Practical Igneous Petrology for Dummies

Transcription

1 Igneous Petrology Cheat Sheet 1 Practical Igneous Petrology for Dummies We can all be dummies when it comes to looking at igneous rocks. There s just a bewildering number of rocks to remember, minerals to be familiar with and textures to recognise. This cheat sheet pack contains some practical advice on how to do about recognising the minerals in a rock in thin section and handspecimen, what are the important properties of a rock to describe and how, and how to come up with a name for your specimen. These cheat sheets are organised into a sequence of tasks that we undertake when examine a sample of igneous rock that lead us to identifiy it. These tasks can be summarised by: First task - What are the minerals in the rock? o Mineral identification in hand specimen. o Mineral identification in thin-section. Second task What texture does the rock have? Third task What is the name of the rock?

2 Igneous Petrology Cheat Sheet 2 Mineral Identification in Hand Specimen Mineral identification in hand-specimen, particularly in igneous rocks, is a very tricky business since there is usually little information to go on. Students will spend hours staring at a rock glumly thinking they should have done a course in chartered accountancy or flower arranging. A professional geologist will simply make an educated guess and pretend they are absolutely confident of their mineral identification. This cheat sheet describes tricks in looking confident when identifying minerals in hand-specimen. A. Trick 1 Have the Right Tools A good hand lens is invaluable in identifying minerals in igneous rocks. Most of the little plastic varieties are practically useless. In the field (i.e. not in practicals!) a good pen knife is also useful for testing hardness. B. Trick 2 Make the Most of the Minerals Properties Remember the Basic Properties Colour often with fine-grained minerals there is not much except colour to go on. The problem is colour is perhaps the least distinctive property of a mineral, however, together with other evidence (such as a general idea of the type of rock) it can be invaluable. There will be some examples below. Hardness Hardness is useful mostly for coarse-grained minerals and can t usually be applied to most igneous rocks. Transparency can you see through the mineral. Is it transparent, translucent or opaque? Cleavage and fracture Sometimes cleavage (planes of fracture) and fracture style (e.g. shell-like concoidal) are dead giveaways for some minerals. Pyroxenes and amphiboles have very characteristic cleavages. Cleavage and fracture Sometimes cleavage (planes of fracture) and fracture style (e.g. shell-like concoidal) are dead giveaways for some minerals. Pyroxenes and amphiboles have very characteristic cleavages. Lustre How the mineral reflects light can be important especially when there is little else to go on. Remember we have the following types of lustre: Adamantine - very gemmy crystals Dull - just a non-reflective surface of any kind Earthy - the look of dirt or dried mud Fibrous - the look of fibers Greasy - the look of grease Metallic - the look of metals Pearly - the look of a pearl Pitchy - the look of tar Resinous - the look of resins such as dried glue or chewing gum Silky - the look of silk, similar to fibrous but more compact Submetallic - a poor metallic luster, opaque but reflecting little light Vitreous - the most common luster, it simply means the look of glass Waxy - the look of wax Crystal System This is a tricky property but very useful for euhedral (well shaped) crystals. Some examples of crystal system and the appearance of crystals are shown on a following page. Crystal Habit This is a subtly different way of describing crystal shape that can be useful in identifying certain minerals which can have characteristic crystal habits. Density Only really useful with large masses of a single mineral. C. Trick 3 Knowing What to Expect Where! Probably the best advice in identifying minerals in hand-specimen is having an idea what minerals you d expect to find in a particular rock. Although there are an enormous number of minerals that can be found important rock forming minerals, which allow us to give a rock a name, are few in number and more important in certain rock types than in others. This sounds like you need to know the rock type before you can identify the minerals. However, it is much easier than this. Two properties of rocks, colour index and grain-size, are so easy to identify even a child could do it. These are enough to allow us to narrow down the identity of the rock forming minerals. Leucocratic rocks (more than 2/3 light minerals green minerals are always considered dark!) Leucocratic rocks include acid rocks such as granite and rhyolite.

3 Igneous Petrology Cheat Sheet 3 Most common light coloured minerals (in order of frequency): Quartz Alkali feldspar Plagioclase feldspar Muscovite Most common dark coloured minerals (in order of frequency): Biotite Amphibole (hornblende most common but also reidbeckite in alkali granites). Pyroxenes Tourmaline (abundant in some granites and pegmatites). Most common opaque minerals: Pyrite Chalcopyrite Azurite Galena Malachite Magnetite Hematite Mesocratic rocks (around ½ light minerals) Mesocratic rocks include intermediate rocks such as diorite and andesite. Most common light coloured minerals (in order of frequency): Feldspars (either alkali or plag can dominate). Quartz Feldspathoids (such as nepheline and leucite). Most common dark coloured minerals (in order of frequency): Hornblende Pyroxenes (augite, hypersthene, pigeonite, diopside) Biotite and/or phlogopite Olivine (rare) Melanocratic rocks (more than 2/3 dark minerals) Melanocratic rocks include basic and ultrabasic rocks such as basalt, gabbro and peridotite. Ultrabasic rocks usually lack any light coloured minerals. Most common light coloured minerals (in order of frequency): Plagioclase feldspar Quartz (rare) Alkali feldspar (rare) Feldspathoids (rare) Most common dark coloured minerals (in order of frequency): Pyroxene (augite, hypersthene, pigeonite, diopside) Olivine Phlogopite (a mica) Garnet Amphiboles (rare) Biotite (rare) D. Trick 4 Know the Key Properties of Rock Forming Minerals

4 Igneous Petrology Cheat Sheet 4 Quartz (SiO2) Colour is as variable but clear quartz is by far the most common color followed by white or cloudy (milky quartz). Purple (Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are also common. Cryptocrystalline varieties (agate, chalcedony etc) can be multicolored. Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous. Crystal System: trigonal. Cleavage and Fracture: No cleavage, concoidal fracture. Crystal Habit: variable but often anhedral. Hexagonal prisms or three sided prismatic crystals are common in veins and pegmatites. Hardness: 7 (less in agate and chalcedony). Occurrence: any leucocratic rock is likely to contain quartz, melanocratic rocks are unlikely to contain quatz. Best Indicators are first the fact that it is very common (always assume transparent clear crystals may be quartz), crystal habit, hardness, striations on the surfaces of crystals, good conchoidal fracture and lack of good cleavage. Alkali Feldspar (KAlSi3O8 - NaAlSi3O8) Color is off-white, most characteristically pink but can be yellow, orange to brown. Luster is vitreous to dull if weathered. Pearly lustre is most common Transparency crystals are usually opaque, may be translucent. Crystal System is monoclinic (orthoclase) or triclinic (microcline, albite). Crystal Habits include blocky or tabular crystals. Crystals have a nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations. Twinning is common. (see above). A psuedo-orthorhombic or psuedo-trigonal variety, found in alpine veins is called adularia, and forms more flattened tabular crystals. Cleavage and Fracture: Two good sets of cleavage nearly at right angles to each other. Concoidal fracture. Hardness: 6 Occurrence: any leucocratic or mesocratic rock may contain alkali feldspar but it is not always present. Melanocratic rocks will not contain alkali feldspars. Best Indicators are its colour when pinkish, its occurrence and the presence of two sets of cleavage. It is most difficult to distinguished from quartz and plagioclase. Cleavage distinguishes it from quartz. Its colour distinguishes it from plagioclase. Plagioclase Feldspar (KAlSi3O8 - NaAlSi3O8) Color is usually white to gray or pale shades of green, yellow or brown. Lustre is vitreous to dull if weathered.. Transparency crystals are translucent to transparent. Crystal System is triclinic Crystal Habits include blocky, or tabular crystals. Twinning is almost universal in all plagioclases. Cleavage and fracture: Cleavage is perfect in one and good in another direction forming nearly right angled prisms. Fracture is conchoidal. Hardness is Occurrence: can occur in leucocratic, mesocratic or melanocratic rocks. Usually is the dominant light coloured mineral in melanocratic rocks. Best Indicators are its colour when greyish or having a green tint. Cleavage distinguishes it from quartz. Its colour distinguishes it from alkali feldspar. Nepheline (feldspathoid) ((Na, K)AlSiO4) Colour is usually off white to gray or brown and occasionally other tints.

5 Igneous Petrology Cheat Sheet 5 Lustre is mostly greasy to dull in weathered specimens. Transparency: Crystals are translucent to more rarely transparent. Crystal System: Hexagonal Crystal Habits: Usually massive or granular. Some prismatic to columnar crystals are found with a simple hexagonal cross section. Cleavage and Fracture: Cleavage is poor, in three directions, prismatic, but rarely seen. Fracture is conchoidal to uneven. Hardness is Occurrence: Alkali-rich (Na, K) leucocratic and mesocratic rocks. It is often found together with alkali feldspars and alkali-rich pyroxenes (see below). It is not found with quartz. Best Indicators are its occurrence and its grey-brown colour. It can be difficult to distinguish from feldspar but lacks good cleavage and has a distinctive greasy lustre. Leucite (feldspathoid) (KAlSi2O6) Color is clear, white or gray, with yellowish and reddish tints possible. Lustre is vitreous or greasy to dull. Transparency: crystals are transparent, translucent to commonly opaque. Crystal System is tetragonal. Crystal Habits include the characteristic trapezohedron. Also granular and massive. Cleavage and Fracture: Cleavage is absent. Fracture is conchoidal Hardness is Occurrence: K-rich leucocratic and mesocratic rocks. It is usually found with K-rich feldspar. It is not found in the presence of quartz. Best Indicators are its occurrence and tetragonal habit. Pyroxenes Any dark mineral with two good sets of cleavage at right angles to each other is probably a pyroxene. They are most likely to be found in melanocratic rocks but can occur in both mesocratic and leucocratic. The descriptions below are for common varieties of pyroxene. Augite (pyroxene) ((Ca, Na)(Mg, Fe, Al)(Al, Si)2O6) Colour is dark green, brown and black Lustre is vitreous to submetallic and even dull. Transparency crystals are mostly translucent or opaque. Crystal System is monoclinic (i.e. a clinopyroxene) Crystal Habits include short prismatic, rarely tabular crystals. The square cross section is distinctive in the prismatic crystals. Also compact, granular, columnar, lamellar and fibrous(rare). Cleavage and Fracture is perfect in two lengthwise directions at right angles. Fracture is uneven. Hardness: 5-6 Occurrence: A common dark mineral (together with hypersthene and pigeonite) in melanocratic rocks. Is also found in mesocratic rocks. Best Indicators are its occurrence, monoclinic crystal shape and cleavage. When in the same rock as hypersthene it can be identified by its green-black colour and its monoclinic crystal shape. Hypersthene (pyroxene) ((Mg, Fe)(Al, Si)2O6) Colour is grey, brown or less commonly green. Lustre is vitreous to pearly. Weathered specimens can have a submetallic luster ("bronzite"). Transparency: Crystals are generally translucent.

6 Igneous Petrology Cheat Sheet 6 Crystal System is orthorhombic (i.e. a clinopyroxene) Crystal Habits include rare individual crystals that have a stubby prismatic habit. More typically massive or in coarse lamellar or fibrous aggregates. Cleavage and Fracture: Cleavage is perfect in two directions at 90 degrees. Fracture is uneven. Hardness is 5-6. Occurrence: A common dark mineral (together with pigeonite and augite) in melanocratic rocks. More Fe-rich orthopyroxenes can, however, be found in mesocratic and leucocratic rocks. Best Indicators are its occurrence, orthorhombic crystal shape and cleavage. When in the same rock as augite it can identified by is grey-brown colour and orthorhombic crystal shape. Most common in intrusive rocks. Pigeonite (pyroxene) ((Mg, Fe)(Al, Si)2O6) Colour is Brown, Greenish brown, Light purple brown, Black. Lustre is vitreous to pearly. Weathered specimens can have a dull lustre. Transparency: Crystals are generally translucent to opaque. Crystal System is monoclinic (i.e. a clinopyroxene) Crystal Habits include rare individual crystals that have a stubby prismatic habit. More typically massive or in coarse lamellar or fibrous aggregates. Cleavage and Fracture: Cleavage is perfect in two directions at 90 degrees. Fracture is concoidal. Hardness is 5-6. Occurrence: A common dark mineral (together with hypersthene and augite) in melanocratic rocks. It is difficult to distinguish from augite, however, it is restricted to extrusive and hypabyssal rocks. Best Indicators are its occurrence, monoclinic crystal shape and cleavage. When in the same rock as augite it can identified by its concoidal fracture. Aegirine (pyroxene) (NaFeSi2O6) Colour is black to greenish or brownish black. Lustre is vitreous to pearly. Transparency: Specimens are generally opaque to translucent. Crystal System is monoclinic. Crystal Habits include long prismatic crystals terminated by a steep asymmetrical pyramid. Also as disseminated grains, compact and fibrous. Cleavage and Fracture: is perfect in two directions at near 90 degree angles. Fracture is uneven. Hardness is Occurrence: Found in alkali-rich leucocratic and mesocratic rocks. Sometimes found with augite. Best Indicators are its occurrence, monoclinic crystal shape and cleavage. When in the same rock as augite it can identified by its more elongate habit. Amphiboles Any dark mineral with two good sets of cleavage at 124 (or 56) degrees to each other is probably an amphibole. They are most likely to be found in leucocratic and mesocratic rocks but can occur in some melanocratic rocks. The descriptions below are for common varieties of amphibole. Hornblende (amphibole) (Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2) Colour is almost always black to dark green. Lustre is vitreous to dull. Transparency: Crystals are generally opaque but thin crystals or exceptional specimens can be translucent. Crystal System is Monoclinic.

7 Igneous Petrology Cheat Sheet 7 Crystal Habits include short stocky prismatic crystals as well as long thin crystal forms. Crystals can have a hexagonal cross-section although rarely symmetrical. Also found granular, massive and occassionally acicular aggregates. Cleavage and Fracture: Cleavage is imperfect in two directions at 56 and 124 degrees. Fracture is uneven. Hardness is 5-6. Occurrence: Hornblende is the most common amphibole and can occur in any of the colour index rocks, although it is most common in mesocratic rocks. Best Indicators are its occurrence, monoclinic crystal symmetry and cleavage. In granites, rhyolites and intermediate volcanics it typically occurs as long thin crystals. It can be difficult to distinguish from biotite and tourmaline. Biotite, however, has a sheet-like habit with only a single good plane of cleavage and tourmaline, which also forms long thin crystals in some granites, has a triangular cross-section. Riebeckite (amphibole) (Na2(Fe, Mg)3Fe2Si8O22(OH)2) Colour is usually dark blue to black. Lustre is vitreous or silky in fibrous forms. Transparency: Crystals are translucent. Crystal System is monoclinic; 2/m. Crystal Habits include slender prismatic to acicular, often aggregated crystals, columnar, granular and fibrous, asbestiform masses. A moss-like aggregate habit has been described when found in some igneous rocks. Cleavage and Fracture: is perfect in two directions at 56 and 124 degree angles. Fracture is splintery to uneven. Hardness is 5 6 Occurrence: Found in alkali-rich leucocratic and mesocratic rocks. Best Indicators are its occurrence, monoclinic crystal symmetry and cleavage. It is difficult to distinguish from hornblende, however, it is likely to be found in the presence of abundant alkali feldspar and often has a distinct bluish tinge. Arfvedsonite (amphibole) (Na3(Fe, Mg)4FeSi8O22(OH)2) Colour is greenish-black, dark gray or black. Lustre is vitreous to dull or silky in fibrous forms. Transparency: Crystals are translucent to opaque. Crystal System is monoclinic.. Crystal Habits include prismatic to acicular or even tabular crystals sometimes in radial stellate aggregates. Some specimens are up to 20 cm in length. Also fibrous, lamellar and massive. Cleavage and Fracture: Cleavage is perfect in two directions at 56 and 124 degree angles. Fracture is uneven to subconchoidal. Hardness is Occurrence: Occurs in alkali-rich rocks similar to riebeckite Best Indicators are its occurrence, monoclinic crystal symmetry and cleavage. It is difficult to distinguish from hornblende, however, it is likely to be found in the presence of abundant alkali feldspar. It can be distinguished from reibeckite by its bluish tinge. Olivine ((Mg, Fe)2SiO4) Colour is a light near emerald green to the more common pale yellowish green; also found colorless, fayalite (Ferich olivine) is greenish brown to black. Lustre is vitreous (except where strongly weathered). Transparency: Crystals are transparent to translucent. Crystal System is orthorhombic. Crystal Habits include flatten tabular to box shaped crystals, but good crystals are rare. More commonly found as grains in alluvial gravels and as granular xenoliths in magnesium rich volcanic rock. Also massive.

8 Igneous Petrology Cheat Sheet 8 Cleavage and fracture: Cleavage is very poor in two directions at 90 degrees, but is much more distinct in fayalite (Fe-rich olivine). Fracture is conchoidal. Hardness is Occurrence: Olivine is a common mineral in most melanocratic rocks. Fayalite (Fe-rich olivine) can, however, be found in mesocratic and leucocratic rocks, particularly in alkali-rich rocks, although it isn t common. Best Indicators are its green colour, orthorhombic crystal symmetry and apparent lack of cleavage. Olivine is relatively easy to distinguish from other minerals, although when weathered it can resemble pyroxene. It does not occur together with quartz. Biotite (mica) (K (Fe, Mg)3 AlSi3O10 (F, OH)2) Colour is black to dark brown and yellow with weathering. Lustre is vitreous to pearly. Transparency crystals are transparent to translucent. Crystal System is monoclinic; 2/m Crystal Habits include tabular to prismatic crystals. Cleavage is perfect in one direction producing thin sheets or flakes. Hardness is 2.5. Occurrence: Most common in leucocratic rocks, but can be found within some mesocratic. Best Indicators are its sheet-like habit, forming books that can readily be pulled apart using your thumbnail, and its colour. It can distinguished from phlogopite by the golden translucent appearance of this mineral in small flakes. Small flakes of biotite will still appear black or dark brown when viewed with a hand lens. Muscovite (mica) (KAl2(AlSi3O10)(F, OH)2) Color is white, silver, yellow, light green and brown. Lustre is vitreous to pearly. Transparency crystals are transparent to translucent. Crystal System is monoclinic. Crystal Habits include tabular crystals. Cleavage is perfect in one direction producing thin sheets or flakes. Hardness is Occurrence: Restricted to leucocratic rocks. Best Indicators are its sheet-like habit, forming books that can readily be pulled apart using your thumbnail, and its colour. It can distinguished from the other micas by its pale colour in small flakes. Phlogopite (mica) (K Mg3 AlSi3O10(OH)2) Colour is pale brown to brown. Lustre is vitreous to pearly. Transparency crystals are transparent to translucent. Crystal System is monoclinic. Crystal Habits include tabular to prismatic crystals. Cleavage is perfect in one direction producing thin sheets or flakes. Hardness is Occurrence: Most common in melaocratic rocks but can occur in alkali rich mesocratic rocks. Best Indicators are its sheet-like habit, forming books that can readily be pulled apart using your thumbnail, and its colour. It can distinguished from biotite by its golden translucent appearance in small flakes. Garnet (X3Y2(SiO4)3)

9 Igneous Petrology Cheat Sheet 9 Only the varieties pyrope and spessartine tend to be found in igneous rocks. Colour: Pyrope is red to reddish purple and sometimes a deep enough red to appear black. Spessartine can be yellow, brown, red or pink. Lustre is vitreous. Transparency crystals are transparent to translucent. Crystal System is isometric/cubic. Crystal Habits include the typical rhombic dodecahedron. Cleavage and Fracture: Cleavage is absent. Fracture is conchoidal. Hardness is Occurrence: Pyrope only occurs in melanocratic rocks, generally mantle ultrabasic rock as part of mantle nodules. Spessartine occurs in leucocratic rocks such as granite and pegmatite and is not particularly common. Best Indicators are its cubic crystal symmetry translucent nature and occurrence.

10 Igneous Petrology Cheat Sheet 10 This tree only applies to the main rock forming minerals. Accessory minerals, and those found in hydrothermal veins and some pegmatites are not included. Once you have found an identification from this tree use the full descriptions on the previous pages to test whether the occurrence and properties of the mineral are consistent. Important some commonly found minerals are not included in this tree. These are alteration products produced by wearthering. These are usually finegrained and present either replacing certain minerals or within veins: Chlorite Common alteration of mafic minerals. Has a typically yellowish green colour. Epidote Common alteration of mafic minerals and feldspar. Has a characteristic iridescent green colour. Serpentine Common alteration product of olivine and pyroxene in mafic and ultramafic rocks. Usually dark green. Zeolites Off white minerals, often fibrous, typically found filling amygdales. Carbonates Commonly found in veins in a wide-range of igneous rocks. Common with serpentine.

11 Igneous Petrology Cheat Sheet 11 Mineral Identification in Thin-Section For Dummies Mineral identification in thin section can be a tricky business and it s easy to forget the basics. All of us tend to be mineral identification dummies at some point whether were students or scientists. This handout sheet contains some handy hints on exactly how to sound like an expert and how to fool anyone. The last sheet (the mineral cheat tree) is particularly useful. A. General Advice Remember the Basic Properties Relief how much the mineral stands out in PPL. If it looks like someone has drawn it with a marker pen then that s high relief. The higher the relief the higher the refractive index. Colour pay attention to it sometimes it can be a give away for certain minerals in PPL. Pleochroism the change in colour of a mineral in PPL when you rotate the stage. Pleochroism is your friend! It can help you big time in choosing between one coloured mineral and another. Birefringence colours the interference colours in XPL. This will tell you the difference in the refractive index of the section of the mineral you re looking at. The annoying thing about birefringence is that it changes with orientation of a crystal, however, it can tell you lots. Crystal shape when you have a euhedral crystal you are laughing. It ll tell you the crystal system, however, how a crystal appears depends on the orientation (see the sheet about crystal shapes). Cleavage and cracks cleavage is a dead give away when combined with the other properties. Again it depends on orientation. Twinning some minerals are a breeze when they have twinning in XPL. Plag, for example, zebra stripe city. Extinction angle the angle between the extinction direction (when the mineral goes black in XPL) and either the elongation direction of laths or the cleavage (see the example on orientations of minerals). Optical sign a bit more tricky but can be used to choose between minerals when the going gets tough (see the sheet on orientation). Interference figures interference figures obtained with the bertrand lens are from HELL!!!! They can, however, be useful in finding the orientation of the crystal in the thin section. Remember When to Give Up! Sometimes in life you just have to say I give up and move to next thing. If you haven t identified a mineral after a few minutes just write down its properties and move on to the next. Who knows you might suddenly realise what it is later. Basically don t waste time struggling with it life s too short and there s better things to spend your time on than optical mineralogy. Identifying a Single Crystal in Thin-Section In many cases you can t just take one crystal in a thin-section and identify the mineral. Most minerals look different in different orientations and are much easier to identify in one orientation than another. Have a look around the section to find a crystal in the right orientation and then figure out the differences (see the sheet on orientation). The tricky part about this is telling which crystals are the same mineral but in different orientations.

12 Igneous Petrology Cheat Sheet 12 Crystal Shapes It is frequently difficult to identify the crystal system of a mineral in thin-section from the crystal shape due to the random sections and subhedral/anhedral shapes. However, for euhedral phenocrysts and when combined with other information such as cleavage you can use crystals shapes to determine the system and the orientation of the crystal axes. Comparing the shapes of a number of phenocrysts often lets you determine the system.

13 Igneous Petrology Cheat Sheet 13 Orientation of Crystals and Different Sections Probably the most complicated part of optical mineralogy is that minerals in the thin-section may be cut at all sorts of different angles and each section will look slightly different. When in doubt (which probably means always) assume the fewest number of minerals possible are present. An Example: Orthopyroxenes and Clinopyroxenes Orthopyroxenes (such as hypersthene and enstatite) are orthorhombic and clinopyroxenes (such as augite and pigeonite) are monoclinic. Telling the difference between the two can be tricky in thin-section but we can use the different orientations of crystals in the section to distinguish them. The diagram below shows the orientation of the crystallographic axes (a, b and c) and optical axes (X, Y and Z) of augite and hypersthene, as well as illustrating the symmetry of the crystal. These diagrams are given in most optical mineralogy books. The appearance of different sections of the mineral in thin section are shown in the diagram seen along the axes. When the N-S polariser direction of the microscope in XPL is parallel to the optical axis that section of the mineral will go into extinction (i.e. go black). Note that in hypersthene (the orthopyroxene) the optical axes are always parallel to the crystallographic axes. All sections of hypersthene will, therefore, have straight extinction. In augite only the sections 001 and 100 have straight extinction. All the other possible sections will have inclined extinction and the largest extinction angle (of degrees) will be seen in the section looking down the b-axis (termed 010 in Miller indices notation). If all sections of pyroxene in a thin-section have straight extinction then the rock contains only orthopyroxene. If all sections have inclined section (perhaps with one or two with straight extinction that just happen to be in the right orientation) then all the pyroxene is likely to be clinopyroxene. Anything between the two suggests both orthopyroxene and clinopyroxene are present.

14 Igneous Petrology Cheat Sheet 14 Interference Figures To obtain an interference figure: (1) Focus on the grain at high mag, making sure the objective is centered, (2) put in the substage condensor (top lever below the stage), (3) put in the bertrand lens, (4) turn up the brightness to max. Don t be put off if its fuzzy and horrible, particularly if its biaxial!!! Uniaxial Interference Figures of Leucite Uniaxial minerals have an optical axis parallel to the c-axis. They are the ones where 2V angle is not listed. Examples include quartz and tourmaline. Biaxial Interference Figures of Biotite Most minerals are biaxial. Even the centered figures change during rotation.

15 Igneous Petrology Cheat Sheet 15

16 Igneous Petrology Cheat Sheet 16 Igneous Textures There are an enormous number of different terms for igneous textures. Those presented here are perhaps the most widely used. Many terms used to describe igneous textures have evolved through usage. Terms Describing Individual Crystals Crystal Shape Euhedral crystal bound by well formed faces Subhedral crystal bound by a few well formed faces Anhedral crystals bound by no well formed faces (i.e. irregular) Crystal Habit Tabular - a term used to describe grains with rectangular tablet shapes. Equant - a term used to describe grains that have all of their boundaries of approximately equal length. Fibrous - a term used to describe grains that occur as long fibers. Acicular - a term used to describe grains that occur as long, slender crystals. Prismatic - a term used to describe grains that show an abundance of prism faces (like the end of a pencil sharpened with a knife). Terms Describing the Crystallinity (number and form of crystals in a rock) Holohyaline glassy rocks with no crystals. Aphantic crystalline but with tiny crystals that can t be distinguished with a handlens. Phaneritic or equigranular crystalline with equal-sized crystals that can be observed with a hand lens. Inequigranular crystalline with unequal-sized crystals that can be observed with a hand lens. Porphyritic a form of inequigranular texture in which some crystals are much larger than those that surround them. Most porphyritic rocks are either extrusive or hypabyssal. Phenocrysts the large crystals in a porphyritic rock. Groundmass or Mesostasis the fine-grained crystals or glass that surrounds phenocrysts in a porphyritic rock. Glomeroporphyritic - if phenocrysts are found to occur as clusters of crystals, then the rock should be described as glomeroporphyritic instead of porphyritic. Seriate a form of inequigranular texture in which crystals show a continuous range in sizes. Granular rocks that lack fine-grained or glassy groundmass. Phaneritic and inequigranular rocks are granular. Terms Describing Grain-Size Note a porpyritic rock with a fine-grained matrix is fine-grained even if the phenocrysts are large! Coarse-grained crystals are > 5mm Medium-grained crystals are 1-5 mm Fine-grained crystals are <1 mm (not including phenocrysts) Terms Applicable to Specific Igneous Rocks Vesicular - if the rock contains numerous holes that were once occupied by a gas phase, then this term is added to the textural description of the rock. Amygdular - if vesicles have been filled with material (usually calcite, chalcedonay, or quartz, then the term amygdular should be added to the textural description of the rock. An amygdule is defined as a refilled vesicle. Pumiceous - if vesicles are so abundant that they make up over 50% of the rock and the rock has a density less than 1 (i.e. it would float in water), then the rock is pumiceous. Scoraceous- if vesicles are so abundant that they make up over 50% of the rock and the rock has a density greater than 1, then the rock is said to be scoraceous. Graphic - a texture consisting of intergrowths of quartz and alkali feldspar wherein the orientation of the quartz grains resembles cuneiform writing. This texture is most commonly observed in pegmatites. Spherulitic - a texture commonly found in glassy rhyolites wherein spherical intergrowths of radiating quartz and feldspar replace glass as a result of devitrification.

17 Igneous Petrology Cheat Sheet 17 Obicular - a texture usually restricted to coarser grained rocks that consists of concentrically banded spheres wherein the bands consist of alternating light colored and dark colored minerals. Terms Describing Microscopic Features of the Rock Myrmekitic texture - an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in plagioclase. This texture is found in granites. Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally surrounded by pyroxene grains. This texture is common in diabases and gabbros. Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of pyroxene grains. Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another mineral. Intergranular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides. Intersertal texture - a texture similar to intergranular texture except that the interstices between plagioclase grains are occupied by glass or cryptocrystalline material. Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material. Coronas or reaction rims - often times reaction rims or coronas surround individual crystals as a result of the crystal becoming unstable and reacting with its surrounding crystals or melt. If such rims are present on crystals they should be noted in the textural description. Zoning Where the composition of the crystal changes outward from the centre in concentric zones. Oscillatory zoning - This sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of different composition as evidenced by extinction phenomena. Sieve texture - This sometimes occurs in plagioclase wherein individual plagioclase grains show an abundance of glassy inclusions. Perthitic texture - Exsolution lamellae of albite occurring in orthoclase or microcline..

18 Igneous Petrology Cheat Sheet 18 The Classification of Igneous Rocks Once you know the mineralogy and texture of an igneous rock classifying it is a trivial task. Leucocratic and Mesocratic Rocks Mesocratic rocks are those along the base of these diagrams. Mesocratic Rocks The diagram above on the left shows quartz-poor and quartz-free fine-grained rocks. On the right are ultrabasic rock which basically lack any light coloured minerals. There are a few basic rocks which are missing: Gabbro coarse-grained rock dominated by Ca-rich feldspar and pyroxene, equivalent to a coarse-grained basalt. Dolerite medium-grained rock dominated by Ca-rich feldspar and pyroxene, equivalent to a medium-grained basalt. Highly alkaline igneous rocks also have their own classification system.

19 Igneous Petrology Cheat Sheet 19 Rock Descriptions in Thin-Section For Dummies Describing rocks in thin-section can be bewildering, however, just follow a few simple rules and you ll find that it takes next to no time at all An Example Rock Description Sketch of Sample X Rules: Make life easy on yourself, in this course I don t want you to draw exactly what you see down the microscope since one field of view will not necessarily describe the rock. Look around first, see what the shapes and relative sizes of phenocrysts/crystals are, look at the range of textures in the groundmass. Once you have the properties of the rock in your mind then draw a labelled schematic diagram of the relationships. Pay special attention to overgrowths, inclusions and zoning since these can help you determine the petrogenesis of the rock. You do not need to draw a circle around the diagram! Description of the rock Rules: Don t simply list the textures and minerals in a rock and consider this to be a description. In this course you should write one to two paragraphs describing the petrology. Rocks descriptions virtually always follow the same pattern 1. First statement states the grainsize (fine,medium,coarse) and the overall texture (e.g. porphyritic). 2. If porphyritic estimate the volume percent phenocrysts. 3. If the rock is porphyritic describe the phenocrysts first in order of decreasing abundance, include habit, shape, size range and an estimate of abundance (do not spend too much time estimating abundance an educated guess will do). Describe any inclusions, zoning or overgrowths on each phenocryst. 4. If the rock is porphyritic then describe the groundmass including an estimate of the volume percents of each phase present and their textural relationships. 5. If the rock is not porphyritic describe each mineral in order of decreasing abundance including inclusions, zoning(if any), overgrowths, and textural relationships to other phases. Do not describe properties used in mineral identification except for crystal shape/habit! 6. You may make a brief statement at the end of the description suggesting its petrogenesis. This should be no more than a couple of sentences. 7. If you are unable to identify a mineral make a brief description of its main properties an unidentified phase with low first order birefringence colours, inclined extinction and a single weak cleavage.