TOPIC 2B Textures stud

Textures of Igneous Rocks: 

TOPIC 2 Textures of Igneous Rocks 1

Overview: 

Overview MICROTEXTURES OF PLUTONIC ROCKS MICROTEXTURES OF VOLCANIC ROCKS HOW TO DESCRIBE AN IGNEOUS ROCK HAND SPECIMEN THIN SECTION TERMS USED TO DESCRIBE IGNEOUS ROCKS DEGREE OF CRYSTALLINITY GRAIN SIZE GRAIN SHAPE MUTUAL RELATIONS OF MINERALS Equigranular textures Inequigranular textures Directive textures Intergrowth textures Exsolution textures Radiate structures Crystal zoning 2

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks 3

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Low temperature mineralization: This vein of calcite was formed as fluids entered an open-space fracture during alteration of this rock. Notice how the orange and red grains of kaersutite (amphibole) match up across the fracture. Sericite alteration: The feldspars in this alaskite from the Boulder Batholith have been largely replaced by fine-grained white mica ( sericite ). 4

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Anorthosite This rock is composed almost entirely of albitic plagioclase . Bronzitite : A bronzitite is an orthopyroxenite dominated by the mineral hypersthene . Almost all of the grains in this photomicrograph are hypersthene . This rock also contains some interstitial plagioclase better seen in plane light. 5

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Clinopyroxinite : This rock is dominated by the clinopyroxene augite . Diabase : A diabase is a basaltic rock with grain size more or less transitional between gabbro (coarse) and basalt (fine). Notice the elongate lath-shaped plagioclase and the colorful clinopyroxene and possible olivine in this rock. 6

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Dunite : Almost all of the grains in this rock are olivine . Note the high order interference colors of olivine and the minor secondary calcite which occurs as veinlets through the sample. Gabbro : Plagioclase and hypersthene ( orthopyroxene ) dominate this rock. This slide may have been cut a little too thick, accounting for the higher-than-expected interference colors of the hypersthene . 7

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Biotite granite: Minerals in this rock include quartz , plagioclase , biotite , and K-feldspar . The same but finer grained. 8

Microtextures of plutonic rocks: 

Granophyre or micrographic texture Micrographic quartz and K-feldspar ( cuneiform shaped intergrowths ) dominate this rock. Myrmekite Myrmekitic texture defined by wormy (rounded) intergrowths of quartz and K-feldspar in plagioclase which is adjacent to K-feldspar. Probably forms as a result of subsolidus exchange. Compare this texture to micrographic texture. Microtextures of plutonic rocks 9

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Norite A norite is a gabbro in which the pyroxene is principally orthopyroxene . This norite from the uppermost portions of the famous Stillwater Complex in Montana also contains some clinopyroxene . Generally, clinopyroxene has higher interference colors than orthopyroxene , as shown in this photomicrograph. Poikilitic (or ophitic ) texture In this photomicrograph, euhedral to subhedral biotite and plagioclase crystals are surrounded by optically-continuous, gray-colored K-feldspar. 10

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Perthitic texture The light gray streaks in this photomicrograph are plagioclase exsolution lamellae in gray K-feldspar . Perthite forms as an originally homogeneous feldspar exsolves two feldspars as temperature falls below the feldspar solvus during subsolidus cooling. In this close-up shot, the plagioclase exsolution lamellae are large enough to exhibit the characteristic polysyntheitic twinning pattern (parallel stripes). In contrast, the K-feldspar host displays splendid "tartan" or "cross-hatched" twinning. 11

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Sub-grains in quartz The crystal structure of this quartz grain has been deformed (probably by low-grade metamorphism) to produce sub-grains. 12

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Tonalite Example 1: This rock contains hornblende, plagioclase, clinopyroxene , biotite , and quartz. Notice how the hornblende rims the small core of relict clinopyroxene in the center of this photo, indicating a reaction relationship. Example 2: This rock contains hornblende, plagioclase, clinopyroxene , biotite , and quartz. 13

Microtextures of plutonic rocks: 

Microtextures of plutonic rocks Sieve texture Sieve texture in corroded, partially resorbed plagioclase cores. It is thought that this texture may be formed in at least two ways: If a plagioclase crystal is placed into a magma in which it is not in equilibrium (by magma mixing), it will become corroded, and melt will penetrate into the crystal structure. The crystal may also become rounded by partial resorption. New plagioclase of a different composition will precipitate from the magma and perhaps form a rim around the corroded core. Alternatively, the same effects could possibly be produced by volatile-loss from decompression as a magma rises to shallower regions in the crust. 14

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks 15

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Amygdaloidal texture The oval feature in this photomicrograph is an amygdule: a formerly open vesicle which has been filled with a secondary mineral(s) precipitated from low-T ground waters which have penetrated into the rock. In this case, the amygdule is probably filled with a zeolite mineral. Volcanic breccia This breccia is composed of fragments of a variety of volcanic materials. 16

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Kimberlite This is a severely altered sample from the only diamond-producing kimberlite in the United States. Sorry, no diamonds in this slide, though! The polygonal shapes are relict crystals of olivine which have completed decomposed to serpentine and other alteration products. Komatiite Komatiites are rare ultramafic volcanic rocks. This sample displays the characteristic " spinifex texture" defined by extremely acicular olivine phenocrysts --probably a sign of rapid crystallization from a significantly- undercooled magma. 17

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Nepheline basalt Many of the phenocrysts in this basalt are nepheline . Rhyolite This crystal-rich rhyolite contains phenocrysts of quartz, K-feldspar ( sanidine ), plagioclase, and biotite in a fine-grained groundmass. Note the interesting shape of the (partially resorbed ? skeletal?) quartz grain in the center of the photograph. 18

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Scoria Scoria is another name for a highly-vesicular (almost "frothy") basalt. The black, ovals features in this photomicrograph are vesicles. Note the acicular, white plagioclase laths throughout and the euhedral , white olivine phenocryst at the lower right. Spherulites in rhyolite Spherulites are radiating masses of fibrous crystals in a glassy matrix. These spherulites are probably composed of alkali feldspars and some polymorph of SiO2, and in this cross-polarized shot, appear as round objects with dark crosses. Note the large phenocryst which forms the nucleus of one of the spherulites at center-left. 19

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Spinifex texture This komatiite sample displays the characteristic "spinifex texture" defined by extremely acicular olivine phenocrysts--probably a sign of rapid crystallization from a significantly-undercooled magma. The texture is named after a type of grass which grows in South Africa in the region where komatiites were first discovered. 20

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Poorly-welded tuff In this sample, the glass shards are starting to get deformed. Note the phenocrysts of quartz (clear) and biotite (dark red) in this rock. Contrast with lightly-compacted tuff and welded tuff. Lightly-compacted tuff In this tuff, the irregularly-shaped glass shards are still relatively undeformed . Also note the phenocryst of quartz (clear) and biotite (dark red) in this slide. Contrast with poorly-welded tuff and welded tuff. 21

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Welded tuff In this sample, the glass shards are fused together in a swirly mass, and the large pumice fragment at center right is flattened. In contrast, quartz phenocrysts are relatively undeformed. Contrast with lightly-compacted tuff and poorly-welded tuff. 22

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Vesicles The black, ovals features in this scoriaceous basalt are vesicles. Note the acicular, white plagioclase laths throughout and the euhedral , white olivine phenocryst at the lower right. Want to see what happens to vesicles after some hydrothermal alteration? Amygdaloidal texture The oval feature in this photomicrograph is an amygdule: a formerly open vesicle which has been filled with a secondary mineral(s) precipitated from low-T ground waters which have penetrated into the rock. In this case, the amygdule is probably filled with a zeolite mineral. 23

Microtextures of volcanic rocks: 

Microtextures of volcanic rocks Vitrophyre A vitrophyre is another name for a phenocryst -bearing obsidian. The phenocrysts in the above photomicrograph are mostly plagioclase. The groundmass is obsidian glass. Can you think of some possible explanations to account for the extremely large difference in grain size in this rock? 24

HOW TO DESCRIBE AN IGNEOUS ROCK: 

HOW TO DESCRIBE AN IGNEOUS ROCK HAND SPECIMEN Note colour, structural features (flow structures etc.), any visible textural features –grain size, porphyritic character etc. Also note the proportion of light and dark minerals, and mention any minerals displaying diagnostic characteristics. The hand specimen description should be completed after examination of the thin section and in the knowledge of what minerals are present. THIN SECTION These notes are for guidance only, and may require adaptation for particular specimens. Write down essential minerals with an indication of their relative abundance ( modal% ). For porphyritic rocks indicate what percentage of the rock is made of phenocrysts. 25

HOW TO DESCRIBE AN IGNEOUS ROCK: 

THIN SECTION (cont.) Description of each essential mineral in order of abundance. State the mineral group and then describe features which are particular to the mineral being described. For example for unknown minerals: Colour – if coloured give pleochroism (if any) Form – crystal shapes, grain size, porphyritic crystals etc. Cleavage – number of cleavages and at what angle they intersect. Relief – high, medium or low Birefringence – 1st, 2nd or 3rd order colours. Extinction – straight or oblique to crystal edges or cleavage – (if oblique give angle) Twinning – if present Alteration – in any. Inclusions – if any Description of accessory minerals. Texture, special features (if any) – SKETCH TEXTURAL FEATURES. Can this tell us anything about how the rock was formed? Can you determine the order of crystallization? Name and classify the rock. 26 HOW TO DESCRIBE AN IGNEOUS ROCK

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

Below is a list of the words most commonly used to describe the textural features of igneous rocks. Some of the terms are illustrated with photomicrographs to help you identify the textures in the rocks you look at. Some however, are uncommon and you may not need to use them. If in doubt, express what you see in simple English!! 1. DEGREE OF CRYSTALLINITY Holocrystalline composed entirely of crystals. Vitreous/holohyaline composed of glass. Hypocrystalline composed of both crystals and glass. 27 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

2. GRAIN SIZE Aphanitic constituents too small to be distinguished with naked eye Phaneritic individual constituents visible to naked eye. Microcrystalline crystals may be distinguished with aid of a microscope. Cryptocrystalline mineral aggregate shown to be crystalline using scanning electron microscope or x-ray techniques but individual crystals are not visible under the microscope . Coarse Grained Average grain diameter > 5mm Medium Grained Average grain diameter 1-5mm Fine Grained Average grain diameter < 1 mm Equigranular most crystals the same size. Inequigranular crystals of different sizes. Coarse Grained Medium Grained Fine Grained 28 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

3. SHAPES OF CRYSTALS Euhedral crystal is bounded by crystal faces. Anhedral no crystal faces developed, often due to other crystals interfering with their growth. Subhedral crystal faces partially developed. 4. MUTUAL RELATIONS OF MINERALS (a) Equigranular textures Allotriomorphic /xenomorphic most crystals anhedral . Hypidiomorphic most crystals subhedral . Panidiomorphic most crystals euhedral . 29 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (b) Inequigranular textures Porphyritic : Large crystals (phenocrysts) in a finer grain or glassy matrix. Microphyritic As above on a microscopic scale. Glomerophyric: Phenocrysts occur in separated clusters. Porphyritic Glomerophyric 30 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (b) Inequigranular textures (cont.) Poikilitic: Smaller crystals of essential minerals enclosed in larger crystals of another mineral. Ophitic: Variety of poikilitic where clinopyroxene (augite) is the host enclosing laths of plagioclase – very common in basalts. Poikilitic Subophitic : As above but pyroxene only partially encloses plagioclase laths. Ophitic 31 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (b) Inequigranular textures (cont.) Intersertal: Wedge shaped interspaces between crystals are filled with glass or late-stage mineral such as analcime. Intergranular: Anhedral crystals of a mineral occupy spaces between sub- euhedral crystals of another. In basalts and similar rocks, clinopyroxene fills gaps between plagioclase. Intersertal Intergranular 32 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (c) Directive textures: Since magma is a fluid, it can flow. If elongate crystals, such as feldspar are present in the magma whilst it is flowing, these crystals may well align themselves in the direction of flow. Flow structures are not confined to extrusive lavas, and can be found in plutonic rocks where “flow” in the magma takes the form of convection currents. Trachytic: Groundmass crystals, usually feldspar, have a sub-parallel arrangement. Very common in trachytes unsurprisingly!! Trachytoid: Alignment of minerals other than feldspar in basic rocks. Trachytic Trachytoid 33 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (c) Directive textures: Hyalopilitic: The alignment of microlites in glassy rocks. Hyalopilitic 34 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (d) Intergrowth textures: Intergrowth textures are when a crystal of one mineral appears to be completely embedded within a crystal of another mineral. The crystals concerned are typically anhedral but one or both may be skeletal, dendritic or radiate. Graphic: Irregular intergrowth of two minerals in which the apparently isolated wedges and rods of one mineral in the other have the appearance of cuneiform writing – visible with the naked eye! Graphic Granophyric : Graphic intergrowth of quartz and alkali feldspar. Granophyric 35 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (d) Intergrowth textures: Myrmekitic: Graphic intergrowths of plagioclase and quartz. Myrmekitic Figure 3-21 . Myrmekite formed in plagioclase at the boundary with K-feldspar. 36 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (e) Exsolution/intergrowth textures: Parallel lamellae, or trains of blebs, of one mineral, and all of the same optical orientation, are enclosed in a single “host” crystal of another mineral. Either forms by the co-precipitation of the two minerals , or via the solid-state exsolution (separation) of the two phases . Perthitic: Parallel streaks and blebs of Na-rich feldspar (albite) within a host of potassium-feldspar (orthoclase or microcline). Very common in some granites, and it may be possible to distinguish several generations of exsolution. Perthitic Antiperthitic Antiperthitic : Parallel streaks and blebs of potassium feldspar (orthoclase or microcline) within a host of Na-rich feldspar ( albite ). Common in syenites and in some granites. 37 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) Pyroxene exsolution textures: In slowly cooled basic rocks (e.g. gabbros, norites) the pyroxenes may show lamellae structures. These are either blebs of orthopyroxene in a host of clinopyroxene or visa versa. Pyroxene exsolution textures 38 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

TERMS USED TO DESCRIBE IGNEOUS ROCKS 4. MUTUAL RELATIONS OF MINERALS (cont.) (f) Radiate structures: Radiate textures are those in which elongate crystals diverge from a common nucleus. They are most frequently found in fine grained rocks, but not exclusively. Spherolitic: Spherolites are approximately spheroidal bodies in a rock, composed of an aggregate of fibrous or elongate crystals of one or more minerals radiating from a nucleus, with glass or crystals in between. Spherolitic Variolitic : A fan-like arrangement of divergent, often branching, fibres . A common arrangement in basic rocks is a fan of plagioclase separated by glass or pyroxene. Variolitic 39

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (g) Crystal zoning Because most common igneous minerals belong to solid-solution series, and equilibrium crystallization is exceedingly rare, many minerals in igneous rocks become zoned during growth. Zoning occurs when the mineral changes its composition in response to changes in magma chemistry. The result is a crystal which has a continuously changing composition from core to rim, which manifests itself as a subtle change in the optical properties of that crystal. Three major varieties of zoning occur: Normal zoning Where the crystal is zoned from the high temperature end member of the solid solution towards the low temperature end member. For example, plagioclase would be zoned from Ca-rich to Na-rich compositions. Reverse zoning Where the crystal is zoned from the low temperature end member of the solid solution towards the high temperature end member. For example, plagioclase would be zoned from Na-rich to Ca-rich compositions. 40 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

4. MUTUAL RELATIONS OF MINERALS (cont.) (g) Crystal zoning Oscillatory zoning Where the composition of the crystal switches from the high temperature to the lower temperature end member a number of times during growth. Continuously zoned crystals Oscillatory zoned crystals 41 TERMS USED TO DESCRIBE IGNEOUS ROCKS

PowerPoint Presentation: 

Figure 3-5. a. Compositionally zoned hornblende phenocryst with pronounced color variation visible in plane-polarized light. Field width 1 mm. b. Zoned plagioclase twinned on the carlsbad law. Andesite, Crater Lake, OR. Field width 0.3 mm. © John Winter and Prentice Hall. 42

PowerPoint Presentation: 

Figure 3-6. Examples of plagioclase zoning profiles determined by microprobe point traverses. a. Repeated sharp reversals attributed to magma mixing, followed by normal cooling increments. b. Smaller and irregular oscillations caused by local disequilibrium crystallization. c. Complex oscillations due to combinations of magma mixing and local disequilibrium. From Shelley (1993). Igneous and Metamorphic Rocks Under the Microscope. © Chapman and Hall. London. 43

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

5. OTHER COMMON TERMS Melanocratic, mesocratic, leucocratic (synonymous with dark-, medium and light colored) indicate the colour index of a rock and hence the relative proportions of dark- to light-coloured mineralsm – the boundaries are at 66% and 33% dark minerals respectively. Rocks may be given a prefix in order to further subdivide a group of related rocks e.g. mela -gabbro, leuco -gabbro. Essential minerals are those which are necessary to the naming of the rock , but may only be present in minor quantities e.g. a crinanite must contain a small amount of analcite. Accessory minerals are those which are present in very small amounts (< 1% by volume ), and can normally be ignored when naming the rock. However, it may be useful in the name to note the present of a particular accessory mineral in a rock, particularly if that mineral is not normally associated with that particular rock type, and this can be done by adding the mineral name as a prefix e.g. quartz gabbro . 44 TERMS USED TO DESCRIBE IGNEOUS ROCKS

TERMS USED TO DESCRIBE IGNEOUS ROCKS: 

TWINNING Carlsbad twins Albite twins Tartan twins Deformation twins 45 TERMS USED TO DESCRIBE IGNEOUS ROCKS

PowerPoint Presentation: 

Figure 3-18. a. Carlsbad twin in orthoclase. Wispy perthitic exsolution is also evident. Granite, St. Cloud MN. Field widths ~1 mm. © John Winter and Prentice Hall. Figure 3-18. b. Very straight multiple albite twins in plagioclase, set in felsitic groundmass. Rhyolite , Chaffee, CO. Field widths ~1 mm. © John Winter and Prentice Hall. 46

PowerPoint Presentation: 

Figure 3-18. (c-d) Tartan twins in microcline. Field widths ~1 mm. © John Winter and Prentice Hall. 47

PowerPoint Presentation: 

Figure 3-19. Polysynthetic deformation twins in plagioclase. Note how they concentrate in areas of deformation, such as at the maximum curvature of the bent cleavages, and taper away toward undeformed areas. Gabbro, Wollaston, Ontario. Width 1 mm. © John Winter and Prentice Hall . 48