Synthesis of K-dominant tourmaline

Abstract

K-dominant tourmaline was synthesized in the system MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -KCl-H 2 O at 700 °C and 4.0 GPa. The crystals were zoned and characterized by less-potassic cores (1.46 wt% K 2 O) and more-potassic rims (up to 3.44 wt% K 2 O). The K-dominant tourmaline rims are represented by the average structural formula (K 0.60(3) o 0.36(3) )(Mg 2.60(7) Al 0.40(7) )(Al 5.98(3) Si 0.02(3) )Si 6 O 18 (BO 3 ) 3 (OH) 3.92(7) O 0.08(8) , which is analogous to the structural formula of dravite and is referred to here as "K-dravite"; the maximum analyzed K content (3.44 wt% K 2 O) represents occupancy of the X site by 0.71 K pfu. The addition of Na to the system in approximately equal molar proportions to K results in the crystallization of K-bearing, Na-rich dravitic tourmaline, dramatically reducing the K content to an average value of 0.47 wt% K 2 O, corresponding to 0.10 K pfu. This suggests that a K-dominated bulk composition is necessary for K-dominant tourmaline crystallization. Compositional zoning shows that solid solution exists between end-member compositions of "K-dravite" [KMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 (OH)] and dravite via the isovalent exchange X K( X Na) -1 , magnesio-foitite via the coupled substitution X K Y Mg( X o Y Al) -1 , and "K-olenite" via the coupled substitution Y MgOH( Y AlO) -1 . Structural refinement of the powder X-ray diffraction data provides a unit-cell volume for the synthesized "K-dravite" of 1580.1(5) Å 3 , which is greater than that determined for K-bearing dravitic tourmaline synthesized at the same conditions [1574.9(4) Å 3 ]. We interpret this to reflect expansion of the crystal structure due to incorporation of the relatively large K + ion.

Synthetic and natural ammonium-bearing tourmaline

“…Any nitrogen-bearing fluid-rock system at sufficiently reducing conditions may contain or produce NH 4 , which, due to its similar ionic radius to Rb, may to certain extent replace the larger monovalent cations such as K, in most silicate structures, particularly in micas and K-feldspar. In contrast, experiments by Berryman et al (2014;and in prep. ) have shown that significant K-incorporation into tourmaline is restricted to high-pressure conditions and K-rich, Na-poor bulk compositions.…”

“…The solid starting material consisted of a homogeneous mixture of SiO 2 , γ-Al 2 O 3 , MgO, and H 3 BO 3 in the stoichiometric proportions of end-member dravite, NaMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 4 , without any sodium, and with excess SiO 2 and H 3 BO 3 . Following the method of tourmaline synthesis from previous studies (Von Goerne and Franz 2000;Meyer et al 2008;Berryman et al 2014) we used excess boric acid in double of the stoichiometric amounts, and excess SiO 2 by 20 mol% to compensate for high fluid solubility of these components. About 15 mg of the solid starting material was loaded into a welded gold capsule 10 mm in length and 3 mm in diameter together with ~3 mg of a 25% NH 4 (OH) solution.…”

“…Recent experimental studies at various pressure and temperature (P-T) conditions have shown that in the system MgO-Al 2 O 3 -SiO 2 -B 2 O 3 -KCl-(NaCl)-H 2 O tourmaline may incorporate up to 0.71 K pfu, provided that the pressure is high enough (4.0 GPa, 700 °C) and the bulk composition Na-poor (Berryman et al 2014;and in prep.). For Na-free bulk compositions, these synthetic tourmalines are mainly solid solutions between the end-member compositions of K-dravite [KMg 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 (OH)], Mg-foitite [o(Mg 2 Al)Al 6 (Si 6 O 18 )(BO 3 ) 3 (OH) 3 (OH)], and Kolenite [KAl 3 Al 6 Si 6 O 18 (BO 3 ) 3 O 3 (OH)].…”

Dumortierite and tourmaline from the Barchi‐Kol diamond‐bearing kyanite gneisses (Kokchetav massif): A Raman spectroscopic study and petrological implications

“…Based on the finding of microdiamond inclusions in K‐rich tourmaline from the Kokchetav massif, Shimizu and Ogasawara suggested that the incorporation of K in the tourmaline structure can extend its stability field up to 6 GPa and 1000°C. A successful synthesis of K‐rich tourmaline has been achieved at 4 GPa and 700°C, which also upheld its high‐pressure origin.…”

Experimental study of the stability and synthesis of the tourmaline supergroup minerals

“…Воскресенской, являются очень высокие Р−Т-параметры и высококонцентрированные растворы, требующие использования дорогостоящего оборудования и материалов. В работе (Goerne et al, 1999) Rosenberg, Foit, 1986;Ekambaram, 1986;Schreyer et al, 2000;Marler et al, 2002;Ertl et al, 2012;Kutzschbach et al, 2017 Rosenberg, Foit, 1986;Ekambaram, 1986;Lynch, 1997;Goerne et al, 1999 Dutrow et al, 1999, Ertl et al, 2012Berryman et al, 2015Berryman et al, , 2016 CaO Воскресенская, иванова, 1975;Taran et al, 1993;Setkova et al, 2011;London, 2011;Рождественская и др., 2012Рождественская и др., , Vereshchagin et al, 2015 ЭКсПеРиМенТАльнОе изучение усТОйчиВОсТи ГЕОХИМИЯ №10 2019 сеТКОВА, БАлицКий, ШАПОВАлОВ и в особенности давлениях до 4 ГПа (Berryman et al, 2014(Berryman et al, , 2016.…”