B. Igneous zeolies
The igneous zeolites usually occur as well-developed crystals, sometimes as large as 1 inch in diameter (Fig. 3.1). They are assumed to have crystallized in cavities and along fractures in basalt from the aqueous solutions corresponding t the last stages of magmatic activity. In most cavities, successive growth of several different zeolite minerals is observed and is usually accompanied by other hydrous minerals. In Nova Scotia basalt, mordenite is reported to comprise 20% of the rock in some localities (15). However, it is rare that the fraction of zeolite mineral in the parent rock exceeds about 5% by volume. Coombs pointed out that the appearance of zeolites in igneous rocks is difficult to interpret on a physicochemical basis. Although some authors have published the paragenetic sequence, the significance of such conclusions is doubtful.