Several new minerals, structurally and chemically similar to the hydrotalcite-manasseite group, have been found in the saline deposits of the central pre-Caspian depression and in those of Middle Asia. Their structures consist of layers of (Mg1-xAlx)(OH)2 plus interlayers of anions and molecular water. In addition to manasseite itself, [(Mg4Al2)(OH)12][(CO3)-3H2O], having unit-cell parameters of a = 3.042, c = 2 × 7.56= 15.12 Å and a calculated density of Dc = 2.15 g/cm3, other varieties were identified whose interlayers contain alternately SO42− or SO42− and CO32− anions. The following new phases were recognized: (1) an 8.85-Å phase having the composition [(Mg4Al2)(OH)12][(SO4)·3H2O], subcell dimensions of a' = 3.04, c' = 8.85 Å, and Dc = 1.96 g/cm3; (2) an 11-Å phase having the composition [(Mg3.96Fe0.06Al1.98)(OH)12][Na0.56(SO4)1.30·7.3H2O], hexagonal unit-cell parameters of a = √3 · 3.042 = 5.28 Å, c = 11.16 Å, and Dc = 1.90 g/cm3; (3) a 16.5-Å mixed-layer phase having the composition [(Mg4Al2)(OH)12][(SO4)0.5(CO3)0.5·3H2O] and an ordered ABAB... alternation of 7.56-Å layers (i.e., a brucite-like layer + a CO32−-containing interlayer) and 8.94-Å layers containing SO42− anions in interlayers; this phase has unit-cell dimensions of a' = 3.05, c' = 16.5 Å and Dc = 2.06 g/cm3; and (4) an 18.5-Å ordered mixed-layer phase having the ideal composition [(Mg8Al4)(OH)24][M+0.5(SO4)1.25(CO3)1.0·9H2O] and an alternation of 7.56-Å, CO3-containing layers and 11-Å layers having SO42− anions and Na and Mg cations in interlayers (M+0.5 corresponds to interlayer cations). This last phase has unit-cell dimensions of a = 3.046, c = 3 × 18.54 = 55.62 Å and Dc = 1.99 g/cm3. Some of the varieties containing SO42− in the interlayers swelled with ethylene glycol or glycerol.

For this diverse family of minerals whose structures are based on brucite-like layers of the composition (R2+1-xR3+x)(OH)2, a unifying system of nomenclature is offered for consideration. In describing these brucite-like structures intercalated with anion- and cation-containing interlayers, the composition of the hydroxide sheet and the interlayers, the periodicity of the layers, and the mixed-layer nature of the material should be described. By means of symbols, a single term might be used to describe the crystal chemistry of any member of the group having a given cation composition for the brucite-like layers, for example, manasseite, a 2H polytype of hydrotalcite that has a repeat distance of 7.56 Å and CO3-containing interlayers, might be designated as 7.56-Å CO3-hydrotalcite-2H.

The formation of iddingsite by the oxidative weathering of Fo80 olivine begins by solution of Mg from planar fissures, 20 Å wide and spaced 200 Å apart, parallel to (001). Oxidation of Fe within the remaining olivine provides nuclei for the topotactic growth of goethite. Cleavage cracks < 50 Å in diameter allow Na, Al, and Ca from adjacent minerals, particularly plagioclase, to enter the altering olivine while Mg and Si diffuse away. In the early stages of weathering, strips of Fe-rich smectite (saponite), 20–50 Å wide and 1–7 layers thick, form bridges 50–100 Å long across the planar fissures. Dioctahedral smectite crystallizes on the margins of wider cleavage-controlled fissures; with further weathering halloysite is formed away from the fissure walls. In the ultimate stages of alteration, the saponite and dioctahedral smectite are lost, leaving a porous, oriented aggregate of goethite crystals each measuring about 50 × 100 × 200 Å (X, Y, Z, respectively), with sporadic veins of halloysite crossing the pseudomorph.

Solutions containing hydroxy-SiAl (HSA) oligocations were prepared by two procedures: (1) treatment of a mixture of orthosilicic acid and AlCl3 with aqueous NaOH, followed by aging of the product; and (2) preliminary preparation and aging of hydroxy-Al13 oligocations followed by reaction of the latter with orthosilicic acid. Ion exchange of Na,Ca-montmorillonite with HSA oligocations yielded pillared, cross-linked montmorillonites (designated as HSA-CLM) showing a maximum d(001) value of 19.5 Å for air-dried samples, and maximum surface areas of ~500 m2/g after outgassing at 250°C/10−3 torr. Corresponding ion exchange of Li-fluorhectorite yielded HSA fluorhectorites (HSA-CLFH) showing a maximum d(001) value of 19.0 Å and a surface area of 355 m2/g. Calculated structural formulae for the HSA-CLM and HSA-CLFH products, based on elemental analysis, showed a gradual increase in the Si/Al ratio in the intercalated HSA oligocations with increasing Si/Al ratio in the pillaring solution. Optimum d(001) values and surface areas of HSA-CLM and HSA-CLFH products were obtained using method 2 and applying a ratio of 1.6–2.5 mmole (Si)Al/g smectite.

The thermal stabilities of HSA-CLM and HSA-CLFH products were determined by heat treatment between 250° and 700°C and subsequent measurement of the d(001) values and surface areas. HSA-CLFH products showed the unusual behavior of increase of d(001) with increase in temperature from 400° to 500°C, and essential constancy of d(001) from 500° to 600°C. The HSA-CLM products showed a gradual decrease in surface area, whereas the HSA-CLFH products prepared with a Si/Al ratio of 1.04–2.18 in the pillaring solution showed constant surface areas with increasing temperature from 250° to 600°C. HSA-CLM and HSA-CLFH show sharply higher acidities compared with those of reference Al-CLM and Al-CLFH samples obtained by pillaring with hydroxy-Al13 oligocations. This increased acidity is probably due to the presence of acidic, surface silanol groups in the HSA oligocations.

Reference smectites were examined to determine relationships between Li uptake, cation-exchange capacity (CEC), and octahedral layer charge after Li saturation and heating at 250°C (Hofmann-Klemen effect). Direct measurements of exchangeable Li after heating led to overestimates of charge reduction due to entrapment of Li in collapsed interlayers. Expansion of interlayers by sequential washings with 1 N MgCl2, 0.01 N MgCl2, and ethanol and subsequent determinations of exchangeable Mg provided accurate measurements of reduced charge. The CEC reductions observed in dioctahedral samples as a result of Li saturation and heating equaled octahedral charge values derived from published mineral formulae, and interlayer charge estimates obtained by alkylammonium exchange confirmed that measured CEC reductions were a consequence of uniform decreases in octahedral layer charge.

Dioctahedral specimens retained 1 to 10 meq/100 g of non-exchangeable Li in excess of CEC reduction and were acidified in direct proportion to their total Fe contents, apparently as a result of the deprotonation of structural hydroxyl groups. Mild acid treatment reprotonated these hydroxyl groups, released excess Li, and resulted in total Li contents comparable to measured CEC reductions. Heating (250°C) Mg-saturated hectorite induced a loss of octahedral Li, acidification, and a reduction of CEC, indicating that Mg had partially replaced octahedral Li. These results suggest that octahedral Li is mobile at low temperatures and that cation movement into or out of the octahedral sheet is favored if the layer charge is reduced.

Ash layers from Searles Lake, California, were sampled in core of drill hole KM-3, which penetrated 693 m of lacustrine sediment deposited in a playa-lake complex over the past 3.2 my. Lake water changed from moderately saline and slightly alkaline (pH ~7.5) to highly saline 2.04 my ago and to highly saline and alkaline (pH ~9.5) 1.28 my ago. As a result of brines flushing downward, the upper 291 m of sediment, spanning the past 1.28 my, contain highly saline, alkaline pore fluid. Silicic ash layers in contact with highly saline, alkaline pore fluid were first altered to phillipsite and meriinoite and then to K-feldspar and searlesite. The transformation of phillipsite and/or meriinoite to K-feldspar required more than 45,000 years and was largely completed in 140,000 years. Tephra layers in contact with moderately saline, slightly alkaline pore fluid vary from uncemented vitric ash containing minor smectite to bentonites in which glass is wholly altered to smectite, clinoptilolite, analcime, and opal. Layers with much fine tephra are more altered than the coarser, better-sorted layers. Alteration is attributed to hydrolysis in essentially a closed hydrologic system, in which the alteration of glass to smectite raised the pH, aSiO2, and (Na+ + K+)/H+ activity ratio to the level where clinoptilolite formed. Some diffusion and/ or fluid flow is, however, indicated by the loss of SiO2 during the alteration of ash layers to smectite and by anhydrite deposited during and after clinoptilolite in some tuffaceous sandstones.

Clinoptilolite from the Fort LaClede deposit in Sweetwater County, Wyoming, shows a moderate selectivity for NH4− over Na+ in aqueous solution. At 30°C, the standard free energy of this replacement reaction is −0.7 kcal/mole at an ionic strength of 0.05 M and −0.8 kcal/mole at 0.5 M. The Na+-NH4+ exchange is complete within 3 days in agitated solution and proceeds to the same extent from the clinoptilolite saturated with either cation.

The Ca2+-Na+ exchange also is complete within 3 days in agitated aqueous solution and proceeds to the same extent from either the calcium or the sodium form of the zeolite. Using test methods which take into account the slower equilibration of Ca2+-loaded clinoptilolite, the cation-exchange capacity is substantially the same over the full range of loading by Ca2+ and Na+. Ca2+ replaces Na+ with decreasing selectivity as Ca2+ loading increases to about 80% at 30°C (95% at 63°C), above which the selectivity reverses. The standard free energy of replacement of two Na+ ions by one Ca2+ ion in 0.05 M solution is −1.2 ± 0.2 kcal/mole at 63°C and −0.3 to −0.8 kcal/mole at 30°C.

A cation-exchange capacity (CEC) method based on ion-sieving properties was developed for the quantitative determination of clinoptilolite in soils. In this method, both zeolitic and non-zeolitic exchange sites in the soil sample are saturated with Na+. The CEC of the non-zeolitic exchange sites is determined by replacing the Na− in these sites with tert-butylammonium ions. The tert-butylammonium ion cannot be exchanged into the zeolitic exchange sites because it is too large to pass through the channels in the clinoptilolite structure. The sample is next washed with NH4OAc to replace the Na+ in the zeolitic exchange sites. The amount of soil zeolite is then estimated by comparing the CEC of zeolitic exchange sites to the total zeolite CEC (175 meq/100 g for pure clinoptilolite). Prior to the CEC analyses, carbonates and organic matter must be removed to minimize interference with the exchange process. A high correlation (r2 = .96) was observed between the abundance of clinoptilolite estimated using the CEC method and the abundance estimated by semiquantitative X-ray powder diffraction analysis.

The CEC procedure was used to quantify clinoptilolite in an Aridic Calciustoll soil from south Texas. About 2–5% clinoptilolite occurs in the A and B horizons, and concentrations progressively increase with soil depth to as much as 20% in the CBk2 horizon.