祁连山高压变质富锰硅质岩中砷硅锰矿的矿物学研究

北祁连山缝合带是一条典型的早古生代大洋型俯冲带,其中发育两条蓝片岩带:含硬柱石的低级蓝片岩带和含榴辉岩的高级蓝片岩带。在清水沟地区含榴辉岩的高级蓝片岩带中的高压变质富锰硅质岩中发现稀有的砷硅锰矿,这是国内首次发现砷硅锰矿。利用光学显微镜、扫描电镜、电子探针、拉曼光谱、透射电镜等方法对其进行了详细的矿物学研究。研究发现,砷硅锰矿密度为3. 69～3. 75 g/cm3,硬度为6～7,与石英、红帘石、蓝闪石、多硅白云母和赤铁矿等矿物共生,二轴晶,正光性,(010)解理完全,从淡黄色到金黄色,多色性强烈;砷硅锰矿的主要成分是Si、Al、Mn,富含As,并含有少量的F、V、P;在拉曼图谱上,砷硅锰矿具有许多明显的特征峰,且显示出砷酸根、钒酸根以及MnO键和Si O3基团的存在;在晶体结构上,砷硅锰矿为斜方晶系,空间群为Pnmm,晶轴参数为a=8. 934、b=5. 817、c=19. 005。这些矿物学特征可以作为砷硅锰矿的鉴定和研究依据。

Mineralogy of ardennite-(As) from HP metamorphic Mn-rich silica rocks in North Qilian Mountain

The North Qilian Mountain suture zone is a typical early Paleozoic oceanic subduction zone, in which two blue schist zones are developed:a low-grade blue schist zone with lawsonite and a high-grade blue schist zone with eclogites. Ardennite-(As), a rare kind of silicate mineral, was found in the high-pressure metamorphic Mn-rich silica rocks in the high-grade blue schist zone containing eclogites in the North Qilian Mountain. This is the first discovery of ardennite-(As) in China. Detailed mineralogical studies were carried out by means of optical microscopy, scanning electron microscopy, electron probe microscopy, Raman spectroscopy and transmission electron microscopy. According to the results obtained, the density of ardennite-(As) is 3.69~3.75 g/cm³ and the hardness is 6~7; ardennite-(As) crystals are biaxial (+) and have perfect cleavage in (010), pleochroism is strong from pale yellow to golden yellow, usually associated with quartz, piemontite, glaucophane, phengite and hematite; the main components of ardennite-(As) are Si, Al, Mn, it is rich in As, and contains a small amount of F, V, P; in Raman spectrum; ardennite-(As) shows many characteristic peaks, probably indicating the presence of arsenate, vanadium, MnO and SiO₃ bonds; in the crystal structure, ardennite-(As) is orthorhombic, the space group is Pnmm, and the crystal axis parameters are a=8.934 Å, b=5.817 Å, c=19.005 Å. These mineralogical features can be used as the basis for the identification and study of ardennite-(As).