![]() Both sites lie close to the mirror plane in the space group Pbnm, but their short distance (1.7 Å) prevents their simultaneous occupancy. 7 found that in topaz-OH hydrogen atoms are spread over two independent crystallographic sites one of these is not found in naturally occurring fluorine-bearing topaz. Conflicting results about the exact space group were reported for the synthetic end-member “topaz-OH” 7, 8, 9. The existence of an upper limit to the OH content is rationalized in terms of hydrogen located in two partially occupied sites whose occupancy limits the OH substitution for F to 50% (“proton-proton avoidance rule”) 6. Usually, natural topaz with OH/(OH + F) < 0.5, crystallizes in the space group Pbnm, with one independent H-site, whereas in synthetic Al 2SiO 4(OH) 2 it displays two non-equivalent H-sites. ![]() For this reason, the study of the OH/F ratio plays a key role to understand the topaz’s formation ambient. A series of hydroxyl-rich topaz (OH-topaz) from X OH = 0.22 up to the pure end-member Al 2SiO 4(OH) 2, were synthesized 5 at high-pressure/high temperature conditions (pressure from 5.5 to 10 GPa, temperature up to 1000 ☌) in the Al 2O 3–SiO 2–H 2O system. Higher OH content was reported for topaz found in ultrahigh-pressure (UHP)-rich Topaz–Kyanite quartzites from Hushan (west of Dongai), (X OH = 0.35), and southern Sulu (X OH = 0.40–0.55), eastern China 4. Its composition ranges from a nearly OH-free end member, Al 2SiO 4F 2, in acid igneous rocks, to Al 2SiO 4F 1.4(OH) 0.6, with X OH = OH/(OH + F) = 0.30, in hydrothermal deposits 3. Topaz is one of the principal fluorine-bearing silicates that occurs as an accessory mineral in fluorine-rich silicate rocks (rhyolites and granites) associated with pneumatolytic/hydrothermal events, and in ultrahigh-pressure rocks 1, 2. The modelled H 2O saturated fluids with the F content not exceeding 1 wt% may represent an anomalous water-dominant / fluorine-poor pegmatite lens of the Padre Paraíso Pegmatite Field. The log(fH 2O/fHF )fluid (1.27 (0.06)) is coherent with the fluorine activity calculated for hydrothermal fluids (pegmatitic stage) in equilibrium with the forming mineral (log(fH 2O/fHF) fluid = 1.2–6.5) and clearly different from pure magmatic (granitic) residual melts. On the basis of this behaviour, we suggest that this temperature may represent the potential initial topaz’s crystallization temperature from supercritical fluids in a pegmatite system. Topaz crystallinity and fluorine content sharply decrease at 1170 K, while mullite phase starts growing. The X OH (0.484) is close to the maximum X OH value (0.5), and represents the OH- richest topaz composition so far analysed in the Minas Gerais district. The fluorine content estimated from neutron diffraction data is ~ 1.03 a.f.u (10.34 wt%), in agreement with the chemical data (on average 10.0 wt%). ![]() Understanding the role of OH/F substitution in topaz is important in order to determine the hydrophilicity and the exchange reactions of fluorine by hydroxyl groups, and ultimately to characterize the environmental redox conditions (H 2O/F) required for mineral formation. In this work we fully characterized blue topaz from Padre Paraíso (Minas Gerais, Brazil) by means of in situ synchrotron X-Ray and neutron powder diffraction measurements (temperature range 298–1273 K) combined with EDS microanalyses. Topaz is one of the main fluorine-bearing silicates occurring in environments where variably acidic (F)/aqueous (OH) fluids saturate the silicate system. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |