Genesis of Jadeite Containing Albite
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摘要: 为详细探讨含钠长石翡翠的成因机制，笔者选取了若干来自缅甸的含钠长石翡翠，对其进行了详细的岩相学、矿物化学等方面的研究。含钠长石翡翠样品属于豆青种，主要由硬玉、钠长石、方沸石和少量的多硅白云母、钡铝硅酸盐等矿物组成。其中的硬玉发育清晰的环带结构，成分从核部至边缘发生规律性的成分变化。翡翠同时受到两期后期流体活动的改造，第一期以钠长石为代表，第二期以方沸石为代表，流体的改造作用使硬玉呈现碎裂状、碎斑状结构和交代穿孔等结构。结果表明，含钠长石翡翠样品表现出从成岩流体中直接结晶的特点，该流体富集Na、Al、Si、K、Ba以及少量的Ca、Fe、Mg等元素，微量元素则相对富集LREE、HFSE和Sr等元素。结合前人的研究结果以及该玉石中的矿物反应关系，笔者推测缅甸翡翠形成的压力和温度范围分别在6～14kbar和300℃～450℃。Abstract: Jadeite is a famous gemstone.The petrogenesis of jadeite is complicated and can be categorized into three main types,magmatic,hydrothermal and metamorphic origin.So far,a consensus has not been reached about the petrogenesis of jadeite.In order to discuss the petrogenesis of jadeite containing albite from Myanmar,the authors select several samples to conduct a detailed research on their petrology and mineral chemistry.The testing methods include optical microscope and scanning electron microscope to observe the petrographic features,X-ray powder diffraction to analyze the mineral contents in a quantificational way,Raman spectroscopy to identify mineral species and electron microscope to measure the compositions of minerals.The jadeite containing albite in this study belongs to the bean green variety with a granular texture.It is semi-transparent and mainly consist of jadeite,albite and analcime with minor phengite and barium aluminosilicates.The Raman peaks of jadeite obtained in this study include 204,375,432,523,699and 992cm-1,while those of albite include 160,185,208,291,479and 507cm-1.From X-ray powder diffraction results show that the jadeites containing albite are mainly composed of jadeite(49.5%-79.7%),albite(20.3%-45.1%)and analcime(0-5.4%),in agreement with the observation under the optical microscope.Most jadeite grains are presented as anhedral and subhedral crystals,with some showing distinct oscillatory zones and their compositions displaying a regular change from the cores to the rims.The jadeite rims are cut through by albite,resulting in a cataclastic texture.Meanwhile,the jadeite grains are alternated by analcime and form poikiloblastic and patchy microstructures.Most albite distributes between the jadeite grains with an anhedral morphology.Minor albite grains present as subhedral and euhedral crystals and develop simple and polysynthetic twins,with local parts corroded by analcime.Analcime is presented as veins in matrix and as inclusions in jadeite.These microstructural features indicate that the former jadeite in these rocks was altered by two later episodes of fluids,represented by albite and analcime respectively.They also suggest that the fluid activities during the formation of the albite jadeites were multi-episodic.The analyses of electron microscope show that jadeite is made by Jd97-99,with low contents of other endmembers,and albite is near pure albite of Ab99An0-1Or0-1,and analcime contains a small amount of CaO.Phengite displays a zoning pattern with its core holding a lower Si content of 3.50%. but a higher Fe/(Fe+Mg)ratio of 0.16,whereas its rim holding a higher Si content of 3.77%. but a lower Fe/(Fe+Mg)ratio of 0.06.The results described above suggest that the albite jadeite from Myanmar directly precipitated from fluids.The oscillatory zones of jadeite reflect that the diagenetic fluids were supplied iteratively during the precipitation of jadeite.The mineral assemblage and compositions suggest that these fluids were enriched with Na,Al,Si,K,Ba and minor Ca,Fe,Mg,etc.Regarding the trace elements,they were relatively enriched with LREE,HFSE and Sr,etc.Based on these data,it can be inferred that these diagenetic fluids mainly came from the dehydration of subducting oceanic crusts and sediments.Combining with the results of oxygen isotopes studies in other papers and the reaction relationships between the minerals in the jadeites containing albite:jadeite+quartz=albite,analcime=jadeite+H2O,4 lawsonite+2 jadeite=paragonite+2 clinozoisite+quartz+H2O,the authors locate their formation P-T conditions at 6-14 kbar and 300℃-450℃.
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Keywords:
- jade containing albite /
- jadeite /
- zoning texture /
- fluid composition /
- P—T condition
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