Characteristics And Petrogenesis Of Albite And Nepheline Jades

Jades have appealed to Chinese people since the medium-stage of Qing Dynasty, and they are regarded as "the king of jades" for their fantastic qualities. Jades are mainly composed by jadeite and/or other sodic to calc-sodic pyroxenes （such as Kosmochlor and omphacite）, which are called as jadeitites in petrology. With the gradual exhaustion of high-quality jades, there come up more and more jades containing subordinate minerals on the market. This leads to big troubles for the quality testing works. In this Master thesis, we selected some typical Myanmar jades that contain secondary minerals to conduct detailed petrology, Lase Raman and infrared spectroscopy, X-ray powder diffraction and electron probe analyses. With these analyses, we discuss the petrogenesis and evolution of the Myanmar jades and draw the following conclusions.1. We first discover and confirm the occurrence of nepheline in the Myanmar jades.2. The constituent minerals and their contents in the selected samples are identified through the X-ray powder diffraction method. Combing the results from the density measurements, the linear relationship between the contents of secondary minerals and the jade densities are constructed. By referring the petrological classification and naming rules （GB/T17412.1/3-1998）, we propose a set of naming rules for jades that contain secondary minerals.3. Based on the distinct secondary minerals, the selected jades can be divided into two groups. One group is featured by albite and analcime, while the other one is by nepheline and amphibole.4. Under the gemology microscope, albite distributes as veins, disseminated texture and blocks, while the nepheline occur as flakes. Most albites are euhedral and subhedral slabs, while nepheline grains are small and their crystal forms cannot be identified. Under reflected lights, the lusters of albite and nepheline are weaker than that of jadeite. With the focused lights, albite and nepheline show distinct discrepancy in transparency:albite is transparent and its cleavages are very distinct, while nepheline is dullish with no cleavages. We also used Raman and infrared spectroscopy to distinguish albite and nepheline. The acquired peaks of albite are mainly291cm-1,479cm-1and507cm-1, while the weaker peaks include160cm-1,185cm-1and208cm-1. The major peaks of nepheline are398cm-1and989cm-1, while the weaker peaks include201 cm-1,347cm-1and465cm-1. The peaks of jadeite include204cm-1,375cm-1,432cm-1,523cm-1,699cm-1and992cm-1.5. The detailed petrological features and mineral chemistry are described as followed:（1） The albite jades are mainly made by jadeite （50-80vol%）, albite （20-45vol%） and analcime （0-5vol%）. The jadeite mainly occurs as anhedral to subhedral crystals, while some grains are characterized with distinct oscillatory zones and their compositions displaying a regular change from the cores to the rims. The jadeite rims are cut through by later albite, resulting in a cataclastic texture. Moreover, the jadeite grains are alternated by analcime, forming poikiloblastic and patchy microstructures. Most albite grains are anhedral, while some are subhedral to euhedral and exhibit simple and polysynthetic twins, with local rims corroded by later analcime. Analcime occurs as veins in matrix and as inclusions in jadeite. These textures indicate that the earlier jadeite in these rocks was altered by two later episodes of fluids, represented by albite and analcime, respectively. They also suggest multi-episodic fluid activities during the formation of the jades. The analyses of electron microscope show:jadeite is mainly made by Jd with low contents of other components; albite is near pure albite of Ab99An0-1Or0-1; analcime contains a small amount of CaO; phengite displays a zoning pattern with its core having a Si content of3.50apfu but a higher Fe/（Fe+Mg） ratio of0.16and its rim holding a higher Si content of3.77apfu but a lower Fe/（Fe+Mg） ratio of0.06.（2） The nepheline jades are granular texture and they are mainly made by jadeite （75-85vol%）, nepheline （75-85vol%）, amphibole （3-5vol%） and accessory Ba-aluminosilicates. Jadeites in the samples have two generations. The earlier one is euhedral and shows regular oscillatory zonings, indicating iterative replenishment of diagenetic fluids during the growth of jadeites. It is partly replaced by later fine-grained jadeite±amphibole, indicating the alteration of later fluid. Based on its occurrences and compositions, nepheline can be divided into two types, i.e. K-poor nepheline and K-rich nepheline, in which the former distributes along the grain boundaries of the euhedral jadeites, whereas the later occupies in the space between the euhedral jadeites. Ba-aluminosilicates scatter within the nepheline matrix. The jadeites in these jades display variable chemical textures. The jadeites with oscillatory zones can be further divided into dark core, grey mantle and bright rim, whose compositions range at Jd97₉9Wo0-1En0-1FsoAeo, Jd95.97W01-2En1-3Fs0-1Aeo and Jd91-92Wo3.4En3-4Fs1-2Aeo, respectively. The fine-grained jadeites are made by Jd92-95Wo1-2En1-2Fs3Aeo-2. The K-rich nepheline exhibits a brighter contrast in the BSE images and contains a composition of Ko.12-o.13Nao.8i-0.84Al0.98-0.99Si1.01-1.03O4; the K-poor nepheline shows a darker contrast in the BSE images and contains a composition of K0Na0.96-0.99Al0.99-1Si1-1.01O4.6. The textural features indicate that the albite and nepheline jades are directly precipitated from fluids. The mineral assemblage and their compositions suggest that the fluids are rich in Na, Al, Si, K, Ba and minor Ca, Fe, Mg for major elements, while the trace elements are characterized with enrichment of LREE, HFSE and Sr, etc. The petrogenetic fluids should come from the dehydration of subducted oceanic crusts and sediments.7. Combining the oxygen isotopes studies in other papers and the reaction relationships between the minerals in the jadeites:jadeite+quartz=albite, analcime=jadeite+H2O,41awsonite+2jadeite=paragonite+2clinozoisite+quartz+H2O, we locate the formation P-T conditions of jadeite at6-14kbar and300-450℃. The albite and nepheline represent the precipitation products of fluids after decompression, in which albite is formed at Si-saturated conditions and nepheline at Si-unsaturated conditions.