Classification of common rocks

by Dr J Floor Anthoni (2000)


-- seafriends home -- all about soil -- Rev 20070718,

Simple properties of rocks for field-testing

Hardness is the ability of one substance to scratch another substance. Geologists use Moh's Hardness scale, which is an arbitrary scale that ranks minerals based on hardness, on a scale from 1 to 10. Minerals with higher numbers are harder. The average pocket knife has a hardness of about 5.5, a copper penny 3.5 and a human fingernail about 2.5. A field geologist has such tools in his pocket. Hardness above 7 is called gemstone hardness (cannot be scratched by quartz). Some minerals are softer in a certain direction only.

1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Potassium feldspar
7 Quartz
8 Topaz
9 Corundum
10 Diamond, the hardest naturally-occurring substance
Density: The density of a mineral is an important natural property, although not easily tested in the field. Heaviest are the gold and platinum metals (density close to 20). Silicates weigh in between 2.5 and 3.5, ores between 4 and 8.

Cleavage: the cleavage of a mineral refers to how it breaks. Depending on the crystal structure, some minerals break in a regular,  predictable manner, whereas others don't. If a mineral breaks in such a way that it leaves smooth, shiny surfaces, then it is said to have cleavage, and those surfaces are called cleavage surfaces. Cleavage can be perfect, good or merely incipient. The more perfect cleavage is, the thinner the sheets are that can be split off. Among the thinnest are flakes of mica. Such minerals form 'books' and their cleavage planes look pearly lustrous.

Fracture: When a mineral is shattered or broken open, fracture surfaces are formed that may not have good cleavage. The appearance of such fracture surfaces is judged conchoidal (rounded), smooth, splintery, hackly, fibrous, even or uneven.

Twinning: Twinning can be defined by the appearance of fine parallel lines, called striations, on the cleavage planes of some minerals. Twinning occurs when a mineral repeatedly changes the direction in which it is growing.

Transparency: According to its transparency to visible light, a mineral is called water-clear, transparent, translucent or opaque. Between these, there are innumerable intermediate stages. Minerals may be translucent at their edges only.

Lustre: Lustre refers to the way a mineral reflects light. It is independent of colour and can occur in various qualities. If a mineral reflects light in a similar way as a metal, it is said to have metallic lustre. Other types of lustre are: glassy (vitreous), pearly, silky, resinous, greasy, waxy and earthy. The degree of lustre is described as splendent, shining, glistening, glimmering, matt, dull.

Colour: Variety of colour is the most striking characteristic of minerals, and in many cases it is their natural colour (yellow sulphur, red cinnabar, green malachite, blue azurite, etc). But alien atoms in small quantities can cause changes in the natural colours of crystals. Some minerals occur in an amazing variation of hues (fluorspar is transparent, white, wine-yellow, honey-yellow to green, blue and violet)

Special light effects: Light is reflected and diffracted by regularly intercalated foreign substances, by fine fractures or by twinning. Labradorescence is a magnificent play of colours like in the blue labradorite. Opalescence is the reflection of light as bright rainbow colours when an opal is turned.

Streak: the colour of a powdered mineral on a white underlay, like pyrite crystals having greenish-black powder on their naturally yellow crystals.

Classification of common rock minerals.
A mineral is an inorganic, natural solid which is found in nature. Its atoms are arranged in definite patterns (an ordered internal structure) and it has a specific chemical composition that may vary within certain limits. A rock is an aggregate of one or more minerals. Notes: Density in kg/litre or g/cm3

Clay layer structures
Structure of clay minerals
The diagrams look at right angles to the sheets making up the main structure of the clay minerals. The silica and associated layers are stacked along the c-axis (up/down). The principal clay minerals are kaolinite, montmorillonite, illite (or mica), and chlorite. The clay minerals are structurally related to the common mineral mica. Their sheets, formed by the joining together of silica tetrahedra in a two-dimensional array, constitute the basic structural units. The deviations from the mica structure and the variations among clay minerals are due to the way the silica sheets are stacked with other chemical layers, and the degree of chemical substitution within both the original silica sheet and the added layers.
The kaolinite sheets are held together by weak hydrogen bonds. Montmorillonite as shown is representative of a group of similar minerals, in which substitutions of iron and magnesium occur at various sites. The iron-rich montmorillonite is the dominant mineral of deep-sea clays of the South Pacific. They share the property of holding water molecules between the sheets, causing (up to double!) expansion and contraction along the c-axis during hydration and desiccation respectively. The montmorillonite minerals also show a high capacity to exchange cations. Illite is the term used for the sedimentary fine-grained equivalent of ordinary mica (muscovite). The chlorite crystal represented here can be modified into an iron-rich form. Deep-sea sediments commonly contain this more iron-rich form.

Vertical dimensions: 7-14 Aengstrom = 0.7- 1.4 nm. Clay platelets can be very thin.
(From Karl K Turekian, Oceans, 1968. After Mason, 1967.)

Notes: Density in kg/litre or g/cm3

Igneous rocks Notes: Density in kg/litre or g/cm3
Classification of igneous rocks
Classification of igneous rocksThis diagram shows the makeup of igneous rocks from the various minerals inside a magma chamber. Density increases from bottom right to top left. 
Intrusive rocks are coarse-grained in texture and crystallise slowly from magma deep in the earth's crust. Extrusive rocks are fine-grained in texture and crystallise quickly from lava on or near the earth's surface. The mineralogy determines the type of rock. Granites and rhyolites consist predominantly of quartz and potash feldspar; gabbros and basalts, predominantly of pyroxene and plagioclase feldspar. Other rock types have intermediate mineral compositions. Note that amphibole = horneblende. Note that the density of the minerals increases from top left (2.6) to bottom right (3.4). Top left: high silica content (acidic); bottom right: low silica content (ultrabasic). The temperature range at which magma solidifies is 1100-700ºC.

(Paul R Pinet in Oceanography, an introduction to Planet Oceanus. 1992.)


The processes inside a magma chamber

As tektonic plates move underneath a continent, they sweep both oceanic sediment and continental sediment downward into the hot mantle, where they heat up violently by processes as yet unknown. The very hot magma is able to melt the continental crust and travel upward through it, cooling in the process. A batch of magma forms, known as a magma chamber, and what happens inside such a batch cauldron is both very complicated, yet simple to understand.

When magma is erupted onto the surface, through the vent of a volcano, it can explode into clouds of ash, because of the enormous pressure of compressed gases like carbon dioxide CO2. This is usually what a young volcano does. As gas pressure diminishes with age, lava pours out, first frothy, cooling rapidly to rhyolite and dacite. Later eruptions are more sedate, resulting in outpourings of andesite. Finally the volcano dies, leaving columns of basalt as a hard crater plug behind. But it is not just the gases that make a difference.

As material leaves the magma chamber, there will be less of it inside to combine with the remaining elements. As can be seen from the igneous rock classification diagram above, the first minerals to leave a magma chamber are also the lightest, that have segregated to the top of the chamber: rhyolite consisting mainly of quartz and feldspars. At the other end of the scale, basalts consist mainly of feldspars and pyroxene, which gives it higher density. As the magma chamber cools, while also losing its pressure, it leaves behind inside the earth a chamber full of peridotite, which consists mainly of the mineral olivene. At this stage, there is not enough pressure left to bring this material to the surface.

A magma chamber may not make it all the way to the surface, cooling entirely inside the crust instead. The chemical process is now slightly different in that not the lightest minerals are 'leaving' the batch but those that solidify first. The remaining liquid minerals can then still react to form different rocks, but the result is a range of 'intrusive' igneous rocks with compositions matching the extrusive series closely (see diagram above).

The process of forming a rock from a solid solution melt
Rock formation from solid solutionThis diagram shows how various minerals are formed from a magma batch with a fixed ratio of two minerals; in this example albite and anorthite. Note that the many elements inside a magma chamber and resultant minerals, complicate this simple example much further. The rectangle shows relative composition horizontally and temperature vertically. The starting mix is 70% liquid albite and 30% liquid anorthite. Cooling starts above point A. Typical of solid solutions, are the two phase curves for each mineral. To the left and above each curve, the mineral is liquid; to the right and below, it is solid. 
As the liquid cools (black arrows from the top down), it arrives at point A. Here the anorthite starts to precipitate, almost purely. In doing so, it increases the albite concentration, and albite moves from A to C while staying liquid. If albite were to precipitate out, its concentration in the melt would decrease, which would move against temperature (up the curve), and is thus impossible. At point C, all anorthite (30%) has solidified slowly. The mix now moves from C to D, rapidly solidifying the 70% albite, which by this time has increased its concentration to 95%. Several types of rock are formed, one on top of the other, as shown by the right-hand diagram.
Note that phase (the liquid/solid boundary) changes not only with temperature but also with pressure, which makes the process of rock formation rather complicated and variable.

Sedimentary rocks
Sediment composition triangle
Composition of sedimentary rockThe diagram shows the range of sedimentary rock types represented as mixtures of three components: calcium (plus magnesium) carbonates, clay minerals (represented by the hypothetical hydrated aluminium and iron oxides as the end member), and silica (silicon dioxide). Sediments and sedimentary rocks have the same ranges of composition.
Iron-rich laterites and aluminium-rich beauxites are the products of intense weathering.
Sandstones are primarily composed of indurated sandy sediments, in many cases dominantly quartz.
Cherts are the sedimentary rock equivalent of biologically deposited siliceous deposits. During the transformation into rock, the amorphous silica, originally deposited by diatoms and radiolarians, is transformed into very hard microcrystalline quartz-rich rock.
Argillaceous (from French: argile = clay) rocks are derived from the lithification of clay-rich muds. Sediments or sedimentary rocks rarely, if ever, have compositions represented by the white area of the triangle.

Metamorphic rocks
Metamorphic rocks have been chemically altered by heat, pressure and deformation, while buried deep in the earth's crust. These rocks show changes in mineral composition or texture or both. This area of rock classification is highly specialised and complex. Metamorphic rock may be of sedimentary origin or stem from igneous rocks. Rocks formed under high temperatures (basalt, gabbro) are less sensitive to metamorphosis than those solidified at low temperatures (quartz & felspar minerals). The following are causes of metamorphism:

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