{"id":143,"date":"2019-06-11T14:48:46","date_gmt":"2019-06-11T14:48:46","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/3-4-classification-of-igneous-rock\/"},"modified":"2021-12-07T19:50:40","modified_gmt":"2021-12-07T19:50:40","slug":"3-4-classification-of-igneous-rock","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/3-4-classification-of-igneous-rock\/","title":{"raw":"3.4 Classification of Igneous Rock","rendered":"3.4 Classification of Igneous Rock"},"content":{"raw":"As has already been described, igneous rocks are classified into four categories: felsic, intermediate, mafic, and ultramafic, based on either their chemistry or their mineral composition. The diagram in Figure 3.4.1 can be used to help classify igneous rocks by their mineral composition. An important feature to note on this diagram is the red line separating the non-ferromagnesian silicates in the lower left (K-feldspar, quartz, and plagioclase feldspar) from the ferromagnesian silicates in the upper right (biotite, amphibole, pyroxene, and olivine). In classifying intrusive igneous rocks, the first thing to consider is the percentage of ferromagnesian silicates. In most igneous rocks the ferromagnesian silicate minerals are clearly darker than the others, but it is still quite difficult to estimate the proportions of minerals in a rock.\r\n\r\nBased on the position of the red line in Figure 3.4.1, it is evident that felsic rocks can have between 1% and 20% ferromagnesian silicates (the red line intersects the left side of the felsic zone 1% of the distance from the top of the diagram, and it intersects the right side of the felsic zone 20% of the distance from the top). Intermediate rocks have between 20% and 50% ferromagnesian silicates, and mafic rocks have 50% to 100% ferromagnesian silicates. To be more specific, felsic rocks typically have biotite and\/or amphibole; intermediate rocks have amphibole and, in some cases, pyroxene; and mafic rocks have pyroxene and, in some cases, olivine.<\/a>\r\n
[caption id=\"attachment_135\" align=\"aligncenter\" width=\"750\"]\"\"<\/a> Figure 3.4.1 A simplified classification diagram for igneous rocks based on their mineral compositions. [Image Description]<\/a>[\/caption]<\/div>\r\nIf we focus on the non-ferromagnesian silicates, it is evident that felsic rocks can have from 0% to 35% K-feldspar, from 25% to 35% quartz (the vertical thickness of the quartz field varies from 25% to 35%), and from 25% to 50% plagioclase (and that plagioclase will be sodium-rich, or albitic). Intermediate rocks can have up to 25% quartz and 50% to 75% plagioclase. Mafic rocks only have plagioclase (up to 50%), and that plagioclase will be calcium-rich, or anorthitic.\r\n
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Exercise 3.5 Mineral proportions in igneous rocks<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\n[caption id=\"attachment_103\" align=\"aligncenter\" width=\"750\"]\"\"<\/a> Figure 3.4.2[\/caption]\r\n\r\nThe dashed blue lines (labelled a, b, c, d)\u00a0 in Figure 3.4.2 represent four igneous rocks. Complete the table by estimating the mineral proportions (percent) of the four rocks (to the nearest 10%).\r\n\r\nHint: Rocks b<\/strong> and d<\/strong> are the easiest; start with those.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
\u00a0Rock<\/th>\r\nBiotite\/amphibole<\/th>\r\nPyroxene<\/th>\r\nOlivine<\/th>\r\nPlagioclase<\/th>\r\nQuartz<\/th>\r\nK-feldspar<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
a<\/th>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/tr>\r\n
b<\/th>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/tr>\r\n
c<\/th>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/tr>\r\n
d<\/th>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nSee Appendix 3 for Exercise 3.5 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\nFigure 3.4.3 provides a diagrammatic representation of the proportions of dark minerals in light-coloured rocks. You can use that when trying to estimate the ferromagnesian mineral content of actual rocks, and you can get some practice doing that by completing Exercise 3.6.\u00a0 Be warned!\u00a0 Geology students almost universally over-estimate the proportion of dark minerals.\r\n\r\n[caption id=\"attachment_137\" align=\"aligncenter\" width=\"1429\"]\"\"<\/a> Figure 3.4.3 A guide to estimating the proportions of dark minerals in light-coloured rocks.[\/caption]\r\n\r\n
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Exercise 3.6 Proportions of ferromagnesian silicates<\/p>\r\n\r\n<\/header>\r\n

\r\n\r\nThe four igneous rocks shown below have differing proportions of ferromagnesian silicates. Estimate those proportions using the diagrams in Figure 3.4.3, and then use Figure 3.4.1 to determine the likely rock name for each one.\r\n\r\n\r\n\r\n\r\n\r\n
\"""\"<\/a><\/td>\r\n\"""\"<\/a><\/td>\r\n\"""\"<\/a><\/td>\r\n\"""\"<\/a><\/td>\r\n<\/tr>\r\n
___<\/strong>%<\/strong><\/td>\r\n___<\/strong>%<\/strong><\/td>\r\n___<\/strong>%<\/strong><\/td>\r\n___<\/strong>%<\/strong><\/td>\r\n<\/tr>\r\n
__________<\/strong><\/td>\r\n__________<\/strong><\/td>\r\n__________<\/strong><\/td>\r\n\u00a0<\/strong>__________<\/strong><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nSee Appendix 3 for Exercise 3.6 answers<\/a>.\r\n\r\n<\/div>\r\n<\/div>\r\nIgneous rocks are also classified according to their textures. The textures of volcanic rocks will be discussed in Chapter 4, so here we\u2019ll only look at the different textures of intrusive igneous rocks. Almost all intrusive igneous rocks have crystals that are large enough to see with the naked eye, and we use the term [pb_glossary id=\"1184\"]phaneritic[\/pb_glossary] (from the Greek word phaneros<\/em> meaning visible) to describe that. Typically that means they are larger than about 0.5 millitmeres (mm) \u2014 the thickness of a strong line made with a ballpoint pen. (If the crystals are too small to distinguish, which is typical of most volcanic rocks, we use the term [pb_glossary id=\"1228\"]aphanitic[\/pb_glossary] (from the Greek word aphanos<\/em> - unseen) The intrusive rocks shown in Figure 3.3.5 are all phaneritic, as are those shown in Exercise 3.6.\r\n\r\nIn general, the size of crystals is proportional to the rate of cooling. The longer it takes for a body of magma to cool, the larger the crystals can grow. It is not uncommon to see an intrusive igneous rock with crystals up to 1 centimetre (cm) long. In some situations, especially toward the end of the cooling stage, the magma can become water rich. The presence of liquid water (still liquid at high temperatures because it is under pressure) promotes the relatively easy movement of ions, and this allows crystals to grow large, sometimes to several centimetres (Figure 3.4.4). Finally, as already described, if an igneous rock goes through a two-stage cooling process, its texture will be porphyritic (Figure 3.3.7).\r\n\r\n[caption id=\"attachment_142\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 3.4.4 A pegmatitic rock with large crystals[\/caption]\r\n

Image Descriptions<\/h3>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
<\/a>Figure 3.4.1 image description: Mineral composition of igneous rocks<\/caption>\r\n
\u00a0Igneous Rocks<\/th>\r\nFelsic<\/th>\r\nIntermediate<\/th>\r\nMafic<\/th>\r\nUltramafic<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
K-feldspar<\/th>\r\n0 to 35%<\/td>\r\n0%<\/td>\r\n0%<\/td>\r\n0%<\/td>\r\n<\/tr>\r\n
Quartz<\/th>\r\n25 to 35%<\/td>\r\n0 to 25%<\/td>\r\n0%<\/td>\r\n0%<\/td>\r\n<\/tr>\r\n
Plagioclase feldspar<\/th>\r\n25 to 50%<\/td>\r\n50 to 70%<\/td>\r\n0 to 50%<\/td>\r\n0%<\/td>\r\n<\/tr>\r\n
Biotite and\/or Amphibole<\/th>\r\n0 to 20%<\/td>\r\n20 to 40%<\/td>\r\n0 to 30%<\/td>\r\n0%<\/td>\r\n<\/tr>\r\n
Pyroxene<\/th>\r\n0%<\/td>\r\n0 to 20%<\/td>\r\n20 to 75%<\/td>\r\n0% to 75%<\/td>\r\n<\/tr>\r\n
Olivine<\/th>\r\n0%<\/td>\r\n0%<\/td>\r\n0 to 25 %<\/td>\r\n25% to 100%<\/td>\r\n<\/tr>\r\n
Intrusive<\/th>\r\nGranite<\/td>\r\nDiorite<\/td>\r\nGabbro<\/td>\r\nPeridotite<\/td>\r\n<\/tr>\r\n
Extrusive<\/th>\r\nRhyolite<\/td>\r\nAndesite<\/td>\r\nBasalt<\/td>\r\nKomatiite<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[Return to Figure 3.4.1]<\/a>\r\n

Attributions<\/h3>\r\n