SILICA MINERALS

undulose extinction

Quartz, SiO₂

Hexagonal, colourless, cleavage generally absent, 1st order birefringence up to pale yellow, low relief, length slow, uniaxial positive. Extinction often undulose or shows 'strain shadows'. Often, quartz grains occur which show uneven or undulose extinction: the crystal does not all go into extinction in one position. This is an effect of distortion of the crystal lattice during tectonic deformation. Normally, this straining occurs prior to incorporation of the grain in the sediment. Stresses in sedimentary sequences are usually insufficient to produce this degree of deformation. Strained quartz may show biaxial figure with low 2V.

Inclusions common: e.g. vacuoles, 'negative crystals', tourmaline, mica, hair-like rutile etc.

Grains which contain two or more crystals are known as polycrystalline quartz. Very common.

Syntaxial secondary overgrowths picked out by 'dust pellicles' are common. A number of quartz types may be recognised:

plutonic or common quartz

Most abundant type derived from granites and granite-gneisses. May be monocrystalline or polycrystalline. Polycrystalline grains tend to be larger. Undulose extinction common. Inclusions generally uncommon.

volcanic quartz

Generally rare, except in volcaniclastic sediments. Derived from acid volcanic rocks. Typically show b -quartz habit of hexagonal bipyramids. Often euhedral, with rounded corners and resorption embayments. Usually monocrystalline and water-clear; inclusions rare except glass and 'negative crystals'

vein or hydrothermal quartz

Common, especially in pebbles. Derived from hydrothermal veins, pegmatites etc. Monocrystalline or coarsely polycrystalline: Semi-composite (cockscomb) extinction is common with crystals in sub-parallel 'sheaves' Vacuoles (fluid inclusions) characteristic. Inclusions may be so concentrated as to give the quartz a very cloudy appearance (cf. altered feldspar: to distinguish the two, look at the inclusions under high magnification). Zoned inclusions and vermicular chlorite are diagnostic.

metamorphic quartz

Quartz of metamorphic origin is common in sedimentary rocks. Most metamorphic quartz is polycrystalline, and may show a variety of forms, often with evidence of stretching or flattening.Grains are often elongate, with crenulate or granulate boundaries and sub-parallel extinction. Extinction may be straight or undulose. Inclusions of metamorphic minerals (e.g. micas, garnets) may be present. Some relatively common varieties are shown at left: stretched metamorphic quartz (top); schistose quartz (centre); recrystallized, granular quartz (bottom)

authigenic quartz

Formed from solutions during diagenesis at low temperature and pressure. May occur as secondary over-growths, drusy cavity andvein infills and geodes; small euhedral crystals, especially in limestones; and nodular masses, especially in dolomites.

N.B. It is not possible to assign many quartz grains to a particular 'type', especially where they are small or monocrystalline. These must be classified as 'common quartz'. Morphology of quartz grains is not an unequivocal indicator of provenance, only a guide to likely sources.

Chert, SiO₂

This term is generally reserved for microquartz i.e. aggregates of microcrystalline quartz, with individual crystals only a few microns across, showing 'pinpoint' birefringence (see left). Chert contains minute water bubbles which lower RI to 1.53-1.54 and give a brownish appearance in transmitted light and silvery in reflected light.

Chalcedony, SiO₂

Sheaf-like bundles of radiating fibres, usually only several microns long may form spherulitic growth with 'Maltese-cross' or 'zebraic' extinction. Contains micro-inclusions of fluid which lower RI

FELDSPARS

orthoclase, KAlSi₃O₈

Monoclinic, colourless, often turbid or brownish with inclusions, may show simple twinning or be untwinned, 1st order birefringence, low relief. RI<balsam; Cleavage is visible; biaxial negative, large 2V. Easily confused with quartz when untwinned, but cloudy appearance due to alteration products such as sericite is typical, and biaxial figure is diagnostic.

microcline, KAlSi₃O₈

Triclinic; 1st order birefringence, 'tartan' twinning is diagnostic, with tapering twin lamellae. Fairly common in sediments.

plagioclase, NaAlSi₃O₈ - CaAl₂Si₂O₈

Triclinic, 1st order birefringence, multiple twins are diagnostic: may be combined with Carlsbad or pericline twins. Most plagioclase in sediments appear to be Na-rich. Often highly altered with vacuoles and sericitisation: this cloudy appearance can be distinguished from that of inclusion rich quartz by observing under high power. It is possible to distinguish tiny crystals of clay minerals.

other feldspars

Other feldspars may occur in sediments, such as perthites (intergrowths of Na-and K-feldspars). sanidine (K,Na)AlSi₃O₈ and anorthoclase (Na,K)AlSi₃O₈ are generally rare, except where derived from nearby volcanic sources.

MICAS

muscovite, KAl₂(AlSi₃O₁₀)(OH)₂

Colourless mica, forming characteristic flakes with perfect cleavage, 2nd order birefringence and extinction parallel to cleavage. Extinction is characteristically speckled and flakes are often deformed in sediments. Authigenic muscovite has a fresh appearance and cross-cuts other textures. Generally forms <2% in sediments.

biotite, K₂(Mg,Fe)₂(AlSi₃O₁₀)(OH)₂

This mica is generally brown, but may also be green. Shows strong birefringence and pleochroism: pleochroic haloes may sometimes be found. Biotite is less stable than muscovite, especially when deformed, and may alter to chlorite or iron oxides. Less common than muscovite in sediments.

CLAY MINERALS (USE HIGH POWER OBJECTIVE)

Form the 'matrix' of many rocks; may be defined as crystals <30 microns in size. Very difficult to identify optically in many cases due to small size, but usually occur as small flakes, aggregates. or radiating growths There is much intergradation due to base exchange and mixed-layer clays.

kaolinite, Al₄Si₄O₁₀(OH)₈

Colourless to pale yellow, forms radiating aggregates of low relief and 1st order birefringence. Commonly forms as alteration product of silicate minerals and rock fragments.

illite / sericite, KAl₂(AlSi₃0₁₀)(OH)₂

May be considered to be small flakes of muscovite <30 microns: however formula is variable due to ionic substitution. Sericite and illite (hydromuscovite) are generally K-deficient. Colourless or yellow irregular matted flakes, often intercalated with other clays. Often shows high first order birefringence due to small size (<30 micron thickness of standard thin section); true birefringence is 2nd order. Often occurs as alteration product of silicates, and is abundant in mudstones.

montmorillonite, Al₄(Si₄O₁₀)₂(OH)₄.nH₂O

with substitution of Al³⁺ by Fe²⁺, Mg²⁺ and Zn²⁺; may also contain Ca²⁺, Na⁺

One of the smectites. Irregular flakes, which may be pale brown or greenish:

low relief with RI<balsam. Birefringence may reach 2nd order, but is generally less than illite. Usually formed by alteration of basic volcanic rocks, often replacing glass shards, and restricted to Mesozoic or younger sediments.

Chlorite Group

A complex group of minerals, related to kaolinite in basic structure, but with much ionic substitution. Usually form aggregates of flakes with green pleochroism and anomalous blue or brown birefringence. Sometimes form vermicular growths. May form due to marine diagenesis, alteration of basic rocks, or onset of low-grade metamorphism

chamosite (Fe₄Al₂)(Si₂Al₂)O₁₀(OH)₈

A chlorite mineral containing ferrous iron, which may form economic deposits. Green, grey, pale brown or colourless: may show weak pleochroism. Usually occurs as oolitic or spherulitic growths showing 'Maltese cross' extinction. Moderate relief and 1st order birefringence.

glauconite

A potassium iron aluminosilicate of variable composition which is structurally analogous to muscovite. Forms fine aggregates of minute crystals which occur as pellets or as infills of fossil cavities. Green and yellow, may show pleochroism; has high birefringence, which is usually masked. Does not form oolitic structures which distinguishes it from chamosite. Diagnostic of marine sediments: often formed of altered faecal pellets.

HEAVY MINERALS

Accessory minerals which form <1% of a sand or sandstone. They survive because they are mechanically durable, and resistant to chemical attack. An important indicator of provenance.

zircon, Zr SiO₄

Colourless or pale colours, forms short tetragonal prisms, with pyramid faces often rounded. Characterised by lack of cleavage, very high relief and birefringence up to 4th order. A very resistant mineral which may undergo several episodes of recycling. May form authigenic overgrowths.

tourmaline

A complex group of hexagonal boro-silicates, of grey, slaty-blue, buff, or olive green colour; characterised by elongate triangular prisms with marked pleochroism, high relief, strong birefringence and parallel extinction. Tourmaline may be polycyclic with well-rounded grains. May form authigenic overgrowths

rutile, TiO₂

Yellowish to reddish brown tetragonal prisms with parallel cleavage. May form needle-like inclusions in quartz. Characterised by very high relief and extreme birefringence which does not show well due to internal reflections. Adamantine lustre in reflected light. Less stable than zircon and tourmaline. May form authigenically.

OPAQUE MINERALS (ore minerals)

These are opaque in transmitted light, and must be studied under reflected light. N.B. Some organic matter is also opaque and may sometimes be distinguished by organic structure.

magnetite, Fe₃O₄

Metallic grey-black in reflected light, may show alteration to red hematite.

ilmenite, FeTiO₃

Metallic grey-black in reflected light: difficult to differentiate from magnetite, but commonly shows alteration to leucoxene (see below)

leucoxene (hydrous Ti Oxides)

Fine aggregates of Ti minerals, formed by alteration of ilmenite, with which it is usually associated. Opaque in transmitted light, white in reflected light.

pyrite, FeS₂

Nearly always authigenic, so often forms euhedral cubes, but may form detrital grains in Precambrian rocks. Brassy-yellow in reflected light.

hematite, Fe₂O₃

Generally an alteration product: may be opaque or translucent red. Shows black to deep red colour in reflected light.

limonite / goethite, FeO(OH)

Hydrated iron oxides, limonite is amorphous, and goethite forms micro-crystals. Limonite is generally translucent and red-brown in reflected and transmitted light. Goethite forms red pseudo-hexagonal crystals a few microns across. These are generally alteration products of mafic minerals and impart red colour to rocks, often coating grains.

CARBONATE MINERALS

Most igneous and metamorphic rocks, and many sedimentary rocks, are composed largely of silicate minerals. However, one important group of rocks consists largely of carbonate minerals. These are the limestones and their metamorphic equivalents, marbles. Additionally, carbonate minerals are often an important component of silicate-dominated sandstones. Rarely, igneous rocks composed chiefly of carbonate minerals occur: these are the carbonatites.

Several carbonate minerals occur commonly in both fragmental sedimentary rocks (mainly as cement) and in limestones. The commonest are:

calcite, CaCO₃; aragonite, CaCO₃; dolomite, CaMg(CO₃)₂;
siderite, FeCO₃; and ankerite, Ca(Mg,Fe,Mn)(CO₃)₂.

Magnesite, MgCO₃, and rhodochrosite, MnCO₃, occur more rarely. Other carbonate minerals include witherite, BaCO₃, and smithsonite, ZnCO₃.

The Calcite Group

Includes calcite, dolomite, siderite, ankerite and magnesite. All crystallize in the trigonal system. Similar optical properties, and often difficult to distinguish, unless the thin section has been stained. The main optical properties of the calcite group minerals are:

Trigonal, colourless or cloudy, perfect rhombohedral cleavages (3 cleavages at 75° ). In PPL, all show 'twinkling' - marked change in relief on rotation of stage. Extinction symmetrical to crystal outlines and cleavage traces. All show extreme birefringence - often 4th, 5th and 6th order pale pinks and greens or off whites, with a 'washed out' appearance. It is often possible to count up through the orders on the wedge-shaped edge of a crystal. Multiple twinning is common: some twin lamellae are thin, and may show lower order interference colours. All have uniaxial negative interference figures, with many rings.

Calcite, CaCO₃

is the commonest carbonate mineral in ancient limestones and in marbles. May contain limited amounts of Mg²⁺, Fe²⁺ and Mn²⁺. In limestones, it occurs as grains, cements and as a replacement of other carbonate minerals. Common as a vein mineral. RIs of calcite are somewhat lower than for other carbonate minerals, giving low relief (negative in some sections). Reacts vigorously with cold, dilute hydrochloric acid.

Dolomite, CaMg(CO₃)₂

is a common component of limestones. Commonly contains Fe²⁺. It does not normally occur as primary sedimentary grains. Most commonly, occurs as a replacement of earlier calcium carbonate (calcite or aragonite). Dolomite may also occur as a cement. Optically very similar to calcite, and not readily distinguishable from it unless the thin section has been stained. An exception is the occurrence of well-formed rhomb-shaped crystals, with cloudy centres and clear, limpid edges. These are typical of dolomite: calcite rarely forms rhombs. Some dolomite crystals have curved faces. Dolomite reacts slowly, if at all, with cold, dilute hydrochloric acid.

Siderite, FeCO₃

occurs commonly in sedimentary rocks, particularly within diagenetic concretions formed in a reducing environments low in sulphate, SO₄^2- ions. Siderite shares the optical properties of the other carbonates, except that its refractive index gives it positive relief in all sections. Alteration often leads to the presence of opaque iron oxides, along cleavage traces, and in and around siderite crystals.

Magnesite, MgCO₃

resembles dolomite and calcite in its optical properties. It is often formed by the alteration of Mg-rich igneous and metamorphic rocks (e.g. serpentinites), but is otherwise rather unusual.

Aragonite

Aragonite differs from the other common carbonate minerals in that it crystallizes in the orthorhombic system. Colourless in thin section, with a single, imperfect cleavage, parallel extinction, and columnar or fibrous structure. Slightly higher relief than calcite, dues its higher RIs, and shows a similar, high order birefringence to calcite. Aragonite has a biaxial negative interference figure with low 2V. In practice, aragonite usually occurs as aggregates of very fine-grained crystals, making determination of its optical properties very difficult. Reacts as vigorously as calcite with dilute hydrochloric acid, but has a higher specific gravity.

A very common component of skeletons of invertebrate organisms, such as mollusc shells and scleractinian coral skeletons. Thus it is a common component of modern carbonate sediments and 'young' limestones. Metastable under normal surface temperatures and pressures, and slowly inverts to calcite with time. For this reason, aragonite is uncommon in ancient limestones. It also occurs as a vein mineral and, rarely, in amygdales of volcanic rocks.

ROCK FRAGMENTS

Not all grains in terrigenous clastic rocks consist of single minerals. Fragments of rocks also occur commonly (also known as lithic fragments). These may often be identified from both their mineralogy and their texture, and some of them will already be familiaru.

Polymineralic grains found in sedimentary rocks: provide information on nature of source area (provenance). May be gravel-sized in some instances, but sand-sized rock fragments are common.

igneous rock fragments

Igneous rock fragments in sediments tend to be volcanic rather than plutonic, as large plutonic grains generally disaggregate into separate minerals. Hence, generally referred to as volcanic rock fragments (VRFs). Some show felted masses of tiny lath-like feldspar crystals, sometimes showing flow textures; if basic they are generally highly altered. Others show a devitrified glass matrix. Phenocrysts, especially of feldspars, distinguish acid VRFs from chert, with which they may be confused.

metamorphic rock fragments (MRFs)

May include slate, phyllite, schist, gneiss, metaquartzite etc.; metamorphic rock fragments generally include quartz, micas. MRFs are often platy or foliated, but hornfelses and metaquartzites are not. It is very difficult to distinguish slate from shale fragments.

sedimentary rock fragments (SRFs)

May include fragments of mudstone, siltstone, sandstone and limestone; chert is a very common indicator of a sedimentary source area. Locally-derived mud and silt clasts are referred to as intraformational.