Cr³⁺/Cr²⁺ in melts in the systems CaO-MgO-Al₂O₃-SiO₂±Na₂O±K₂O doped with Cr added as 0.5 wt% Cr₂O₃ were determined as a function of oxygen fugacity (fO₂) at 1400°C by XANES spectroscopy of their quenched glasses, using the intensity of the shoulder on the Cr K-edge due to the 1s→4s transition. The addition of Na and K to the system CMAS increases Cr³⁺/Cr²⁺ at constant temperature and fO₂, in good agreement with the predictions from the “ideal optical basicity”. The new results have been combined with previous results to calibrate a model for Cr³⁺/Cr²⁺ in silicate melts as a function of temperature, pressure and melt composition: log₁₀(Cr³⁺/Cr²⁺) = ¼ ΔQFM + 3031/T - 2.26 + (843 P – 158 P²)/T + ∑c_ZX_Z

where X_Z are the mole fractions of the oxide components Z defined on the single-cation basis and ∑c_ZX_Z = 2.00 X_NaO0.5 + 1.00 (X_MgO + X_Fe2+O) + 0.37 (X_AlO1.5 + X_Fe3+O1.5) + 2.12 X_KO0.5 + 2.44 X_CaO + 3.69 X_TiO2, T is temperature in K, P is pressure in GPa, and ΔQFM is the difference between the fO₂ of the silicate melt and the Quartz-Fayalite-Magnetite buffer at 10⁵ Pa, given by log₁₀fO₂(QFM) = 8.58 – 25050/T, relative to the conventional standard state of pure O₂ at 10⁵ Pa. The effect of pressure is markedly non-linear, so the model should not be extrapolated above 4 GPa. Combining this model with the similar one for Fe³⁺/Fe²⁺ in silicate melts gives: log₁₀(Cr³⁺/Cr²⁺) = log₁₀(Fe³⁺/Fe²⁺) + 3031/T - 0.9 + (1317 P – 219 P²)/T + ∑j_ZX_Z

where ∑j_ZX_Z = 1.00 (X_MgO + X_Fe2+O) + 0.37 (X_AlO1.5 + X_Fe3+O1.5) -1.59 X_KO0.5 +0.04 X_CaO + 3.69 X_TiO2.

High Cr²⁺/∑Cr in silicate melts is promoted by high temperature and low pressure, as well as low Fe³⁺/∑Fe. For a parental MORB melt composition at 1250˚C (1523 K), 10⁵ Pa, with Fe³⁺/∑Fe = 0.10, the model predicts Cr²⁺/ΣCr = 0.27. The effect of pressure is very large: the Cr²⁺/ΣCr in the above example would drop to 0.03 at 2 GPa and 1250°C. The Cr²⁺ present in Fe³⁺-containing melts at magmatic temperatures decreases on cooling because of the electron exchange reaction: Cr²⁺ + Fe³⁺ = Cr³⁺ + Fe²⁺.

确定了体系CaO-MgO-Al ₂ O ₃ -SiO ₂ ±Na ₂ O±K ₂ O的熔体中的Cr ³⁺ / Cr ²⁺掺杂了0.5 wt％的Cr ₂ O ₃作为氧的函数通过淬火玻璃的XANES光谱在1400°C下的逸度（f O ₂），使用由于1s→4s转变而导致的Cr K边缘上的肩部强度。将Na和K添加到系统CMAS中可在恒定温度下增加Cr ³⁺ / Cr ²⁺，并增加f O _2。，与“理想的光学碱度”的预测非常吻合。新的结果与先前的结果相结合，以校准硅酸盐熔体中Cr ³⁺ / Cr ²⁺作为温度，压力和熔体成分的函数的模型：log ₁₀（Cr ³⁺ / Cr 2 ⁺）=¼ΔQFM+ 3031 / T-2.26 +（843 P – 158 P ²）/ T + ∑c _Z X _Z

其中X _Z是基于单阳离子定义的氧化物组分Z的摩尔分数，_{∑c Z} X _Z = 2.00 X _NaO0.5 + 1.00（X _MgO + X _{Fe2 + O}）+ 0.37（X _AlO1.5 + X _{Fe3 + O1.5}）+ 2.12 X _KO0.5 + 2.44 X _CaO + 3.69 X _TiO2，T为温度，单位为K，P为压力，单位为GPa，ΔQFM为硅酸盐熔体的f O ₂与硅酸盐熔体之间的差石英-铁橄榄石-磁铁矿缓冲液（10 ⁵ Pa，由log ₁₀ f O _2给出）（QFM）= 8.58 – 25050 / T，相对于10 ⁵ Pa的常规标准纯O ₂状态。压力的影响明显是非线性的，因此不应在4 GPa以上外推模型。将此模型与硅酸盐熔体中Fe ³⁺ / Fe ²⁺的相似模型结合起来，可得出：log ₁₀（Cr ³⁺ / Cr 2 ⁺）= log ₁₀（Fe ³⁺ / Fe 2 ⁺）+ 3031 / T-0.9 + （1317 P – 219 P ²）/ T + ∑j _Z X _Z

其中∑j _Z X _Z = 1.00（X _MgO + X _{Fe2 + O}）+ 0.37（X _AlO1.5 + X _{Fe3 + O1.5}）-1.59 X _KO0.5 +0.04 X _CaO + 3.69 X _TiO2。

高温和低压以及低Fe ³⁺ / ∑Fe促进了硅酸盐熔体中的高Cr ²⁺ / ∑Cr。对于在1250˚C（1523 K），10 ⁵ Pa下Fe3 ⁺ / ∑Fe = 0.10的母体MORB熔体成分，模型预测Cr ²⁺ /ΣCr= 0.27。压力的影响非常大：上述示例中的Cr ²⁺ /ΣCr在2 GPa和1250°C时将降至0.03。存在于岩浆温度下的含Fe ³⁺的熔体中存在的Cr ²⁺在冷却时由于电子交换反应而降低：Cr ²⁺ + Fe ³⁺ = Cr ³⁺ + Fe ²⁺。