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This study focused on the mineralogical and physico-chemical properties of heulandite/clinoptilolite (Heu/Cpt)-rich zeolitic tuffs SE of Ankara (Turkey) and evaluated their potential for use as adsorbents. The main Heu/Cpt minerals correspond to the intermediate type and the Cpt-type of the heulandite series. Minor chabazite, erionite and rarely analcime and accessory feldspars, mica, smectite, and quartz were also detected. Three types of Heu were recognized according to their thermal behaviour. Cpt (type-III Heu) was thermally stable at 600°C, whereas type-I Heu collapsed and type-II Heu partially collapsed. The Si/Al ratios of the Hul/Cpt ranged from 3.54 to 5.03, the Γ parameter (Γ = Ca/Al + 0.115*Al) varied between 0.78 and 1.85 and Al + Fe3+varied between 6.27 and 8.00. The Na, K and Ca contents of Heu/Cpt varied widely and the Heu had an intermediate composition between types II and III. The Mg + Ca content is greater than the Na + K content in Heu-type II and smaller in Heu-type III zeolites. The BET surface area and the pore-size of the samples varied between 10.15 and 194.27 m2/g, and between 1.41 and 12.42 nm, respectively. The N2adsorption and desorption isotherms were comparable, but the adsorption/desorption capacity is greater in type III than in type II Heu zeolites. The cation exchange capacities of the Heu/Cpt-rich samples ranged from 185 to 280 meq/100 g.

Diagenetic alteration in two tuff horizons (the lower and upper tuff units) in the GoÈrdes region of Turkey led to the formation of mainly heulandite-clinoptilolite-type zeolites and also clay and silica minerals and K-feldspar. Possible variations in the climate and geological environment followed by variations in the hydrological system and the composition of the Miocene lake water and groundwater at that time resulted in the mineralogical facies. The lower tuffs are unaltered or slightly altered in the northern part and are more altered, particularly to zeolite (clinoptilolite) in the southern part of the study area. Heulandite and K-feldspar are mainly authigenic minerals in the upper tuff unit. The ammonium-exchange capacities of the 16 samples were investigated to understand the effects of the mineralogical composition on their ion-exchange capacities, which vary in the range 0.19-2.00 mEq/g. It is observed that the ion-exchange capacities are strongly dependent on the zeolite contents of the rocks. The correlation coefficient of this linear relation is 0.86. The most significant increase in ammonium-exchange capacity (from 0.2 to ~1.0 mEq/g) was observed when the zeolite contents increased from 0 to 30 wt.%. The second increase in exchange capacity was observed for samples containing >80 wt.% zeolite. Although there is no significant effect from the other authigenic minerals, smectite has a positive effect, and K-feldspar and opal-CT have almost no effect on the ion exchange capacities.

Laumontite is a common zeolite mineral indicative of low-grade metamorphism of lavas and volcaniclastic sediments. Stoichiometric laumontite (CaAl2Si4O12·4.5H2O) dehydrates in air at 298.15 K, 1 bar to leonhardite (CaAl2Si4O12·3.5H2O) via loss of water from the W1 crystallographic site. Consideration of reported X-ray diffraction and calorimetric data indicates that the standard molal volume and entropy for dehydration of laumontite to leonhardite + liquid water are ∼13 cc/mol and ∼8 cal/molK, respectively, at 298.15 K, 1 bar. Equilibrium between laumontite and leonhardite occurs at 70 to 80% relative humidity at 298.15 K, 1 bar, corresponding to a standard molal Gibbs energy and enthalpy of reaction of ∼170±40 and ∼2630±100 cal/mol, respectively. Calculated univariant equilibrium for this reaction is nearly linear from 46±3°C at 1 bar to 235±3°C at 5000 bar. Comparison of geological observations with these results suggests that laumontite forms as ‘leonhardite’ during metamorphism and diagenesis. Consideration of ‘leonhardite’, instead of laumontite, in low-grade metamorphic phase relations facilitates prediction of the relative stabilities of zeolites in natural and geologic systems, where calculated entropies of reaction incorrectly predict that assemblages bearing fully hydrated laumontite are stable at lower temperatures than the zeolites heulandite and stilbite.

A zeolite found in a soil profile developed in Danian chalk was studied. The results showed that it is a heulandite, temperature-stable to above 600°C with a Si/Al ratio of 2.88 and almost completely K-exchanged. The mineral probably formed under burial diagenetic conditions in the chalk formation.

The unstable glassy component of the Oligocene tuffaceous sediments of the Metaxades area, Thrace, Greece, has undergone extensive zeolitic diagenetic alteration. The authigenic minerals occur as crypto- to micro-crystalline aggregates making up most of the matrix in the altered tufts and as precipitates in cavities produced by dissolved glass fragments. Glass-shard pseudomorphs are ubiquitous and most of them are partly filled by one or more of the minerals smectite, heulandite, mordenite and silica. The heulandite group zeolites are mostly high-silica calcium-rich heulandites showing intermediate thermal behaviour between most heulandites and clinoptilolites. Their Si/AI ratios are similar to clinoptilolite (4.74-5.19) but their divalent/monovalent cation ratios (1.5-3.24) are partly superposed onto the ratios of heulandite group 1 and 2 and differ considerably from the values of the relevant ratio in clinoptilolite. Rare K-rich clinoptilolite crystals have been identified in one sample only. Based on field observations, compositions and paragenetic relationships of coexisting authigenic minerals and the absence of critical phases such as laumontite, analcime or authigenic albite, it is suggested that the Metaxades pyroclastic rocks underwent burial diagenesis intermediate between Iijima's (1978) zones II and III commonly developed in silica saturated environments. K E VWO R D S: heulandite, clinoptilolite, mordenite, burial diagenesis, zeolite, Thrace, Greece.

Heavy metals and radionuclides are both hazardous cationic species, and their interaction with soils have received increasing attention in recent years. Ion exchange is regarded as one of the key processes that regulate these hazardous species within the environment, but it can also be the process used for the decontamination of polluted soil, (ground)water or effluent. An ion-exchanger, often a zeolite, which is specific to the type of contamination, can be used to selectively remove the pollutant. For example, Clinoptilolite, which is used to remove Cs-137 and Sr-90 from soils and milk, or Phillipsite, which has been used to recover Cd from acid mine drainage. Due to this selective recovery of the cationic contamination, zeolites concentrate and therefore reduce the waste volume. For these reasons we decided to model cationexchange and adsorption behaviour of a range of zeolites with varying structural properties, including ring size and Si/A1 ratio. The zeolites studied are; zeolite A (LTA), zeolite RHO, Heulandite and its isostructure Clinoptilolite. The cationic species used in our calculations were selected for their varying ion size, charge along with their environmental importance (Table 1). Zeolite A was chosen as a model structure to verify our simulation techniques as there is ample experimental crystallo-graphic data available to compare our results with. In addition, atomistic calculations have been used successfully before to study structural and reactivity properties of zeolites, including Na-exchanged zeolite A. Atomistic simulations could aid the understanding of the ion-exchange properties of zeolites by studying the bulk and surface structures of the different zeolites for; (i) cation adsorption sites, (ii) effect of aluminium distribution, (iii) cation substitution, (iv) cation diffusion pathways and activation energies, (v) effect of water at neutral, acid and basic pH.

SummaryZeolites (eight species) and the associated secondary minerals (minerals akin to zeolites, chlorites and related minerals, silica, and calcite) in the Deccan Traps of Western India have been examined in some detail on the basis of chemical, optical, and X-ray studies. The three zeolite zones (laumontite, scolecite, and heulandite, in ascending order) suggested by Walker have been recognized. Efforts have been made to understand their genesis. On available field and laboratory data it is suggested that the different zones of zeolitization are the result of increasing depth and the action of the circulating fluids on the rocks. The activity of such solutions was probably enhanced in the wake of the structural disturbances and intrusive activity that affected the rocks of this area.

Summary. Mapping of the distribution of zeolites in the Antrinl basalku has revealed that gmelinite is confined to a narrow zone ahmg the eastern seaboard of Antrim. This zone comprises less than 1% of the m~ea of the b~alts, but in it gmelirfite is extremely ~bundant. The gmelinite zone is superimposed upon the lava pile, and the minerals in it are clearly later than the laves ; i~ is in the form of a dome, elongated north-west and traceable for 18 or 24 miles in this direction, and is found in the b,a,ual parts of the lava pile, locally reaching a height of sonm 500 feet in the pile. The zone may be related in its position and origin to the Islandmagee fault zone. Gmelinite frequently forms a parallel overgrowth on chabazite. Superimposed upon the southern part of the gmclinite zone is a zone of late chabazite and heulandite containing, inter alia, parallel overgrowths of late chabazite on simple or twinned gmelinite. A STUDY of the amygdale minerals in the Tertiary basalts of the Garro~l ])lateau area (W~lker, 1951) showed that the zeolites and the crystal habits of chabazite have a regular vertical distribution in the lavas. ]'he present paper concerns the distribution of the zeolite gmelinite. This mineral is confined to a narrow belt of lavas along the eastern seaboard of Antrim, referred to in this paper as the gmelilfite zone, but within this restricted zone it is extremely abundant. Superinlposed upon the gmclinite zone in its southern part, and encroaching also upon the other adjacent zeolite zones, is a zone of late chabazite and heulandite, which is also described below. In what follows, after the minerals have })eeu described, ~hc shape and relations of the gmelinite zone are considered, and the conclusons to be drawn from this study reviewed.

SummaryClinoptilolite-heulandite is reported from Jurassic, Cretaceous, and Palaeocene sediments in south-east England. It occurs mainly in 1–20 μm size fractions, and is associated with abundant montmorillonite and often also with glauconite and amorphous silica or low-temperature disordered cristobalite-tridymite. Its mode of occurrence and the associated minerals suggest an authigenic origin, but there is no evidence that the source material was volcanic.

In 1890, L. V. Pirsson 2 described under the name of mordenite a crystalline mineral, isomorphous with heulandite, occurring in a highly weathered amygdaloidal basalt in the Hoodoo Mts., Wyoming. His analysis (1, table I) showed that the mineral had a composition near that of the original mordenite of H. How (1864) and his assumption that it represented a platy-crystalline phase of that normally fibrous mineral was generally accepted.

SummaryA heulandite from Nelson Creek, Skamania County, Washington, has 30 H2O per unit cell instead of the normal 24; the excess water is lost on gentle heating. The b and c cell-dimensions are also rather larger than normal. Normal heulandite also occurs in the same area.