Ma Yunlu, Bao Xinjian, Liu Xi
School of Earth and Space Sciences, Peking University, Beijing 100871, China.
Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education of China, Beijing 100871, China.
Molecules. 2021 Feb 7;26(4):872. doi: 10.3390/molecules26040872.
MgAlO-spinel has wide industrial and geological applications due to its special structural and physical-chemical features. It is presumably the most important endmember of complex natural spinel solid solutions, and therefore provides a structural model for a large group of minerals with the spinel structure. There exists a well known but still inadequately understood phenomenon in the structure of MgAlO-spinel, the Mg-Al cations readily exchanging their positions in response to variations of temperature, pressure, and composition. A large number of experiments were performed to investigate the Mg-Al cation order-disorder process usually quantified by the inversion parameter (representing either the molar fraction of Al on the tetrahedral T-sites or the molar fraction of Mg on the octahedral M-sites in the spinel structure), and some thermodynamic models were thereby constructed to describe the - relation. However, experimental data at some key were absent, so that the different performance of these thermodynamic models could not be carefully evaluated. This limited the interpolation and extrapolation of the thermodynamic models. By performing some prolonged annealing experiments with some almost pure natural MgAlO-spinel plates and quantifying the values with single-crystal X-ray diffraction technique, we obtained some critical equilibrium values at down to 773 K. These new - data, along with those relatively reliable values at relatively high from early studies, clearly indicate that the CS94 Model (a model constructed by Carpenter and Salje in 1994) better describes the Mg-Al cation order-disorder reaction in MgAlO-spinel for a wide range of . On the basis of the CS94 Model, a geothermometer was established, and its form is = 21362 × - 12143 × + 6401 × - 10 ( standing for the closure temperature of the Mg-Al cation exchange reaction). This geothermometer can be used to constrain the thermal history of the geological bodies containing MgAlO-spinel.
镁铝尖晶石因其特殊的结构和物理化学特性而具有广泛的工业和地质应用。它可能是复杂天然尖晶石固溶体中最重要的端元矿物,因此为一大类具有尖晶石结构的矿物提供了结构模型。在镁铝尖晶石结构中存在一种众所周知但仍未得到充分理解的现象,即Mg-Al阳离子会随着温度、压力和成分的变化而轻易交换它们的位置。人们进行了大量实验来研究通常由反转参数(表示尖晶石结构中四面体T位上Al的摩尔分数或八面体M位上Mg的摩尔分数)量化的Mg-Al阳离子有序-无序过程,并据此构建了一些热力学模型来描述反转参数与其他参数的关系。然而,一些关键反转参数的实验数据缺失,因此无法仔细评估这些热力学模型的不同性能。这限制了热力学模型的内插和外推。通过对一些几乎纯的天然镁铝尖晶石薄片进行长时间退火实验,并使用单晶X射线衍射技术对反转参数值进行量化,我们获得了低至773K的一些关键平衡反转参数值。这些新的反转参数数据,连同早期研究中相对可靠的较高温度下的反转参数值,清楚地表明CS94模型(由Carpenter和Salje于1994年构建的模型)在很宽的温度范围内能更好地描述镁铝尖晶石中Mg-Al阳离子的有序-无序反应。基于CS94模型,建立了一个地质温度计,其形式为 = 21362 × - 12143 × + 6401 × - 10(代表Mg-Al阳离子交换反应的封闭温度)。这个地质温度计可用于约束含镁铝尖晶石地质体的热历史。