Composition of Analcime from Sedimentary and Burial Metamorphic Rocks¹

Data for correlation of unit cell size with composition of natural and synthetic analcimes are reviewed and are plotted on a determinative graph. Unit cell sizes are given for about 40 analcime samples from sedimentary and burial metamorphic rocks from 13areas. The range in cell sizes i s found to be from a_o = 13.721Å to 13.679Å, corresponding to a range in composition of approximately Na₁₆Al₁₆Si₃₂O₉₆ · 16H₂O to Na₁₃Al₁₃Si₃₅O₉₆ · nH₂O.

Silica-rich analcimes apparently form by reaction of acid volcanic ash with saline water. Analcimes formed in the absence of quartz or other silica minerals are less siliceous than about Na_14.5Al_14.5Si_33.5O₉₆ · nH₂O. Irrespective of this, relatively silica-poor analcimes form either by direct precipitation or by reaction of clay minerals and perhaps other materials with waters of high alkalinity. Dolomite is often present in the resulting assemblage. Analcime formed during burial metamorphism tends to be of intermediate composition close to Na₁₄Al₁₄Si₃₄O₉₆nH₂O and may represent a closer approach to the quartz-analcime equilibrium over an appreciable range of temperatures and pressures.

Cell dimensions of analcime from the Murihiku Supergroup of New Zealand are very much alike, even in samples separated stratigraphically by as much as 4600 m. In other suites, there are wide variations in composition between analcimes from closely spaced horizons. Factors which affect analcime composition evidently include the composition of the parent material and the chemical environment in which the analcime crystallizes. The silica content of natural analcime is not solely a function of temperature and has little significance as a geothermometer.