Igneous Classification Igneous rocks can be classified according

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!

Plutonic vs. Volcanic

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).

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.

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.

Classification by composition • There are several classifications, of individual rocks or rock suites. • By silica percentage: %Si. O 2 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 Al 2 O 3>Na 2 O+K 2 O+Ca. O Peraluminous Muscovite, biotite, topaz, corundum, garnet, tourmaline Na 2 O+K 2 O+Ca. O>Al 2 O 3 Metaluminous Melilite, biotite, pyroxene & Al 2 O 3 > Na 2 O+K 2 O hornblende, epidote Al 2 O 3 ~ Na 2 O+K 2 O Subaluminous Olivine, pyroxenes Al 2 O 3 < Na 2 O + K 2 O Peralkaline Sodic pyroxenes & amphiboles

Classification by composition • By Alkali-Lime index: for a suite of rocks, Ca. O and Na 2 O+K 2 O are plotted against Si. O 2. Generally, Ca. O decreases with increasing Si. O 2 while Na 2 O+K 2 O increases. Suites are classified by the Si. O 2 where the intersection occurs: Rock Suite Calcic Calc-alkaline Alkali-calcic Alkaline <51% Alkali-Lime Index Illustrative rock series >61 %Si. O 2 Mid-ocean ridge basalts 56 -61% Continental margin arc series 51 -56% Some intraoceanic island arcs Intraplate continental melts

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 = Si. O 2); 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 = Mn. O); Elementi in traccia (presenti in percentuali inferiori allo 0, 1%). Tali elementi vengono misurati in parti per milione (ppm). 10000 ppm = 1%.

Analisi chimiche rappresentative di rocce vulcaniche (elementi maggiori e minori) B-A Ol th TH Al LC And Dac Riol Co 69, 7 73, 2 75, 2 1, 0 0, 4 0, 2 0, 1 16, 0 15, 2 14, 0 12, 0 Si. O 2 45, 4 49, 2 53, 8 49, 1 46, 2 60, 0 Ti. O 2 3, 0 2, 3 2, 0 1, 5 Al 203 14, 7 13, 3 13, 9 17, 7 14, 4 Fe 2 O 3 4, 1 1, 3 2, 6 2, 8 4, 1 1, 9 1, 1 0, 6 0, 9 Fe. O 9, 2 9, 7 9, 3 7, 2 4, 4 6, 2 1, 9 1, 7 1, 2 Mn. O 0, 2 0, 1 0 0, 2 0, 0 0, 1 Mg. O 7, 8 10, 4 4, 1 6, 9 7, 0 3, 9 0, 4 0. 0 Ca. O 10, 5 10, 9 7, 9 9, 9 13, 2 5, 9 2, 7 1, 3 0, 3 Na 20 3, 0 2, 2 3, 0 2, 9 1, 6 3, 9 4, 5 3, 9 4, 8 K 20 1, 0 0, 5 1, 5 0, 7 6, 4 0, 9 3, 0 4, 1 4, 7 P 205 0, 4 0, 2 0, 4 0, 3 0, 4 0, 2 0, 1 0, 0 0, 1 1, 2 N. B. La Si. O 2 è sempre l’ossido più abbondante!

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

Oltre alla classificazione su base mineralogicapetrografica (triangoli di Streckeisen) e quella su base chimica (diagramma TAS), esiste un ulteriore criterio classificativo: la normativa. classificazione 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.