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Igneous Rocks Classifying rocks • Rock classification usually involves determining two parameters: – Texture: the size, shape, and relationship of the mineral grains that make up the rock – Composition: the amount/proportion of various minerals that make up the rock • True for all rocks but we’ll start with igneous rocks Igneous rocks… • Form from magma • Lava is magma that reaches the surface and loses gasses (mostly H2O and CO2) • If magma crystallizes before reaching the surface it forms an intrusive (plutonic) rock • When lava crystallizes it forms extrusive (volcanic) rock • Intrusive and extrusive rocks have different textures Intrusive igneous rocks • • • • Cool slowly Crystals grow larger Texture is phaneritic (lit. “visible grains”) Must give a grain size: – – – – Visible but less than 1 mm = fine grained (f.g.) Between 1 and 5 mm = medium grained (m.g.) Greater then 5 mm = coarse grained (c.g.) Greater than 2 cm = pegmatitic (see below) • Don’t have to give “phaneritic” Extrusive igneous rocks • Cool rapidly • Crystals are small (generally too small to see) • Texture is aphanitic (lit. not visible grains) • Where lava cools very quickly (quenches) the rock will be a glass (non-crystalline) • Textural term is glassy Phaneritic texture Aphanitic texture Porphyritic rocks • When magma cooling rate changes the rock will commonly have two different grain sizes • Term used is porphyritic • The larger grains are phenocrysts which sit in a groundmass • Must indicate the grain size of the groundmass (aphanitic, phaneritic or (rarely) glassy) to denote complete texture – e.g., f.g. porphyritic, aphanitic porphyritic, etc. Origin of porphyritic texture Porphyritic texture Porphyritic textures Groundmass (aphanitic) Phenocrysts Porphyritic textures Phenocrysts Groundmass (phaneritic) Other textural terms • Volcanic rocks commonly have bubble holes (vesicles) caused by escaping gas. Texture is vesicular • Use different terms for light or dark coloured vesicular rocks. – Light coloured vesicular rocks generally have many tiny holes. Texture is pumaceous, rock is pumice. – Dark coloured vesicular rocks have fewer and larger holes. Texture is scoriaceous, rock is scoria. Pumice Scoria Other special (textural) terms • Amygdaloidal: textural term used when vesicles have became filled with minerals deposited from solutions percolating through the rock • Tuff is a pyroclastic rock formed from volcanic fragments (ash) • Obsidian is a volcanic glass • Pegmatite is an intrusive rock with very large grains (cm size) – typically due to crystallisation from water-rich magma. Texture is pegmatitic Determining texture Determining texture Determining texture 1 cm Composition • Rocks are named according to the composition (proportion of minerals). The proportion of dark Fe-Mg (ferromagnesian) minerals is an important criterion • Felsic rocks contain less than 10% Fe-Mg mins. • Intermediate rocks 10-30, 30-40% Fe-Mg mins. • Mafic rocks have 40-90% Fe-Mg mins. • Ultramafic rocks have >90% Fe-Mg mins. Composition Rhyolite Granite Dacite Granodiorite Amphibole Type of magma Felsic Intermediate F(elsic), I(intermediate), M(afic), U(ltramafic) • Is the rock felsic, intermediate, mafic, or ultramafic? – Determined on the basis of percentage Fe-Mg minerals – Chart gives ranges of % Fe-Mg minerals. – In general one can associate % Fe-Mg mins with colour. The darker the rock, the higher the % Fe-Mg minerals. – Relatively easy to determine for phaneritic rocks – With aphanitic rocks must go entirely on colour: felsic rocks are buff, pink or red (felsic glass, obsidian, is black); intermediate rocks vary from shades of grey to green; mafic rocks are dark green or grey to black. Name of the feldspar • Name the feldspar. Two choices: – K-feldspar (pink, cream) – Plagioclase (white, grey or blue), striations on cleavage surfaces – Note that in aphanitic rocks you may not be able to see any feldspar (too fine grained). Therefore report feldspar as n.d. = not determined. This is not the same as saying there is none. – Glassy rocks have no minerals, i.e. no feldspar Fe-Mg mineral • Name the Fe-Mg mineral(s). • There are four choices: – – – – – Biotite (usually in felsic rocks) Amphibole (predominant in intermediate rocks) Pyroxene (in mafic and ultramafic rocks) Olivine (in mafic and ultramafic rocks) Note: In the aphanitic rocks you may not be able to see the Fe-Mg mineral. Report n.d. (not determined) – Glassy rocks have no Fe-Mg minerals. Identifying the Fe-Mg minerals • Biotite: Black, shiny, flakey • Amphibole: Black/dark green, shiny (visible cleavage surfaces), not flakey • Pyroxene: Black/dark green, dull (cleavage not readily visible), not flakey • Olivine, apple green, glassy % quartz • Give the percentage quartz – In phaneritic rocks this is relatively easy; quartz is the grey vitreous (glassy-looking) mineral – Note that % quartz varies inversely with the amount of Fe-Mg mineral. • Felsic rock contain significant amount of quartz and a little Fe-Mg mineral (usually biotite) • Mafic or ultramafic rocks no quartz and lots of FeMg mineral (pyroxene and/olivine) – In aphanitic rocks report “n.d.” not 0% – Obsidian has no minerals (i.e. 0% quartz). I(ntrusive) or E(xtrusive) • Is the rock intrusive (I) or extrusive (E)? – Rule of thumb: phaneritic rocks are intrusive, aphanitic or glassy rocks (including porphyritic aphanitic or porphyritic glassy rocks) are extrusive – Except, basalts (which are extrusive) can be fine grained (phaneritic) Name • Use: – – – – the charts (on handout), examples (at back of lab), poster (back of lab, this powerpoint file), book. • Note that the name is, in some respects, the least important column of this lab. • This lab is your first introduction to igneous rocks and it is far more important to become familiar with them than to name them. Microscopes and pumice • Look through microscopes at slides • Answer questions • DON’T MOVE THE SLIDES OR ROTATE THE STAGE • The drawings on the board show you where to find points of interest in the field of view • Don’t confuse colourless and transparent… • With pumice “experiment” think of density – as a whole. One final point… • It will help to use the hand lens (particularly to differentiate biotite from amphibole) • Hand lenses have a very short focal length so, to be of use, you must hold them close to your eye (and the sample) Questions?