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Igneous Classification

Igneous Classification. Igneous rocks can be classified according to composition , mineralogy , texture and/or locality(!). The first distinction is between volcanic and plutonic rocks.

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Igneous Classification

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  1. Igneous Classification • Igneous rocks can be classified according to composition, mineralogy, texture and/or locality(!). • The first distinction is between volcanic and plutonic rocks. • Volcanic rocks are erupted at the Earth’s surface and cool very quickly. There is insufficient time to grow large crystals. This leads to formation of glass or very fine-grained rocks, or to phenocrysts (crystals that grew before eruption) in a fine groundmass. • Plutonic rocks crystallize at some depth, and therefore lose heat relatively slowly. Crystals have time to grow after nucleation, and the resulting rocks generally have individual crystals large enough to see unaided. • Rocks of exactly the same composition and mineralogy get different names in their volcanic and plutonic forms, because they look different!

  2. Plutonic vs. Volcanic

  3. Classification by mineralogy • The standard classification scheme uses the mineralogy of the rock (how much quartz, how much plagioclase, etc.) • There is one important twist…for volcanic rocks you usually cannot measure the actual minerals present (or it may be a glass and there are no minerals present). • In this case, instead of the actual minerals, you classify based on normative mineralogy • The norm is a calculation based on the bulk composition of a volcanic rock, for what minerals would be present if it were fully crystallized. • The standard norm calculation is called the CIPW norm, after Cross, Iddings, Pirsson, and Washington (1902).

  4. Classification by mineralogy Mineral content (actual or normative) of the rock by volume is divided into Quartz (Q), Alkali feldspar (A), Plagioclase (P), Feldspathoids (F), and Mafic minerals like amphibole, biotite, pyroxenes, and olivine (M). For rocks with M < 90%, the Streckeisen double-triangle is used. It shows names defined by Q-A-P-F recalculated to 100%. Many of these names are really obscure; don’t try to learn all of them.

  5. Classification by mineralogy For rocks with mostly mafic minerals, a different scheme is used. The proportion of olivine, orthopyroxene, clinopyroxene, and plagioclase locate a rock using the appropriate Streckeisen ternary diagram.

  6. Classification by composition • There are several classifications, of individual rocks or rock suites. • By silica percentage: • %SiO2 Designation % Dark Minerals Designation Example rocks • >66 Acid <40 Felsic Granite, rhyolite • 52-66 Intermediate 40-70 Intermediate Diorite, andesite • 45-52 Basic 70-90 Mafic Gabbro, basalt • <45 Ultrabasic >90 Ultramafic Dunite, komatiite • By alumina saturation (this controls which dark minerals show up): • Chemistry Designation Distinctive Minerals • Al2O3>Na2O+K2O+CaO Peraluminous Muscovite, biotite, topaz, • corundum, garnet, tourmaline • Na2O+K2O+CaO>Al2O3 MetaluminousMelilite, biotite, pyroxene • & Al2O3 > Na2O+K2O hornblende, epidote • Al2O3 ~ Na2O+K2O Subaluminous Olivine, pyroxenes • Al2O3 < Na2O + K2O Peralkaline Sodic pyroxenes & amphiboles

  7. Classification by composition • By Alkali-Lime index: for a suite of rocks, CaO and Na2O+K2O are plotted against SiO2. Generally, CaO decreases with increasing SiO2 while Na2O+K2O increases. Suites are classified by the SiO2 where the intersection occurs: Rock Suite Alkali-Lime Index Illustrative rock series Calcic >61 %SiO2 Mid-ocean ridge basalts Calc-alkaline 56-61% Continental margin arc series Alkali-calcic 51-56% Some intraoceanic island arcs Alkaline <51% Intraplate continental melts

  8. Riassumendo…. I componenti chimici di una roccia vengono espressi in tre modi: Elementi maggiori (presenti in percentuali superiori all’1% in peso). Tali elementi vengono misurati con l’ossido di riferimento (es. Si = SiO2); Elementi minori (presenti in percentuali comprese tra lo 0,1 e l’1% in peso). Anche tali elementi vengono misurati con l’ossido di riferimento (es. Mn = MnO); Elementi in traccia (presenti in percentuali inferiori allo 0,1%). Tali elementi vengono misurati in parti per milione (ppm). 10000 ppm = 1%.

  9. Analisi chimiche rappresentative di rocce vulcaniche (elementi maggiori e minori) N.B. La SiO2 è sempre l’ossido più abbondante!

  10. Classificazione delle rocce vulcaniche – Classificazione su base CHIMICA TAS = Total Alcali vs. Silica Na HAWAIITE MUGEARITE BENMOREITE Serie K TRACHIBASALTO POTASSICO SHOSHONITE LATITE

  11. Oltre alla classificazione su base mineralogica-petrografica (triangoli di Streckeisen) e quella su base chimica (diagramma TAS), esiste un ulteriore criterio classificativo: la classificazione normativa. La classificazione normativa si basa su una serie di calcoli proposti da un gruppo di studiosi americani nei primi anni del ‘900 ed è nota come NORMA CIPW(dalle iniziali degli studiosi Cross, Iddings, Pearson e Washington). Il calcolo normativo si basa sulla composizione chimica della roccia e ricostruisce i minerali teorici (VIRTUALI) che si sarebbero potuti formare a partire da quella certa composizione di fuso.

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