Abstract
Alkali-free tourmalines synthesized in the system MgO-AI2O3-SiO2-B2O3-H2O in the presence of excess B2O3 and H2O at 1 kbar have been characterized by optical and X-ray measurements, partial chemical analyses, and infrared spectra. Infrared absorption bands in the OH-stretching region are more similar to those of elbaite than dravite.
| Syn. Temp. . | Refractive Index . | . | Cell Dimensions . | . | Partial Analysis (Mole%) . | . | . |
|---|---|---|---|---|---|---|---|
| (°C) | ε | ω | α (A) | c(A) | SiO2 | Ai2o3 | MgO |
| 600 | 1.605(1) | 1.624(1) | 15.884(2) | 7.128(1) | 49.2 | 32.8 | 18.0 |
| 450 | 1.607(1) | 1.624(1) | 15.847(5) | 7.108(3) | 44.2 | 34.9 | 20.9 |
| Syn. Temp. . | Refractive Index . | . | Cell Dimensions . | . | Partial Analysis (Mole%) . | . | . |
|---|---|---|---|---|---|---|---|
| (°C) | ε | ω | α (A) | c(A) | SiO2 | Ai2o3 | MgO |
| 600 | 1.605(1) | 1.624(1) | 15.884(2) | 7.128(1) | 49.2 | 32.8 | 18.0 |
| 450 | 1.607(1) | 1.624(1) | 15.847(5) | 7.108(3) | 44.2 | 34.9 | 20.9 |
Assuming stoichiometric boron, variables in site occupancies include numbers of (1) vacancies at alkali sites, (2) IVA1, (3) VIA1, and (4) protons. Although chemical data serve to constrain site occupancies only within broad limits, structural formulas are univariant in that determination of any one variable would fix the others. Comparison of cell parameters of alkali-free tourmaline with those of elbaite and dravite suggest approximate VI(Mg/Al) ratios which correspond to the structural formulas:
Alkali site occupancy (IXMg) and Al content are higher while the proton content is lower at 450°C than at 600°C. Alkali-free tourmaline synthesized at 600°C is an Al-rich, alkali-defective dravite.
