COMPOSITIONAL RANGES OF CLINOPTILOLITE AND HEULANDITE: CONTROL BY FLUID COMPOSITION OR PETROLOGIC CONSTRAINTS?

Based on extensive analytical data primarily from Yucca Mountain (YM), Nevada, Bish and Boak (2001) described distinct chemical trends for clinoptilolite (CPT) and heulandite (HEU). CPT shows Ca/Ca+Na+K ranging from 0.02-0.92 but restricted Si/(Si+Al) ratios (0.80-0.86); HEU shows restricted Ca/Ca+Na+K from 0.67-0.92, but wider ranging generally lower Si/(Si+Al) ratio (0.74-0.85). Compositional ranges of naturally occurring samples are more constrained than those in the laboratory, where Ca, Na and K can be almost completely exchanged, regardless of Si/Al ratio. Some compositions in the CPT-HEU zone of overlap show intermediate thermal behavior, suggesting intergrowth of two phases rather than true compositional overlap. Three explanations for the apparent restricted compositional ranges of CPT and HEU are: (1) water chemistry dictates variation; (2) phase equilibrium limit variations, and 3) crystal structure limits variation. At YM, high Si/Al HEU forms in very shallow horizons, whereas low Si/Al HEU forms in deeper zones (near or below the disappearance of opal-CT), especially in the eastern area, where deep-seated carbonates may influence water composition. Ca-poorer Si-rich CPT occurs at intermediate depth in the west and east areas. These geographic and depth controls support option 1 to explain compositions of CPT/HEU at YM. However, Stonecipher (1978) showed limits to Si/Al in Na,K-rich CPT similar to those from YM. It is not clear that water chemistry is exclusively responsible for the lack of low Si/Al, alkali-rich CPT. Option 2 implies that for CPT, low Si/Al compositions may be unstable with respect to another Na/K-mineral with limited Ca solid solution, such that low Si/Al HEU can form. Option 3 implies structural constraints, as documented by Ross et al. (1992) for some natrolite group minerals, in which covariation of alkali/ alkaline earth elements with Si/Al ratio reflect occupancy of intraframework channels. Without data from other localities, it may be difficult to distinguish among crystallographic, petrologic,and geochemical constraints on CPT/HEU compositions at YM. Suites of data on chemical variation of natural zeolites are uncommon in the literature, but offer substantial benefits in understanding the structure and stability of these complex solid-solution minerals.