An introduction to igneous rocks – Part 1

Basalt. Image Wikipedia

What is an igneous rock?

It’s hard, may be pale or nearly black, but what’s in it?

This entry is aimed as a brief introduction to igneous mineralogy/petrology to the ‘beginners’ and it may be useful to have available a mineralogy and/or petrology text, see the ‘Books’ below the title bar.

An igneous rock is essentially a collection of potassium-, sodium-, calcium-(ie alkalis), iron- and magnesium-(ie ferromagnesian) silicates and alumino-silicates, free quartz and ferro- and ferro-titanium/chromium (and other metal) oxide and occasionally sulphide, minerals that have solidified from the molten state, ie magma.

These minerals are grouped in several ways according to their relative importance to the rock mass: Essential or Primary are those from which the rock is primarily composed, eg Quartz, Mica and Feldspar in granite, or who’s presence gives name to a specific rock type, eg Reibekite Microgranite (as on Ailsa Craig – used to make the best curling stones!); Accessory may be present but have no bearing on the rock type, eg Zircon, Apatite in granites; Secondary produced by later weathering or hydrothermal alteration of the original essential minerals,eg Kaolin from the alteration of feldspar in granite or Chlorite from the hydrothermal alteration of primary ferromagnesian minerals.

A further classification is based on the silica saturation of the rock; silica saturated Acidic, silica poor Basic; this classification does not refer to the amount of free silica – ie Quartz – in the rock, but to the total silicate in the minerals present. In addition, Intermediate rocks are those showing mixed acid and basic characteristics; Ultra-basic (or Ultra-mafic) are silica depleted and contain rare oxides. Note, the acid-basic categorisation is not that of chemists redox pH divisions.

Examples of these groups are:

Acid : Rhyolite

Intermediate : Andesite. Note Dacite and Trachyte lavas fall between Intermediate and Acid

Basic : Calc-alkaline Basalt, High-alumina Basalt, Tholiitic Basalt

Ultra-basic : Picritic basalt

The minerals are in 6 main groups: Feldspars/Feldspathoids, Amphiboles, Pyroxenes, Micas, Olivines and oxides (of silicon and metals). The lighter coloured minerals are termed Felsic, the darker ferromagnesian, Mafic; the relative proportions roughly determining the colour of the rock; hence acid rocks which have a high felsic content are generally paler than the basic types with higher mafic minerals.

It may be relevant here to digress to the effects of decreasing silica content on mineralogy as mentioned earlier. With reference to the potassic and sodic feldspar/feldspathoids, the silica saturated end members, feldspar, are Orthoclase and Albite and by the removal – ie silica depletion – of one SiO2 molecule, two stages of felspathoids are produced thus, (ie feldspathoids being silica poor feldspar):

Orthoclase – KAlSi3O8                      Albite – NaAlSi3O8

Leucite – KAlSi2O6                           Jadeite – NaAlSi2O6

Kalsilite – KAlSiO4                            Nepheline – NaAlSiO4

Picture from: mii.org Feldspar microcline.

Feldspars

Feldspars and Feldspathoids comprise the bulk of the felsic minerals, their relationships mentioned above, but feldspars are the larger rock-forming group and are subdivided into potassic (K feldspar) and sodic-Na and calcic-Ca (combined Na and Ca form the Plagioclase sub-group) varieties. They are generally pale coloured, whites, greys to pinks and almost colourless.

K feldspars, predominantly Orthoclase and Sanidine, are characteristic of the more acidic rocks – dacite, trachyte and rhyolites

The Plagioclase group are a chemical continuous substitution series of 6 recognised minerals between the two end members Albite (Na end) and Anorthite (Ca end). The more sodic members are associated, in general, with more acid rocks, calcic with basic. The minerals of the 6 divisions are identified by name and analysis notation of the Albite (Ab):Anorthite(An) ratio thus:

Albite          Ab100An0 to Ab90An10

Oligoclase   Ab90An10 to Ab70An30

Andesine     Ab70An30 to Ab50An50

Labradorite  Ab50An50 to Ab30An70

Bytownite    Ab30An70 to Ab10An90

Anorthite     Ab10An90 to Ab0An100

Pyroxenes and Amphiboles combined, are the main mafic rock forming mineral groups in volcanic rocks and as in the feldspars, both exhibit chemical substitution series between end-members in their respective groups.

Pyroxenes are a large complex group of chain silicates – so called from the molecular strucure of the minerals – and they are subdivided on a crystallographic basis into 2 sub-groups, Ortho- and Clino-pyroxenes (of the Orthorhombic and Monoclinic crystal groups respectively. There are 7 crystallographic groups: Cubic, Tetragonal, Orthorhombic, Monoclinic, Triclinic, Hexagonal and Trigonal; the differences being according to the crystal symetry ie the relative positions of the crystal rotation axes).

The main orthopyroxenes having Enstatite -En – (MgSiO3) and Ferrosilite – Fs – (FeSiO3) as end members, the intervening mineral Hypersthene in older texts is now also referred as Orthopyroxene. The minerals are identified by their En:Fs ratio.

The Clinopyroxenes, again Mg and Fe silicates, but in some minerals with Ca, Al or Na. Augite, Diopside, Pigeonite and Aegrine are the main minerals.

The main Pyroxenes are

Augite           Most common pyroxene in basalt, andesite; contains Al and Ca

Diopside        in Basic rocks; contains Ca

Pigeonite       As Augite; contains Al

Aegrine         Alkali pyroxene, in more acid rocks; contains Na and Fe

Hypersthene  In Intermediate and Basic rocks

Enstatite       In Intermediate and Basic rocks

The Amphiboles are another large common group of rock forming minerals, chemically comparable with the pyroxenes, the main differences being in the crystal structure with Amphiboles arranged as a double chain and the presence of an hydroxyl radical (OH) in the molecule thus for example an orthorhombic equivalent member of each:

Amphibole   Anthophyllite Mg7Si8O22(OH)2

Pyroxene     Enstatite MgSiO3

Again, the minerals crystallise in the orthorhombic and monoclinic groups and substitution series are between the end-members.

By far the most important Amphibole in igneous rocks is Hornblende (a CaMgFeAl silicate), in the more acid divisions from acid Andesite to Rhyolite; the other members being more commonly associated with metamorphic rocks.

ALAN C

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