Scapolite : alkali atom configurations , antiphase domains , and compositional Yariations
Electrostatic energy calculations on an intermediate scapolite indicate that Na+ should prefer to be adjacent to Clanions and Ca2* to be adjacent to CO3radicals. The calculations also indicate that short-range ordering of NarCl and CaaCO3 clusters is energetically favored and might give rise to antiphase domains. Electrical neutrality considerations on the anion/radical site in scapolite suggest that the unusual compositional variations observed in the marialite-meionite series are the result of crystallographic constraints on the two possible independent exchange reactions in scapolite: NaCICaCO-3 and NaSie6-Ai. The compositions of naturally occurring scapolites may be explained in terms of the scapolite composition that represents local charge balance between Ca2*, Na* cations and Cl-, COIanions.
Mineralogical Zoning in a Scapolite-Bearing Skarn Body on San Gorgonio Mountain , California
Concentrically zoned skarns on Mt. San Gorgonio have been studied in detail from the point of view of field and petrographic relations. The skarns are mineralized blocks of marble of the Precambrian San Gorgonio Igneous-Metamorphic Complex, which are within the contact zone between the Complex and the Middle Jurassic Cactus Quartz Monzonite. They consist of cores of calcite-forsterite-diopside marble, surrounded by concentric zones of diopside, actinolite * epidote * calcite + quartz, epidote + garnet * calcite * quartz, and scapolite + calcite + quartz. The genesis of the skarns involved three steps. (l) Regional metamorphism formed tremolite and forsterite from calcite, dolomite, and quartz. (2) The intrusion of the quartz monzonite stabilized diopside * forsterite relative to tremolite * calcite. Reaction between quartz monzonite and marble resulted in the formation of a zone of diopside * wollastonite and a zone of sodic scapolite, by the diffusion of Ca, Mg, and Si across the original contact. Components rejected from this zone formed ferroan diopside in the scapolite zone and in the quartz monzonite, and altered the scapolite to a more calcic form. This process involved a net volume loss of 56.6 percent. (3) Inf i l t rat ion of f luids, possibly of magmatic origin, formed epidote and garnet from scapolite, and actinolite from diopside.
On phase equilibria in metamorphosed siliceous dolomites
Basing on calculations and experiments the equilibrium curves of 15 reactions occuring during metamorphism of siliceous dolomites are presented. In a T—XCO2 diagram these curves intersect at four isobaric invariant points. This is consistent with the fact, that different reaction sequences are known from the nature.