长期以来，地外物质中磷酸钙成分的矿物有称为陨磷钙钠石的，也有称为白磷钙石的，原因是它们的成分和结构相近，其所含少量Na和Mg在结构中的存在状态不详。广州地化所的谢先德等与美国亚利桑那大学的杨和雄等合作，对我国随州陨石中具理想成分的陨磷钙钠石首次进行了晶体结构精测。主要成果有： 

　　（ⅰ）确定Mg和Na在该矿物中并非无序分布，而是有序占位，查明其结构单元是由一个组成为[Mg(PO₄)₆]^16–风车状复合阴离子，被组成为[Ca₉Na(PO₄)]¹⁶⁺ 的一些隙间复合离子团相连。 

　　（ⅱ）明确了陨石、月岩和火星岩石等地外物质中无水磷酸钙矿物应统称为陨磷钙钠石，而地球上含有（OH）的磷酸钙则应称为白磷钙石，从而为这两种矿物名称不再混用提供了可靠依据。 

　　（ⅲ）进一步确定了钙磷酸盐矿物中几种不同的隙间复合离子团，即：陨石中的陨磷钙钠石为[Ca₉Na(PO₄)]¹⁶⁺；地球上的白磷钙石为[Ca₉(PO₃[OH])]¹⁶⁺；陨石中石富钙陨磷钙钠为[Ca₉(Ca_0.5e_0.5)(PO₄)]¹⁶⁺，月球上的富稀土陨磷钙钠石为：[(Ca_9-xREE)_x(Na_1-xe_x)(PO₄)^]16+。 

　　（ⅳ）根据随州陨石中的陨磷钙钠石均高压相变为涂氏磷钙石，从而为在地球上寻找白磷钙石的高压相、在陨石中和在月球上寻找富钙陨磷钙钠石和富稀土陨磷钙钠石的高压相提供了科学依据。 

　　有关研究成果已在American Mineralogist（2015）上发表。 

　　Figure 1. Backscattered electron image showing a large merrillite(Mer) grain in the Suizhou meteorite.

Ol = olivine, Pyx = Low Capyroxene,Mas = maskelynite, M = FeNi metal. 

　　Figure 2. The arrangement of the bracelet-and-pinwheel structural units in Suizhou merrillite.

A [(Mg,Fe)(PO₄)₆]^16– structure unit is indicated separately at the lower-right corner.

　　Advances in the study of crystal structure of Ca-phosphate minerals in extraterrestrial materials 

　　 Owing to the chemical and structural similarities between merrillite and whitlockite，there has been a considerable overlap in the use of these two mineral names in the literature. By means of electron microprobe analy-sis, single-crystal X？ray diffraction, and Raman spectroscopy, Xiande Xie of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, in collaboration with Hexiong Yang of the University of Arizona conducted the first structure determination of merrillite with a nearly ideal chemical composition from the Suizhou meteorite. It was found that the Suizhou merrillite is trigonal with space group R3c and unit-cell parameters a = 10.3444(3), c = 37.0182(11) ？, and V = 3430.5(2) ？³. Its crystal structure, refined to R1 = 0.032, is characterized by a structural unit consist-ing of a [(Mg,Fe)(PO₄)₆]^16– complex anion that forms a “bracelet-and-pinwheel” arrangement. Such structural units are linked by interstitial complexes with a formula of [Ca₉Na(PO₄)]¹⁶⁺, which differs from that of [Ca₉(PO₃[OH])]¹⁶⁺, [Ca₉(PO₃F)]¹⁶⁺, [Ca₉(Ca_0.5e_0.5)(PO₄)]¹⁶⁺, or [(Ca_9–xREE)_x(Na_1–xe_x)(PO₄)]¹⁶⁺ in terrestrial whitlockite, terrestrial/ extraterrestrial bobdownsite, meteoritic Ca-rich merril-lite, or lunar REE-rich merrillite, respectively. The Suizhou merrillite is found to transform to tuite at high pressures, pointing to the likelihood of finding REE-bearing tuite on the Moon as a result of shock events on REE-merrillite. 

　　（矿物学与成矿学重点实验室&科技与规划处供稿）