*2.1. Sample Synthesis and Characterization*

A total of five coesite samples were synthesized at the *P-T* conditions of 5 and 7.5 GPa and 1273 K with heating durations of 9–12 hours (Table 1), using welded Pt capsules in sintered MgO octahedron assemblies in the 1000-ton multi-anvil press at China University of Geosciences (Wuhan). The corner truncation of the 25.4-mm tungsten carbide cubes was 12 mm for synthetic experiments at 5 GPa and 8 mm for the runs conducted at 7.5 GPa, respectively. Temperature was monitored with a W5Re95–W2Re74 (C-type) thermocouple, and a graphite furnace was used in our experiments. Analytical reagent SiO2, Al(OH)3, B(OH)3 (purity of >99.99%) were adopted as the starting materials to synthesize hydrous coesite samples with different compositions: Si-pure (Run 503), B-doped (R663), Al-doped (R749), and B,Al-doped (R694 and R712). Excess liquid water (1 μL) was added in each capsule to guarantee the water fugacity. Single crystals up to 300 μm were recovered from these synthetic experiments, while no other crystallized phases were detected (by Raman spectra) in the run products.

In-situ analyses of the trace elements of B and Al in these synthetic coesite samples were conducted on an Agilent 7900 inductively coupled plasma mass spectrometry (ICP-MS) (Agilent Technology, Tokyo, Japan) combined with a Yb femtosecond laser ablation (fs-LA) system (GeoLas 2005, Lambda Physik, Göttingen, Germany), without applying an internal standard [29]. The ICP-MS works at a power of 1350 W with a plasma and an auxiliary gas flow rate of 15.0 and 1.0 L/min, respectively; while the fs-LA system (λ = 257 nm) is operated at a repetition rate of 8 Hz and a pulse length of 300 fs. The spot size is 24 μm with an energy density of 2.8 J/cm2, and a mixture of He and Ar is used as the carrier gas. The element contents of B and Al were calibrated against multiple-reference materials (BCR-2G, BIR-1G, and BHVO-2G) using the 100% oxide normalization method [30] with the detection limits of 0.1 ppmw for B and 0.8 ppmw for Al, and the determined average B and Al concentrations in these reference materials show relative deviations of −5 to about −10% from the recommended values [31]. The derived B and Al concentrations are listed in Table 1.

Because the Al concentration in R749 is as high as about 400 ppmw as indicated by fs-LA–ICP-MS, we further checked the Al composition by a JEOL JXA-8100 electron probe micro analyzer (EPMA) (JEOL Ltd., Akishima, Japan), which is equipped with four wavelength-dispersive spectrometers (WDS). The EPMA system is operated at an accelerating voltage of 15 kV and a beam current of 5 nA, while the spot size is reduced to 10 nm to minimize the fluctuations of X-ray intensity as well as sample damage [32]. The certified mineral standards of pyrope garnet (for Al) and olivine (for Si) were adopted for quantification using ZAF wavelength-dispersive corrections. Totally, twelve points were selected for measurements on the sample of R749, and the derived Al2O3 content is 0.0742 ± 0.0096 wt.% with a detection limit of 100 ppmw (corresponding to 393 ± 51 ppmw for the Al element), which is consistent with the result from fs-LA–ICP-MS within the experimental uncertainties.


**Table 1.** Starting composition and synthetic condition for each run. The B and Al element concentrations are mainly measured by femtosecond laser ablation

measured by ICP-MS; *b*: determined by an electron probe micro analyzer (EPMA).

 5

> *a*:

 1273

 9

 —

 —

392.7 ± 50.8 *b*

1166 ± 151 *b*

7.2 ± 2.9

 48 ± 19

**R749:**

SiO2 (96) + Al(OH)3 (4) + 1 μL H2O

#### *Crystals* **2019** , *9*, 642
