Other
Scientific paper
Jun 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984gecoa..48.1331w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 48, Issue 6, pp.1331-1344
Other
13
Scientific paper
An end member of the tourmaline series with a structural formula (Mg 2 Al)Al 6 (BO 3 ) 3 [Si 6 O 18 ](OH) 4 has been synthesized in the system MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -H 2 O where it represents the only phase with a tourmaline structure. Our experiments provide no evidence for the substitutions Al Mg + H, Mg 2H, B + H Si, and AlAl MgSi and we were not able to synthesize a phase "Mg-aluminobuergerite" characterized by Mg in the (3a)-site and a strong (OH)-deficiency reported by and (1975). The alkali-free tourmaline has a vacant (3a)-site and is related to dravite by the + Al for Na + Mg substitution. It is stable from at least 300°C to about 800°C at low fluid pressures and 100% excess B 2 O 3 , and can be synthesized up to a pressure of 20 kbars. At higher temperatures the tourmaline decomposes into grandidierite or a boron-bearing phase possibly related to mullite ("B-mullite"), quartz, and unidentified solid phases, or the tourmaline melts incongruently into corundum + liquid, depending on pressure. In the absence of excess B 2 O 3 tourmaline stability is lowered by about 60°C. Tourmaline may coexist with the other MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -H 2 O phases forsterite, enstatite, chlorite, talc, quartz, grandidierite, corundum, spinel, "B-mullite," cordierite, and sinhalite depending on the prevailing PTX-conditions. The (3a)-vacant tourmaline has the space group R3m with a =15.90 Å, c = 7.115 Å, and V = 1557.0 Å 3 . However, these values vary at room temperature with the pressure-temperature conditions of synthesis by ±0.015 Å in a , ±0.010 Å in c , and ±4.0 Å 3 in V , probably as a result of MgAl order/disorder relations in the octahedral positions. Despite these variations intensity calculations support the assumed structural formula. Refractive indices are n o = 1.631(2), n E = 1.610(2), n = 0.021. The infrared spectrum is intermediate between those of dravite and elbaite. The common alkali and calcium deficiencies of natural tourmalines may at least partly be explained by miscibilities towards (3a)-vacant end members. The apparent absence of (3a)-vacant tourmaline in nature is probably due to the lack of fluids that carry boron but no Na or Ca.
Schreyer Werner
Werding Günter
No associations
LandOfFree
Alkali-free tourmaline in the system MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -H 2 O does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Alkali-free tourmaline in the system MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -H 2 O, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Alkali-free tourmaline in the system MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -H 2 O will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1854221