Compositionally, scapolite is essentially plagioclase feldspar with chloride ions and carbonate and sulfate radicals in interstices of the structure. The extra anions are balanced by extra sodium or calcium (and some potassium) atoms, so the formula can be written as plagioclase plus halite, calcite, or gypsum.
The formula suggests environments containing both feldspar plus volatiles: typically contact metamorphic and metasomatic environments.
|In the view at left, blue tetrahedra
consist of silicon or aluminum surrounded by oxygens.
Yellow spheres represent sodium or calcium, purple
represents anions at elevation 0 and 1/2c.
There are two types of four-membered tetrahedral rings.
The structure is obviously tetragonal. a = 12.1 Angstrom units, c = 7.6
|In the side view at left, we see how alkali metal ions are contained in small tetrahedral cages and large anion radicals in the large openings. Here the varying size of the ions represents distance from the plane of the diagram.|
Created 6 March, 2002, Last Update 14 December 2009
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