Abstract
Gonnardite, phillipsite and gismondine occur as euhedral to subhedral crystals in the cavities of calcareous xeno-lith. Thomsonite and a scolecite-like zeolite often replace nepheline crystals, and rarely fill up minute vesicles in nephelin-ite. The chemical analyses were made by EDS. The most Na-rich gonnardite is (Na6.61Ca1.36)Σ7.79 [Al9.11Si10.84O40] nH2O, and the most Na-poor one is (Na5.21Ca1.72)Σ6.93 [A19.47Si10.47O40]· nH2O. Phillipsite-K and phillipsite-Ca are rec-ognized. The former is (K2.11Ca1.71Na0.93Ba0.10)Σ4.85[Al6.73Si9.29O32] nH2O in association with Na-rich gonnardite, and the latter is (Ca1.92K1.32Na0.79Ba0.53)Σ4.56[Al7.13Si8.90O32]·nH2O in association with Na-poor gonnardite. Gismondine has a narrow compositional variation with minimum Ca/(Na+K+Ca)= 0.92 and Al/Si = 0.95-0.97 in tetrahedral framework, and Na- and K-bearing one is (Ca0.96Na0.04K0.04)Σ1.04[Al1.96Si2.03O8] ·nH2O. Thomsonite also has a narrow composition-al variation with Ca/(Na+K+Ca) = 0.62-0.69 and Al/Si = 0.95-0.99. One of thomsonite existing in boundary of nepheline and gonnardite is (Ca2.03Na1.14)Σ3.177[Al4.91Si5.01O20]· nH2O. Due to very minute grain of a fibrous zeolite, exact identi-fication of species by X-ray powder diffraction was not made. As the chemical composition is close to known scolecite, it is tentatively called as scolecite. It is small compositional variation except minor substitution for Ca by Na in case of re-placing nepheline. The composition of vesicle filling one is, (Ca0.99Na0.05)Σ1.04[A12.02Si2.98O10]· nH2O. These zeolites found in cavities have been formed under the reaction with nephelinite magma and calcareous xenolith.
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