- Slides: 21
IGNEOUS ROCK TEXTURES
ROCK TEXTURES DEFINED The use of the term “texture” as applied to rocks is not at all intuitive. One might expect rock textures to include possibilities such as smooth, rough, silky, hard, soft, waxy, or other such properties, but this is not the case. Whether applied to igneous or other rocks, the term texture embraces the size, shape, and arrangement of mineral grains [the general term we use rather than “crystals”, although mineral grains are crystals, by definition]. It is generally by the texture that a geologist will first recognize whether a rock is igneous, sedimentary, or metamorphic, before they get down to the business of identifying the rock or naming it.
IGNEOUS ROCK TEXTURES - PRINCIPLE The fundamental principle behind igneous rock textures is that grain size is controlled by cooling rate. Thus, rapid cooling at the Earth’s surface of extrusive molten material, or lava, results in the growth of smaller crystals, or prevents crystal growth altogether. Conversely, slow cooling within the Earth’s crust of intrusive molten material, called magma, results in the growth of fewer but larger crystals, because atoms are able to migrate through the liquid to attach themselves to crystals that have already begun to form. The many igneous rock textures are simply variations on or modifications of this principle.
Igneous Extrusive, or Volcanic, Rocks These rocks, formed by cooling and solidification (not always crystallization) of lava, are typically fine-grained, to the extent that detailed analysis of the mineralogy of these rocks is only possible with the high magnification of a petrographic microscope, or even higher resolution techniques. As we shall see in the file on igneous rock classification, the colour of the rock is an important clue to its bulk or average composition, which controls its mineralogy, and other clues may tell us what minerals are present.
Aphanitic Crystals are uniformly finegrained and interlocking, with individual crystals invisible to the unaided eye. This is the generic, base texture for most igneous extrusive rocks, the fine-grained nature being a result of rapid cooling that prevents growth of large crystals. Even with a hand lens, virtually nothing can be identified, and such rocks can be very frustrating to deal with.
Porphyritic Large, evident crystals called phenocrysts [red arrows] are surrounded by an aphanitic matrix or groundmass [blue arrows]. This texture represents two stage cooling. Slower cooling of magma within the crust leads to growth of the phenocrysts, whose early growth leads to the development of well-formed faces. Once erupted as lava, the remaining liquid crystallizes as the aphanitic groundmass.
Vesicular Vesicles are simply bubbles produced by gases escaping from lava as it solidifies. The vesicles [red arrows] are in a full range of sizes, enclosed in an aphanitic groundmass [blue arrows]. The few slightly larger crystals [green arrows] are of the mineral olivine, a common constituent of Hawaiian basalt volcanic rocks such as this one.
Vesicles and Xenoliths This sample from the previous slide shows a more complex face. The red, blue, and green arrows are the same as in the previous slide. Volatiles dissolved in magma under high pressure within the Earth escape when lava is erupted, like CO 2 from soda pop. A xenolith [literally, “foreign rock”, purple arrow] is a fragment of a previously crystallized rock incorporated in the magma as it nears the surface.
Scoria – 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].
Pumice This is an extremely vesicular felsic [low Fe+Mg, high silica content] volcanic rock. This material will actually float on water because it has so many bubbles or vesicles. Gases do not escape easily from high viscosity [stiff] felsic lavas, and so a light-coloured volcanic froth is produced. Pumice is used in the cosmetic industry as an exfoliant, effective at removing calluses due to the abrasive nature of the glass-hard frothy surface.
Amygdaloidal – 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.
Glassy Very rapid cooling may prevent crystal growth altogether, and we get natural volcanic glass, called obsidian. Note the conchoidal fracture [green arrows] we would expect for glass. Note the contrast between the black fresh surface [yellow arrows] and the rusty brown weathered surface [red arrows], which reveals the minor iron content that stains the glass black. This is a black igneous rock whose colour index [% mafic minerals] is nevertheless zero.
Glassy This texture is more likely in felsic [high silica content, not necessarily as quartz] lavas, which are viscous or stiff. This sharp natural glass can be fashioned into effective weapon tips. Obsidian is an exception to the definition of rocks, which among other things are aggregates of one or more minerals, which by definition are crystalline solids. Glass is an amorphous solid, which means it lacks the ordered arrangement of atoms that characterizes crystals.
Igneous Intrusive, or Plutonic, Rocks The coarse crystal size associated with slow cooling means that the hand specimen properties of minerals can be easily applied and exploited to identify the minerals present in the rock. This may extend as far as being able to recognize cleavage intersection angles in the few crystals that may be favourably oriented in the sample under consideration.
Phaneritic – 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 Staple 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 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 and Porphyritic textures are classically developed in extrusive rocks, but it is not restricted to them. This mafic intrusive rock is called a gabbro, a mixture of pyroxene and plagioclase feldspar. The bulk of the rock is phaneritic, with grains approximately 1 -2 mm across. However, there is a megacryst, a large pyroxene crystal, whose extent is indicated by the yellow arrows.
Phaneritic 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.
Pegmatite This term applies to extremely coarsegrained igneous intrusive rocks, usually of a felsic composition. Some restrict the term to rocks with a grain size exceeding 2 cm, but others allow more latitude. In the late stages of cooling, volatiles tend to be concentrated in the magma. This lowers magma viscosity, accounting for the abnormally large crystals. In this specimen, there are crystals of an unusual turquoise variety of potassium feldspar called amazonite [red arrows].
Pegmatite Finer crystals on the upper surface of the specimen may represent more rapid cooling at the exterior of the intrusion; field data could confirm this. The large crystals [see cleavage face, yellow arrow] may be gem quality crystals, often of minerals that are significant sources of unusual elements such as lithium, fluorine, and boron. Pegmatites are thus attractive as well as economically significant.