IGNEOUS ROCK IGNEOUS ROCK TEXTURESTEXTURESPrepared by Dr. F. Clark,Prepared by Dr. F. Clark,

Department of Earth and Atmospheric Department of Earth and Atmospheric Sciences, University of AlbertaSciences, University of Alberta

Oct. 05Oct. 05

ROCK TEXTURES DEFINEDROCK TEXTURES DEFINED

The use of the term “texture” as applied to rocks is not The use of the term “texture” as applied to rocks is not at all intuitive. One might expect rock textures to at all intuitive. One might expect rock textures to include possibilities such as smooth, rough, silky, include possibilities such as smooth, rough, silky, hard, soft, waxy, or other such properties, but this is hard, soft, waxy, or other such properties, but this is not the case. Whether applied to igneous or other not the case. Whether applied to igneous or other rocks, the term texture embraces the size, shape, rocks, the term texture embraces the size, shape, and arrangement of mineral grains [the general term and arrangement of mineral grains [the general term we use rather than “crystals”, although mineral we use rather than “crystals”, although mineral grains are crystals, by definition]. It is generally by grains are crystals, by definition]. It is generally by the texture that a geologist will first recognize the texture that a geologist will first recognize whether a rock is igneous, sedimentary, or whether a rock is igneous, sedimentary, or metamorphic, before they get down to the business metamorphic, before they get down to the business of identifying the rock or naming it.of identifying the rock or naming it.

IGNEOUS ROCK TEXTURES - IGNEOUS ROCK TEXTURES - PRINCIPLEPRINCIPLE

The fundamental principle behind igneous rock The fundamental principle behind igneous rock textures is that grain size is controlled by cooling textures is that grain size is controlled by cooling rate. Thus, rapid cooling at the Earth’s surface of rate. Thus, rapid cooling at the Earth’s surface of extrusive molten material, or lava, results in the extrusive molten material, or lava, results in the growth of smaller crystals, or prevents crystal growth of smaller crystals, or prevents crystal growth altogether. Conversely, slow cooling within growth altogether. Conversely, slow cooling within the Earth’s crust of intrusive molten material, the Earth’s crust of intrusive molten material, called magma, results in the growth of fewer but called magma, results in the growth of fewer but larger crystals, because atoms are able to migrate larger crystals, because atoms are able to migrate through the liquid to attach themselves to crystals through the liquid to attach themselves to crystals that have already begun to form. The many that have already begun to form. The many igneous rock textures are simply variations on or igneous rock textures are simply variations on or modifications of this principle.modifications of this principle.

Igneous Extrusive, or Volcanic, Igneous Extrusive, or Volcanic, RocksRocks

These rocks, formed by cooling and solidification These rocks, formed by cooling and solidification (not always crystallization) of lava, are typically (not always crystallization) of lava, are typically fine-grained, to the extent that detailed analysis fine-grained, to the extent that detailed analysis of the mineralogy of these rocks is only possible of the mineralogy of these rocks is only possible with the high magnification of a petrographic with the high magnification of a petrographic microscope, or even higher resolution techniques. microscope, or even higher resolution techniques. As we shall see in the file on igneous rock As we shall see in the file on igneous rock classification, the colour of the rock is an classification, the colour of the rock is an important clue to its bulk or average composition, important clue to its bulk or average composition, which controls its mineralogy, and other clues which controls its mineralogy, and other clues may tell us what minerals are present.may tell us what minerals are present.

AphanitiAphaniticcCrystals areCrystals are uniformly uniformly fine-grained fine-grained and and interlocking, interlocking, with with individual individual crystals crystals invisible to invisible to the unaided the unaided eye.eye.This is the generic, base texture for most igneous extrusive rocks, the This is the generic, base texture for most igneous extrusive rocks, the fine-grained nature being a result of rapid cooling that prevents fine-grained nature being a result of rapid cooling that prevents growth of large crystals. Even with a hand lens, virtually nothing can growth of large crystals. Even with a hand lens, virtually nothing can be identified, and such rocks can be very frustrating to deal with.be identified, and such rocks can be very frustrating to deal with.

PorphyritiPorphyriticcLarge, Large, evident evident crystals crystals called called phenocrysts phenocrysts [red arrows] [red arrows] are are surrounded surrounded by an by an aphanitic aphanitic matrix or matrix or groundmass groundmass [blue [blue arrows].arrows].This texture represents two stage cooling. Slower cooling of magma This texture represents two stage cooling. Slower cooling of magma within the crust leads to growth of the phenocrysts, whose early within the crust leads to growth of the phenocrysts, whose early growth leads to the development of well-formed faces. Once erupted growth leads to the development of well-formed faces. Once erupted as lava, the remaining liquid crystallizes as the aphanitic groundmass.as lava, the remaining liquid crystallizes as the aphanitic groundmass.

VesicularVesicularVesicles are Vesicles are simply simply bubbles bubbles produced by produced by gases gases escaping escaping from lava as from lava as it solidifies.it solidifies.

The vesicles [red arrows] are in a full range of sizes, enclosed in The vesicles [red arrows] are in a full range of sizes, enclosed in an aphanitic groundmass [blue arrows]. The few slightly larger an aphanitic groundmass [blue arrows]. The few slightly larger crystals [green arrows] are of the mineral olivine, a common crystals [green arrows] are of the mineral olivine, a common constituent of Hawaiian basalt volcanic rocks such as this one.constituent of Hawaiian basalt volcanic rocks such as this one.

Vesicles Vesicles and and XenolithXenolithssThis sample This sample from the from the previous slide previous slide shows a more shows a more complex face. complex face. The red, blue, The red, blue, and green and green arrows are the arrows are the same as in the same as in the previous slide. previous slide. Volatiles dissolved in magma under high pressure within the Earth Volatiles dissolved in magma under high pressure within the Earth escape when lava is erupted, like COescape when lava is erupted, like CO22 from soda pop. A xenolith from soda pop. A xenolith [literally, “foreign rock”, purple arrow] is a fragment of a previously [literally, “foreign rock”, purple arrow] is a fragment of a previously crystallized rock incorporated in the magma as it nears the surface.crystallized rock incorporated in the magma as it nears the surface.

Scoria – a Highly Vesicular BasaltScoria – a Highly Vesicular Basalt

If basalt, which is a mafic [high Fe+Mg content, low silica content] volcanic rock, is highly vesicular, then we have scoria, which for lack of a better way to describe it, resembles the guts or interior of an Aero chocolate bar. Note how the iron content in this chemically unstable mafic rock weathers to produce a rust-coloured surface [brown arrows], versus the greenish grey fresh surface [cut surface; green arrows].

PumicePumiceThis is an This is an extremely extremely vesicular felsic vesicular felsic [low Fe+Mg, [low Fe+Mg, high silica high silica content] content] volcanic rock. volcanic rock. This material This material will actually will actually float on water float on water because it has because it has so many so many bubbles or bubbles or vesicles.vesicles.Gases do not escape easily from high viscosity [stiff] felsic lavas, Gases do not escape easily from high viscosity [stiff] felsic lavas, and so a light-coloured volcanic froth is produced. Pumice is used in and so a light-coloured volcanic froth is produced. Pumice is used in the cosmetic industry as an exfoliant, effective at removing calluses the cosmetic industry as an exfoliant, effective at removing calluses due to the abrasive nature of the glass-hard frothy surface.due to the abrasive nature of the glass-hard frothy surface.

Amygdaloidal – With Filled VesiclesAmygdaloidal – With Filled Vesicles

It may be that a vesicular rock, such as this basalt [red arrows point to vesicles set in the aphanitic groundmass highlighted by blue arrows], has fluids circulating through the vesicles, from which minerals may precipitate or crystallize. These deposits, with rounded outlines reflecting their origin as bubble-filling, are called amygdules [purple arrows]. Phenocrysts would be angular in outline.

GlassyGlassyVery rapid Very rapid cooling may cooling may prevent crystal prevent crystal growth growth altogether, and altogether, and we get natural we get natural volcanic glass, volcanic glass, called obsidian. called obsidian. Note the Note the conchoidal conchoidal fracture [green fracture [green arrows] we arrows] we would expect would expect for glass.for glass.Note the contrast between the black fresh surface [yellow arrows] and Note the contrast between the black fresh surface [yellow arrows] and the rusty brown weathered surface [red arrows], which reveals the the rusty brown weathered surface [red arrows], which reveals the minor iron content that stains the glass black. This is a black igneous minor iron content that stains the glass black. This is a black igneous rock whose colour index [% mafic minerals] is nevertheless zero.rock whose colour index [% mafic minerals] is nevertheless zero.

GlassyGlassyThis texture is This texture is more likely in more likely in felsic [high felsic [high silica content, silica content, not necessarily not necessarily as quartz] as quartz] lavas, which are lavas, which are viscous or stiff. viscous or stiff. This sharp This sharp natural glass natural glass can be can be fashioned into fashioned into effective effective weapon tips.weapon tips.Obsidian is an exception to the definition of rocks, which among other Obsidian is an exception to the definition of rocks, which among other things are aggregates of one or more minerals, which by definition are things are aggregates of one or more minerals, which by definition are crystalline solids. Glass is an amorphous solid, which means it lacks crystalline solids. Glass is an amorphous solid, which means it lacks the ordered arrangement of atoms that characterizes crystals.the ordered arrangement of atoms that characterizes crystals.

Igneous Intrusive, or Plutonic, Igneous Intrusive, or Plutonic, RocksRocks

The coarse crystal size associated with slow cooling The coarse crystal size associated with slow cooling means that the hand specimen properties of means that the hand specimen properties of minerals can be easily applied and exploited to minerals can be easily applied and exploited to identify the minerals present in the rock. This identify the minerals present in the rock. This may extend as far as being able to recognize may extend as far as being able to recognize cleavage intersection angles in the few crystals cleavage intersection angles in the few crystals that may be favourably oriented in the sample that may be favourably oriented in the sample under consideration.under consideration.

Phaneritic – With Evident CrystalsPhaneritic – With Evident Crystals

Igneous intrusive rocks have evident crystals [the Greek word phaneros means visible or evident] that one can easily distinguish with the unaided eye, even if one doesn’t have the skill to identify what minerals they are. In these samples, one sees grey glassy quartz, black biotite and amphibole, and cream-coloured potassium feldspar.

Phaneritic – the Igneous Intrusive Phaneritic – the Igneous Intrusive StapleStaple

Although these samples have smaller crystals than the previous two, the individual crystals or mineral grains are still readily distinguished without magnification. The coarse crystal size makes many igneous intrusive rocks quite attractive, and they are also durable and reasonably stable chemically. This makes them good choices for grave markers and facing stone for buildings.

Phaneritic – Seeing Cleavage on GrainsPhaneritic – Seeing Cleavage on Grains

The coarse grain size of phaneritic intrusive rocks allows us to see cleavage faces developed on many grains. Essentially, when the rock is stressed and breaks when samples are taken, many of the randomly oriented crystals have their cleavage directions more or less parallel to the face of the sample, and so they break along cleavage. This gives excellent reflections [blue arrows] from potassium feldspar in this case.

Phaneritic Phaneritic and and PorphyritiPorphyriticcPorphyritic Porphyritic textures are textures are classically classically developed in developed in extrusive extrusive rocks, but it rocks, but it is not is not restricted to restricted to them.them.This mafic intrusive rock is called a gabbro, a mixture of pyroxene and This mafic intrusive rock is called a gabbro, a mixture of pyroxene and plagioclase feldspar. The bulk of the rock is phaneritic, with grains plagioclase feldspar. The bulk of the rock is phaneritic, with grains approximately 1-2 mm across. However, there is a megacryst, a large approximately 1-2 mm across. However, there is a megacryst, a large pyroxene crystal, whose extent is indicated by the yellow arrows.pyroxene crystal, whose extent is indicated by the yellow arrows.

Phaneritic and PorphyriticPhaneritic and Porphyritic

The same specimen as in the previous slide, we take advantage of the development of cleavage to highlight the megacryst [width indicated by yellow arrow], which is somewhat obscure in the previous image. The cleavage direction meets the surface of the specimen at something like 30 degrees, but the crystal still breaks along cleavage in a series of several steps, which reflect at the same time.

PegmatitPegmatiteeThis term This term applies to applies to extremely extremely coarse-grained coarse-grained igneous igneous intrusive rocks, intrusive rocks, usually of a usually of a felsic felsic composition. composition. Some restrict Some restrict the term to the term to rocks with a rocks with a grain size grain size exceeding 2 exceeding 2 cm, but others cm, but others allow more allow more latitude.latitude.

In the late stages of cooling, volatiles tend to be concentrated in In the late stages of cooling, volatiles tend to be concentrated in the magma. This lowers magma viscosity, accounting for the the magma. This lowers magma viscosity, accounting for the abnormally large crystals. In this specimen, there are crystals of abnormally large crystals. In this specimen, there are crystals of an unusual turquoise variety of potassium feldspar called an unusual turquoise variety of potassium feldspar called amazonite [red arrows]. amazonite [red arrows].

PegmatitPegmatiteeFiner crystals Finer crystals on the upper on the upper surface of surface of the specimen the specimen may may represent represent more rapid more rapid cooling at the cooling at the exterior of exterior of the intrusion; the intrusion; field data field data could confirm could confirm this.this.The large crystals [see cleavage face, yellow arrow] may be gem The large crystals [see cleavage face, yellow arrow] may be gem quality crystals, often of minerals that are significant sources of quality crystals, often of minerals that are significant sources of unusual elements such as lithium, fluorine, and boron. Pegmatites unusual elements such as lithium, fluorine, and boron. Pegmatites are thus attractive as well as economically significant.are thus attractive as well as economically significant.