Clinoptilolite Subgroup

Clinoptilolite is the name applied to specimens with unknown dominance of the extra-framework cations. It comprises clinoptilolite-Ca, clinoptilolite-K and clinoptilolite-Na.

Note on the distinguishing differences between clinoptilolite and heulandite (http://rruff.info/uploads/CM35_1571.pdf ):

"Zeolite mineral species shall not be distinguished solely on the basis of the framework Si : Al ratio. An exception is made in the case of heulandite and clinoptilolite; heulandite is defined as the zeolite mineral series having the distinctive framework topology of heulandite and the ratio Si : Al < 4.0. Clinoptilolite is defined as the series with the same framework topology and Si : Al ≥ 4.0.

Application of the rule:
Many zeolites have a widely variable Si : Al ratio, but this, in itself, is not regarded as providing adequate grounds for recognition of separate species. The exception is based on entrenched usage of the names heulandite and clinoptilolite, and their convenience for recognizing an important chemical feature. The cutoff value adopted (following Boles 1972) is arbitrary in a continuous range of compositions. The usual 50% compositional rule cannot be applied, as there are no clearly defined Si,Al end-member compositions for heulandite and clinoptilolite. Thermal stability has been used by some investigators to distinguish clinoptilolite from heulandite. This is a derivative property, however, suggested by Mumpton (1960) as an aid to identification, and it is not appropriate as the basis for definition. Alietti (1972) and Boles (1972) have shown that there is no gap in composition either in framework or extra-framework cation contents between heulandite and clinoptilolite, and that samples transitional in composition show intermediate properties in terms of thermal stability.

Clinoptilolite:
The cation content is highly variable. Ca-, Na-, and K-dominant compositions are known, and Sr, Ba, and Mg are in some cases substantial. TSi in the range 0.80–0.84. Minerals with the same framework topology but with TSi < 0.80, Si/Al < 4.0 are classified as heulandite, with which clinoptilolite forms a continuous series. Monoclinic, C2/m, or C2, or Cm.

Heulandite:
The cation content is highly variable. Ca-, Na-, K-, and Sr-dominant compositions are known, and Ba and Mg are in some cases substantial. TSi in the range 0.71 to 0.80."