Philip E. Rosenberg

Coupled substitutions in the tourmaline group

AbstractStatistical analysis of 136 natural tourmaline compositions from the literature reveals the presence and extent of coupled substitutions involving several cations and structural sites. In schorls and dravites these are a dehydroxylation type substitution (1) (OH)−+R2+ = R3++O2− and an alkali-defect type substitution (2) R++R2+ = R3++□, Al3+ being the predominant R3+ action. Substitution (1) which represents solid solution towards a proton-deficient end-member, R+ R33+R63+(BO3)3 Si6O18O3(OH), accounts for three times as much of the observed compositional variability as does (2) which represents substitution toward a hypothetical alkali-free end-member, □(R22+R3+) R63+(BO3)3Si6O18(OH)4. The occurrence of both of these substituions produces intermediates between end-member schorl/ dravite, R+ R32+R63+(BO3)3Si6O18(OH)4, and a new series within the tourmaline group, R1−x+R33+R63+(BO3)3Si6O18O3−x (OH)1+x.In addition to dehydroxylation type, 2(OH)−+Li+ = R3++202−, and possibly alkali-defect type, 2R++Li+ = R3++2□, substitutions, a third type Li++O2− = (OH)−+□, occurs in the elbaites giving rise to Li-poor, proton-rich species. All three substitutions serve to reduce the Li-content of natural elbaite which, as a result, does not attain the composition of the ideal end-member, Na(Li1.5Al1.5)Al6(BO3)3Si6O18(OH)4. Substitution from elbaite and schorl/dravite toward R1−x+R33+R63+(BO3)3Si6O18O3−x(OH)1+x is very extensive and may be complete.Substitution toward R1−x+R33+R63+(BO3)3Si6O18O3−x(OH)1+x results in improved local charge balance. The mean deviation

The nature, formation, and stability of end-member illite: A hypothesis

{\Delta \zeta \left( {\text{O}} \right)}

Illite equilibria in solutions: I. Phase relationships in the system K2OAl2O3SiO2H2O between 25 and 250°C

from oxygen charge saturation

Rare-earth element geochemistry of fluorite-carbonate deposits in western Montana, U.S.A.

\left( {\zeta \left( {\text{O}} \right) = 2.0} \right)

Non-magmatic fracture-controlled hydrothermal systems in the Idaho Batholith: South Fork Payette geothermal system

is at a maximum in end-member schorl, dravite and elbaite. Substitutions (1) and (2) progressively decrease