斑晶(phenocryst)是一种早期形成、相对较大且通常明显大于火成岩基质颗粒的晶体,这种在晶体大小上有明显差异的岩石被称为斑岩,使用形容词“斑状”来描述它们。由于斑晶要么生长于岩浆内早期阶段,要么是在成岩后再结晶,因此,通常具有自形形式。一般情况下,并不使用斑晶这一术语,除非可直接观察到晶体,有时声明直径需大于0.5毫米[1],低于该水平但仍大于基质晶体的斑晶称为微斑晶,而非常大的斑晶则称为巨斑晶,有些岩石中同时含有微斑晶和巨斑晶[2],在变质岩中,类似于斑晶的晶体称为变斑晶。

花岗岩通常有大长石斑晶,这块花岗岩产自勃朗峰瑞士一侧,具有白色大斜长石斑晶,三斜晶系矿物, 切开时呈梯形形状。1欧元硬币直径2.3厘米)用于显示刻度。

斑晶更多地出现在较轻(含二氧化硅更高)的火成岩中,如长英岩安山岩,尽管它们出现在包括超基性岩在内的整个火成岩光谱中。在一些伟晶岩中发现的最大晶体通常是比其他矿物大得多的斑晶。

斑晶分类编辑

 
弗吉尼亚州蓝岭山脉始新世中期斑状隐晶质长英岩的显微照片。斜长石斑晶(白色)和角闪石斑晶(黑色;与斜长石共生)位于显示流状构造的斜长石板条的精细基质中。

岩石可根据斑晶的性质、大小和丰度来进行分类,并在确定岩石名称时,往往会注意是否存在斑晶。无斑隐晶岩是指没有斑晶[3],或更多的是斑晶含量小于1%(按体积计)的岩石[4],而有时形容词“斑岩的”(phyric)被用来代替术语“斑状”以表示斑晶的存在。斑状岩的命名通常用矿物名来修饰,一般按矿物丰度递减的顺序来称呼。因此,当橄榄石在玄武岩中形成原生斑晶时,其名称可从玄武岩细化为“斑状橄榄石玄武岩”或“橄榄石斑状玄武岩”[5]。与此类似,以橄榄石为主要斑晶,但同时含少量斜长石斑晶的玄武岩可称为“橄榄石-斜长石斑岩玄武岩”。

在更复杂的命名法中,只约含1%斜长石斑晶,但含有4%橄榄石微斑晶的玄武岩可能被称为无斑隐晶质至疏晶质斜长石-橄榄石斑岩玄武岩,在此斜长石排在橄榄石之前,是因为其晶体更大[6]。通常将一种岩石分类为无斑隐晶质,还是疏晶质斑岩所面临的问题是,是否含有大量超过最小尺寸的晶体[7]

利用斑晶的分析编辑

地质学家利用斑晶来帮助确定岩石的起源和转变,因为晶体的形成部分取决于压力和温度。

其他特征编辑

斜长石斑晶通常表现出环带性,其钙质内核被逐渐增多的质外壳所包围,这种环带反映了岩浆成分随结晶过程产生的变化[8]。如果晶体边缘显示出比晶体核芯更低的温度成分,则称为正环带;而逆环带描述了一种更不寻常的情况,即边缘显示出比核心更高的温度成分;振荡环带则显示低温和高温成分之间的周期波动[9]

在奥长环斑花岗岩中,正长石斑晶被包裹在钠斜长石,如奥长石外壳内。

在浅层侵入体火山流中,喷发或浅层成岩前形成的斑晶被细粒至玻璃状的基质包围,这些火山斑晶通常显示流动条带,平行排列的条状晶体。 这些特征为岩石的起源提供了线索。类似地,晶粒内微裂缝和晶体间的任何共生也提供了额外的线索[10]

另请查看编辑

备注编辑

  1. ^ The minimum size boundary is arbitrary and not precise. It is based upon observation and may vary depending upon whether technical aids, such as a hand lens or a microscope are used or not. One analyst used a 100 µm limit on the size of crystals as that was the minimum that could be point-counted accurately by optical means. Murphy, M. D.; Sparks, R. S. J.; Barclay, J.; Carroll, M. R.; Brewer, T. S. Remobilization of andesite magma by intrusion of mafic magma at the Soufriere Hills Volcano, Montserrat, West Indies. Journal of Petrology. 2000, 41 (1): 21–42. doi:10.1093/petrology/41.1.21 . 
  2. ^ Smith, George I. Geology and Volcanic Petrology of the Lava Mountains, San Bernardino County, California. United States Geological Survey professional paper 457. Washington, D.C.: United States Geological Survey. 1964: 39. OCLC 3598916. 
  3. ^ Gill, Robin. Igneous Rocks and Processes: A Practical Guide. Hoboken, New Jersey: Wiley. 2011: 34. ISBN 978-1-4443-3065-6. 
  4. ^ Some use a 1% boundary condition, Sen, Bibhas; Sabale, A. B.; Sukumaran, P. V. Lava channel of Khedrai Dam, northeast of Nasik in western Deccan Volcanic province: Detailed morphology and evidences of channel reactivation. Journal of the Geological Society of India. 2012, 80 (3): 314–328. doi:10.1007/s12594-012-0150-8.  and Ocean Drilling Program, Texas A & M University. Proceedings of the Ocean Drilling Program. Part A, Initial report 140. National Science Foundation (U.S.). 1991: 52. , while others suggest a limit of 5%. Piccirillo, E. M.; Melford, A. J. The Mesozoic Flood Volcanism of the Paraná Basin: Petrogenetic and Geophysical Aspects. São Paulo, Brazil: Universidade de São Paulo, Instituto Astronômico e Geofísico. 1988: 49. ISBN 978-85-85047-04-7.  and Moulton, B. J. A.; et al. Volcanology of the Felsic Volcanic Rocks of the Kidd-Munro assemblage in Prosser and Muro Townships and Premininary Correlations with the Kidd Creek Deposit, Abitibi Greenstone Belt, Ontario. Geological Survey of Canada, Current Research, No. 2008-18. Ottawa: Geological Survey of Canada. 2008: 19. ISBN 978-1-100-10649-6. 
  5. ^ Gill, Robin. Igneous Rocks and Processes: A Practical Guide. Hoboken, New Jersey: Wiley. 2011: 21. ISBN 978-1-4443-3065-6. 
  6. ^ Byerly, Gary R.; Wright, Thomas L. Origin of major element chemical trends in DSDP Leg 37 basalts, Mid-Atlantic Ridge. Journal of Volcanology and Geothermal Research. 1978, 3 (3): 229–279. doi:10.1016/0377-0273(78)90038-0. 
  7. ^ Gangopadhyay, A. M. I. T. A. V. A.; Sen, Gautam; Keshav, Shantanu. Experimental Crystallization of Deccan Basalts at Low Pressure: Effect of Contamination on Phase Equilibrium (PDF). Indian Journal of Geology. 2003, 75 (1/4): 54 [2021-11-04]. (原始内容存档 (PDF)于2010-06-15). 
  8. ^ Williams, Howel; Turner, Francis J.; Gilbert, Charles M. Petrography: An introduction to the study of rocks in thin sections. San Francisco: W. H. Freeman. 1954: 102–103. ISBN 978-0-7167-0206-1. 
  9. ^ "Crystal zoning." Oxford Reference. Accessed 8 Aug. 2020. https://www.oxfordreference.com/view/10.1093/oi/authority.20110803095651756页面存档备份,存于互联网档案馆).
  10. ^ Cox, S. F.; Etheridge, M. A. Crack-seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures. Tectonophysics. 1983, 92 (1): 147–170. doi:10.1016/0040-1951(83)90088-4. 

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