Compositionally, scapolite is essentially plagioclase feldsparwith chloride ions and carbonate and sulfate radicals ininterstices of the structure. The extra anions are balanced byextra sodium or calcium (and some potassium) atoms, so theformula can be written as plagioclase plus halite, calcite, orgypsum.
The formula suggests environments containing both feldsparplus volatiles: typically contact metamorphic and metasomaticenvironments.
|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 15 January 2020