraction (<2 mm) shows a rather broad and diffuse 001 reflection spread between 10.3 and 13.6°2θ. Upon treatment, the 001 reflection of halloysite expands up to 10.94 Å and 11.9 Å corresponding to ethylene glycol and dimethyl formamide treatment, respectively. After these treatments, kaolinite appeared with its characteristic basal spacing (~7 Å). The percentage of halloysite in halloysite-intercalated kaolinite ranged between 80 and 90%. Heating to 350°C, produces a kaolinite-like structure (~7.1 Å) that developed to a metakaolinite-structure when heated to 550°C. Morphologically, halloysite appears as well developed tubes composed entirely of SiO2 and Al2O3, while kaolinite is characterized by very fine platelets arranged in book-like or rosette-like shapes. A differential thermal analysis curve of the studied halloysite showed an endothermic peak at ~138°C due to the dehydration of interlayer water of halloysite. The small shoulder at ~540°C and the endothermic peak at ~593°C is attributed to the dehydroxylation of halloysite, kaolinite and alunite. On the other hand the exothermic peak that appeared at 995°C is due to the formation of new phases such as mullite and/or spinel. The infrared vibrational spectrum is typical of highly disordered halloysite and kaolinite.

Halloysite was formed as a result of alteration of the overlying glauconite suggesting intensive chemical alteration during a humid wet period that prevailed in the Bahria Oasis during the late Eocene. Glauconite alteration releases K, Fe, silica and alumina. Iron forms at least part of the iron ore in the El-Gideda mine while alumina forms halloysite as well as alunite when interacted with silica in an acidic environment.

The Biga Peninsula of NW Turkey is host to six major halloysite deposits in the Go¨nen, Yenice and Balya districts. Mineralization took place in areas of Permian limestone blocks where the Triassic Karakaya Complex is in contact with early Miocene calc-alkaline volcanic rocks. Hypogene halloysite mineralization was controlled by the intersection of minor faults in the vicinity of clay deposits. During the Pleistocene, activity of the North Anatolian Fault (NAF) brought ascending geothermal solutions through the fault zones to the surface, which led to hydrothermal alteration and halloysite formation. N-MORB normalized element values for each halloysite deposit and the volcanic rocks suggest genetic links. Alunite and halloysite were formed in the Turplu area where upwelling hydrothermal waters contained major H2S and SO2 acids. Only halloysite mineralization occurred in outflow areas of the same fossil geothermal field.

Pyrite and alunite samples from the Turplu deposits have δ34S values of 0.6–1.8% and 4.8–7.9%, respectively, with values for gypsum of 3.1–3.5%. The δ34S values of pyrite suggest that local meteoric waters had partially mixed with the dominant fluid during the closure stage of fossil hydrothermal activities. The range of δD values of halloysite samples from Turplu is –58.4 to –68.6%. The δ18O values for halloysite are in the range 16.7–18.1%. All halloysite deposits in the study areas are either overlying or adjacent to limestone blocks, and these provide excellent drainage for the discharging geothermal waters. Subsurface drainage systems in the karstic environment and the SO2-bearing thermal waters indicate the importance of acidic waters and the continuous leaching of elements in forming relatively pure hydrated halloysite. A steam-heated dissolution-precipitation model is proposed for the occurrence of all halloysite and alunite deposits. Sulphur gases (H2S-SO2) of hypogene origin rose from deep in the fault zone to the surface where they encountered oxygenated groundwater at the water table. The occurrence of H2SO4 in this hydrothermal system enhanced the acidity of geothermal waters provoking advanced argillic alteration. Hypogene alunite deposits also have large P2O5 contents, suggesting a parent material with a magmatic origin deeper than the alkaline tuffs. Halloysite is a fast-forming metastable precursor to kaolinite.

The high-temperature (HT) behaviour of a sample of natural alunite was investigated by means of in situ HT single-crystal X-ray diffraction from room temperature up to the dehydroxylation temperature and consequent collapse of the crystal structure. In the temperature range 25–500°C, alunite expands anisotropically, with most of the contribution to volume dilatation being produced by expansion in the c direction. The thermal expansion coefficients determined over the temperature range investigated are: αa = 0.61(2) × 10–5 K–1 (R2 = 0.988), αc = 4.20(7) × 10–5 K–1 (R2 = 0.996), αc/αa = 6.89, αV = 5.45(7) × 10–5 K–1 (R2 = 0.998). At ∼275–300°C, a minor discontinuity in the variation of unit-cell parameters with temperature is observed and interpreted on the basis of loss of H3O+ that partially substitutes for K+ at the monovalent A site in the alunite structure. Increasing temperature causes the Al(O,OH)6 sheets, which remain almost unaltered along the basal plane, to move further apart, and this results in an expansion of the coordination polyhedron around the intercalated potassium cation. Sulfate tetrahedra act as nearly rigid units, they contract a little in the lower temperature range to accommodate the elongation of the Al octahedra.

Minamiite has been discredited and renamed natroalunite-2c to show a double unit-cell structure and natroalunite can be designated as natroalunite-1c to show a single unit-cell structure. Kintoreite can be designated as kintoreite-1c to show the same single unit-cell structure, and IMA 1993-039 is a new superstructure of kintoreite and can be designated as kintoreite-2c to show a double unit-cell structure. Beaverite has been renamed beaverite-(Cu). The Zn-bearing beaverite of Sato et al. (2008) has been named “beaverite-(Zn)”, but data for the mineralhave not been approved by the CNMNC. Orpheite has been discredited as P-rich hinsdalite. Proposal 07-D was approved by the CNMNC.

An assemblage of alunite-supergroup minerals (ASM), rhabdophane-group minerals (RGM), goethite and associated clay minerals occurs in Permian A-type porphyritic microgranite in the eastern part of the Velence Hills, Hungary. The secondary sulfates/phosphates include jarosite, Pb-rich jarosite and alunite, corkite, hinsdalite and rhabdophane-(Ce), -(La) and -(Nd). Detailed electron probe microanalysis and Raman spectroscopy reveal a wide miscibility among RGM end-members and show a rhabdophane–tristramite–brockite solid solution with extensive compositional variation. Moreover, ASM show heterogeneous composition and complex substitution mechanisms within the alunite, beudantite and plumbogummite groups. The formation of this rare mineral assemblage reveals extensive remobilization of rare-earth elements (REE), Th, U, P, S, Fe and Pb under supergene conditions. Compositional variations and substitution trends of the RGM investigated indicate that Th, U, Ca and Pb are incorporated into the rhabdophane structure by a (Ca, Pb)2+ + (Th, U)4+ ↔ 2REE3+ substitution mechanism. Consequently, we suggest the following end-member formulae for RGM containing divalent and tetravalent cations: (Ca0.5Th0.5)PO4·H2O for brockite, (Pb0.5Th0.5)PO4·H2O for grayite and (Ca0.5U0.5)PO4·2H2O for tristramite. The ASM and RGM originated from total leaching of the primary magmatic REE, Th, U and P minerals in the microgranite [most probably allanite-(Ce), fluorapatite and possibly also xenotime-(Y)], together with input of Pb and S in low-temperature, acid sulfate solutions, connected with an adjacent Palaeogene andesite–diorite intrusion and the accompanying hydrothermal sulfide mineralization.

Few reports exist on the use of Diffuse Reflectance Infrared Fourier Transform Spectrometry coupled with Mass Spectrometry (DRIFTS-MS) in situ to monitor the dehydroxylation of kaolinitic clays. The use of DRIFTS-MS in situ allows study of the effect of heat treatment on the dehydroxylation, identifying intensities and temperatures at which the hydroxyl groups are released, forming metakaolinite and meta-alunite. The effluent gases from the infrared cell were analysed by mass spectrometry. The decrease in intensity of the bands at 3694, 3669, 3650 and 3621 cm−1 associated with the −OH stretching vibration modes of AlVI−OH−AlVI of kaolinite began at 450°C. Two additional bands at 3513 and 3485 cm−1 are associated with the vibration of AlVI−OH of alunite that also began to disappear during thermal treatment. Monitoring of the fractions m/e 17 and 18 using a mass spectrometer revealed that the intensity of these fractions increased starting at 450°C. Therefore, it is possible to study the dehydroxylation process of clays during thermal treatment.Chemical and mineralogical characterization of a kaolinitic clay (KN) fromMexico showed that the clay consists of 64.8% kaolinite, 11.0% alunite and 24.4% quartz based on PXRD, EDS, TG/DTA, TEM and FTIR results, and suggested that the material might have potential for use in the manufacture of ceramics, refractory bricks or geopolymers.