While both zeolites and clays are aluminosilicate minerals, zeolites are distinguished by their porous, three-dimensional microporous network structure with interconnected cavities and channels, unlike the layered structure of clays which allows for swelling and shrinking in water. Clinoptilolite can absorb and desorb H20 indefinitely without changing its form.

Figure 1. Molecular geometry of a silicate (SiO4) tetrahedron and an aluminate (AlO4) tetrahedron. Both structures are tetrahedral, with a central silicon or aluminum atom bonded to four oxygen atoms. 

Clinoptilolite is a heulandite-type zeolite mineral that is a nontoxic microporous hydrated aluminosilicate. It is characterized by a stable 3-D framework of linked SiO4 and AlO4 tetrahedra, with intervening open channels and cages. Partial substitution of Si4+ by Al3+ results in an overall negatively charged crystal lattice, largely neutralized with diverse exchangeable extra framework cations (commonly K+, Na+, Ca2+, and/or Mg2+) that are loosely bound within the open pore structure. Water molecules also occupy open framework cavities.

Figure 2. Clinoptilolite structure (generated by VESTA [3])

Zeolites occur in several geologic settings of differing ages, but nearly all mineable deposits are of Cenozoic age. The predominant zeolite precursors worldwide are acid to intermediate volcanic glass from ashfall tuffs interbedded with lacustrine sediments, from vitroclastic tuffs in volcanic rocks or from hydrothermally altered vitrophyres and vitroclastic tuffs. 

Figure 3. Clinoptilolite surface mining operation.

Clinoptilolite occurrences unrelated to volcanic ash are less common and typically linked to feldspar, feldspathoid, biogenic silica, or clay mineral alteration. Sheppard (1973) identified six scenarios of zeolite formation: (1) in hydrologically closed (saline/alkaline) systems, (2) in open (freshwater/groundwater) systems, (3) during burial diagenesis/metamorphism, (4) by hydrothermal activity, (5) in deep marine environments, and (6) by weathering of soils.

The lattice properties, combined with the resulting sorptive character, make clinoptilolite an important, high demand commodity, accounting for the predominant part of natural zeolite production. Clinoptilolite is used in concrete and mortar as a pozzolan, as a paper and rubber filler, in soil conditioning and fertilizing in horticulture and agriculture, and in treating gas and exhaust gas, water and wastewater, and organic and nuclear waste. It is also important both for animal husbandry and for medical purposes in humans. Chemically modifying the surface of clinoptilolite and loading the framework with different cations allows many further uses.