Jeremejevite as a precursor for olenitic tourmaline: consequences of non-classical crystallization pathways for composition, textures and B isotope patterns of tourmaline
Jeremejevite as a precursor for olenitic tourmaline: consequences of non-classical crystallization pathways for composition, textures and B isotope patterns of tourmaline
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Jeremejevite 作为橄榄石碧玺的前体：非经典结晶途径对碧玺成分、结构和 B 同位素模式的影响
DOI：
10.1127/ejm/2017/0029-2604
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发表时间：
2017
影响因子：
2.1
通讯作者：
Heinrich
中科院分区：
文献类型：
--
作者：
Kutzschbach;Wunder;Meixner;Heinrich
许多关于电气石合成的研究都报告了在晶体的核心区域存在尖锐的晶内化学梯度和广泛的孔隙度，这两种情况仍然缺乏可靠的解释。使用的例子，我们表明，这些功能可能是解释的前体阶段的发生在tourcraft形成。在SiO_2-Al_2O_3-B_2O_3-NaCl-H_2 O体系中，以石英-Al_2O_3-H_3BO_3固体混合物和NaCl溶液为原料，在700 ℃/40 kbar下进行了时间相关的活塞-圆筒合成实验，运行持续时间为0.5 h、2.5 h和216 h。在所有的实验中都形成了明显的环带状角闪石（[4]富硼核，[4]贫硼边缘），其丰度随着运行时间的增加而增加。另外的固体产物相柯石英和水铝石的量随时间而减少。广泛的孔隙度是公认的jeetrojevite和早期生长的针状tourcover的核心。结构关系表明，橄榄状电气石的增长是以牺牲在这个系统中作为晶体前体的jealitic jevite为代价的。可能的反应是：3Al6（BO3）5（OH）3+（12−2x）SiO2（aq）+2yNaCl（aq）→2NayAl3Al6（Si6−xBx）O18（BO3）3（O3−x，OH1+x）+（4.5 −x）B2O3（aq）+（2.5−x）H2O+2yHCl（aq）。基于Rayleigh分馏模型，我们估算了在700 °C/40 kbar下，Δ 11 Bjer流体的δ 11 = 2.8‰时，硼镁石与流体之间的最小B同位素分馏。由于水硅镁石的逐步溶解，流体δ11B值随运行时间的增加而不断降低。因此，伴随着钾镁矾的溶解而生长的橄榄石将在钾镁矾中产生分散或反向的硼同位素模式（重核，轻边），这不能由简单的瑞利分馏产生。类似的反应，涉及jeetrojevite或其他前体相可能解释化学环带和多孔结构的tourcester核心区在许多实验研究报告。在纳米比亚Erongo山脉伟晶岩中天然tourplasty过度生长的jeetrojevite的出现，引起了jeetrojevite也可能作为tourplasty形成的自然系统中的前体的假设。
Many studies dealing with the synthesis of tourmaline report a sharp intragranular chemical gradient and extensive porosity in the core zones of the crystals, both of which still lack a reliable explanation. Using the example of olenitic tourmaline, we show that these features are likely explained by the occurrence of a precursor phase during tourmaline formation. Time-dependent piston–cylinder synthesis experiments were performed in the system SiO2–Al2O3–B2O3–NaCl–H2O at 700 °C/40 kbar with run durations of 0.5 h, 2.5 h and 216 h, starting from quartz–Al2O3–H3BO3solid mixtures and NaCl solutions. Sharply zoned olenitic tourmaline ([4]B-rich cores,[4]B-poor rims) formed in all experiments, with its abundance increasing with increasing run duration. The amount of the additional solid product phases coesite and jeremejevite decreased with time. Extensive porosity is recognized in jeremejevite and in the cores of early grown acicular tourmaline. Textural relationships indicate that olenitic tourmaline grows at the expense of jeremejevite which acts as a crystalline precursor in this system. A possible reaction is: 3Al6(BO3)5(OH)3+(12−2x)SiO2(aq)+2yNaCl(aq)→2NayAl3Al6(Si6−xBx)O18(BO3)3(O3−x,OH1+x)+(4.5−x)B2O3(aq)+(2.5−x)H2O+2yHCl(aq).The transformation likely proceeds via a dissolution/re-precipitation mechanism, which triggers the sharp chemical zonation in olenite. Based on Rayleigh fractionation modelling, we estimate a minimum B isotope fractionation between jeremejevite and fluid with Δ11Bjer-fluid≈ −2.8‰ at 700 °C/40 kbar. Due to the progressive dissolution of jeremejevite, the fluids δ11B values continuously decrease with increasing run duration. Hence, olenite growth concomitant with the dissolution of jeremejevite will produce scattered or inverse boron isotope patterns (heavy cores, light rims) in tourmaline, which cannot result from simple Rayleigh fractionation. Similar reactions involving jeremejevite or other precursor phases might explain chemical zonation and porous textures in tourmaline core zones reported in many experimental studies. The occurrence of natural tourmaline overgrowing jeremejevite in pegmatites of the Erongo Mountains, Namibia, gives rise to the assumption that jeremejevite might also act as a precursor for tourmaline formation in natural systems.
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