斑晶

斑晶（phenocryst）是一種早期形成、相對較大且通常明顯大於火成岩基質顆粒的晶體，這種在晶體大小上有明顯差異的岩石被稱為斑岩，使用形容詞「斑狀」來描述它們。由於斑晶要麼生長於岩漿內早期階段，要麼是在成岩後再結晶，因此，通常具有自形形式。一般情況下，並不使用斑晶這一術語，除非可直接觀察到晶體，有時聲明直徑需大於0.5毫米^[1]，低於該水平但仍大於基質晶體的斑晶稱為微斑晶，而非常大的斑晶則稱為巨斑晶，有些岩石中同時含有微斑晶和巨斑晶^[2]，在變質岩中，類似於斑晶的晶體稱為變斑晶。

斑晶更多地出現在較輕（含二氧化矽更高）的火成岩中，如長英岩和安山岩，儘管它們出現在包括超基性岩在內的整個火成岩光譜中。在一些偉晶岩中發現的最大晶體通常是比其他礦物大得多的斑晶。

斑晶分類

弗吉尼亞州藍嶺山脈始新世中期斑狀隱晶質長英岩的顯微照片。斜長石斑晶（白色）和角閃石斑晶（黑色；與斜長石共生）位於顯示流狀構造的斜長石板條的精細基質中。

岩石可根據斑晶的性質、大小和豐度來進行分類，並在確定岩石名稱時，往往會注意是否存在斑晶。無斑隱晶岩是指沒有斑晶^[3]，或更多的是斑晶含量小於1%（按體積計）的岩石^[4]，而有時形容詞「斑岩的」（phyric）被用來代替術語「斑狀」以表示斑晶的存在。斑狀岩的命名通常用礦物名來修飾，一般按礦物豐度遞減的順序來稱呼。因此，當橄欖石在玄武岩中形成原生斑晶時，其名稱可從玄武岩細化為「斑狀橄欖石玄武岩」或「橄欖石斑狀玄武岩」^[5]。與此類似，以橄欖石為主要斑晶，但同時含少量斜長石斑晶的玄武岩可稱為「橄欖石-斜長石斑岩玄武岩」。

在更複雜的命名法中，只約含1%斜長石斑晶，但含有4%橄欖石微斑晶的玄武岩可能被稱為無斑隱晶質至疏晶質斜長石-橄欖石斑岩玄武岩，在此斜長石排在橄欖石之前，是因為其晶體更大^[6]。通常將一種岩石分類為無斑隱晶質，還是疏晶質斑岩所面臨的問題是，是否含有大量超過最小尺寸的晶體^[7]。

利用斑晶的分析

地質學家利用斑晶來幫助確定岩石的起源和轉變，因為晶體的形成部分取決於壓力和溫度。

其他特徵

斜長石斑晶通常表現出環帶性，其鈣質內核被逐漸增多的鈉質外殼所包圍，這種環帶反映了岩漿成分隨結晶過程產生的變化^[8]。如果晶體邊緣顯示出比晶體核芯更低的溫度成分，則稱為正環帶；而逆環帶描述了一種更不尋常的情況，即邊緣顯示出比核心更高的溫度成分；振盪環帶則顯示低溫和高溫成分之間的周期波動^[9]。

在奧長環斑花崗岩中，正長石斑晶被包裹在鈉斜長石，如奧長石外殼內。

在淺層侵入體或火山流中，噴發或淺層成岩前形成的斑晶被細粒至玻璃狀的基質包圍，這些火山斑晶通常顯示流動條帶，平行排列的條狀晶體。這些特徵為岩石的起源提供了線索。類似地，晶粒內微裂縫和晶體間的任何共生也提供了額外的線索^[10]。

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備註

^ 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  .
^ 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.
^ Gill, Robin. Igneous Rocks and Processes: A Practical Guide. Hoboken, New Jersey: Wiley. 2011: 34. ISBN 978-1-4443-3065-6.
^ 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.
^ Gill, Robin. Igneous Rocks and Processes: A Practical Guide. Hoboken, New Jersey: Wiley. 2011: 21. ISBN 978-1-4443-3065-6.
^ 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.
^ 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]. （原始內容存檔 (PDF)於2010-06-15）.
^ 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.
^ "Crystal zoning." Oxford Reference. Accessed 8 Aug. 2020. https://www.oxfordreference.com/view/10.1093/oi/authority.20110803095651756 （頁面存檔備份，存於互聯網檔案館）.
^ 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.

參考文獻

Best, Myron. Igneous and Metamorphic Petrology second. Oxford, England: Blackwell Publishing. 2002. ISBN 978-1-4051-0588-0.
Williams, Howel; Turner, Francis J.; Gilbert, Charlse M. Petrography: An introduction to the study of rocks in thin sections. San Francisco: W. H. Freeman. 1954. ISBN 978-0-7167-0206-1.
The Integrated Ocean Drilling Program (IODP). (2001) Proceedings of the Ocean Drilling Program, Vol. 187 Initial Reports.[1] （頁面存檔備份，存於互聯網檔案館）

[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]. （原始內容存檔 (PDF)於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 （頁面存檔備份，存於互聯網檔案館）.

[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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