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
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
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.
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
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.
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 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.
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.
the colour of a powdered mineral on a white underlay, like pyrite crystals
having greenish-black powder on their naturally yellow crystals.
of common rock minerals. A
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
is an aggregate of one or more minerals.
elements. Minerals consisting of a single element.
Platinum. Pt. density
21.46, hardness 4-4.5. Found as granules in sand from some igneous rocks.
Most noble of metals.
Gold. Au. Density 19.3,
hardness 2.5-3. Often found in association with pyrite, chalco pyrite and
arsenopyrite in quartz veins. When found in streams, the gold is in small
flat particles of varying sizes. It is very ductile (heat and electricity)
and malleable. Used for ornaments because of its malleability.
Silver Ag. Density 10-11,
Copper. Cu. Density 8.95,
hardness 2.5-3. First used for making tools, later mixed with tin to make
Sulfur. S. Density 2.07,
Graphite. C. Density
2.09-2.21, hardness 1-2. Found in compact masses, large deposits. Its high
melting point and electrical conductivity makes it suitable for high temperature
electrodes. It is used as a dry lubricant in high temperature applications.
Diamond. C. Density 3.50,
hardness 10. Lustrous, transparent, colourless, yellow or green. Brittle
(-O). Oxides are common in geochemical environments poor in silica.
Silicates form easily from a magma, so if silica is used up in a magma
chamber, then the oxides remain to be formed. Their structure is complex:
octahedral and dodecahedral crystals. Very stable against weathering, but
dissolving slowly in hydrochloric acid (HCl).
Fe3O4. Density 5.17, hardness 6. Magnetic iron oxide. Large deposits are
segregated from igneous magmas at high temperatures and are mined near
the surface as iron ores. Can contain chromium or manganese.
Maghemite. Fe2O3. Iron
Hematite. Fe2O3. Iron
sesquioxide, ferric iron oxide. Non-magnetic.
Ilmenite group ATiO3.:
Ilmenite FeTiO3, titanic
iron ore, a black sand containing 36.8% iron and 31.6% titanium, is the
principal ore for titanium, but is hard to melt. Ilmenite is part of basic
igneous rocks such as gabbro and norite.
Density 5.26, hardness 5-6. Rust. Non-magnetic. Oxidized iron minerals.
Is an important iron ore, found in many shapes and colours, from ochre
to blood red.
Goethite. HFeO2. Non-magnetic
Alumina. Al2O3. Aluminium
Beauxite. A mixture of
various oxides such as boehmite, diaspore, hydrargillite, alumogel, etc.,
mixed with iron hydroxides which impart the red colour.
Corundum. Al2O3. Aluminium
oxide. Density 4-4.1, hardness 9. An important mineral, used as precious
stone (red ruby and blue sapphire), and its greyish variety as emery for
abrasive and refractory products.
Spinel group AB2O4.
oxide. MgAl2O4. Density 3.55, hardness 7.5-8.
Gibbsite. Aluminium hydroxide.
Al(OH)3. Found in aluminium ore.
Brucite. Magnesium hydroxide.
Mg(OH)2. One of magnesium's ores.
(-CO3) metal-ion solid solutions. Usually medium or low hardness. Soluble
in hydrochloric acid (HCl)
Calcite. Density 2.71,
hardness 3. Most common, most abundant of all minerals. It has many forms
of crystallisation (up to 700 forms!). Fine-grained, dissolves easily in
rainwater, especially when loaded with carbon dioxide in the form of carbonic
acid (H2CO3). Stalactites, stalacmites, etc.
CaCO3. Limestone in a metamorphic crystalline (or granular) state, and
capable of taking a polish. Used in sculpture and architecture.
Aragonite CaCO3. Orthorhombic
Dolomite. Calcium Magnesium
carbonate. (Ca,Mg)CO3. CaMg(CO3)2. Is the chief source of magnesium. Used
for agricultural fertiliser, steel manufacturing, and as a filler in paint,
putty and rubber. Marble composed of dolomite has beautiful colours.
Gypsum. CaSO4.2H2O. Density
2.31, hardness 2. A hydrated form of calcium sulphate, occurring naturally
and used in the building industry and to make plaster of Paris. Easily
identified because it can be scratched by fingernail and is light. Also:
selenite, sericolite, alabaster.
Density 3.1, hardness 5. A naturally occurring crystalline mineral of calcium
phosphate and fluoride, used in the manufacture of fertilisers. A fairly
common mineral found in pegmatites, metamorphic and igneous rock, and in
Also: lithiophilite Li,MnFePO4,
xenotime YPO4, monazite CePO4, linbethenite Cu2(OH)PO4,olivenite CU2(OH)AsO4,
adamite Zn2(OH)AsO4, cornetite Cu3(OH)3PO4, descloizite, brazilianite,
crandallite, and many others.
Halite (Salt). NaCl.
Sylvite. KCl. Potassium
salt, occurs in compact masses.
milk white or reddish granular masses.
Fluorite. Calcium fluoride.
CaF2. Density 3.18, hardness 4, isometric crystals. Very wide range of
Also: cryolite, atacamite, cotunnite,
(SiO2) (-SiO4) (-Si3O8) (-SiO3). Arranged by heavy element content
(the Bowen series)
Olivine. Density 3.27-3.37,
hardness 6.5-7. Ultrabasic. (Fe,Mg,Mn) silicates in solid solution. olivine
= forsterite + fayalite. Density 3.3-4.4. Tetrahedral structure. Green.
Resistant to weathering but susceptible to metamorphism. Much of the Earth
is made out of this mineral, and it is a major component of the mantles
of other terrestrial planets. It is usually a greenish crystal, often found
as inclusions in basaltic lavas. Large crystals are called chrysolite and
are used for jewellery.
Fayalite. Fe2SiO4. dark-green
(Ca,Fe,Mg,Na,Al,Ti) silicates in solid solution. White. Density 2.8-3.7.
Common in meteorites. 10% of crust. Single chain tetrahedral structure.
Moderately resistant to weathering.
= wollastonite + enstatite in solid solution
Augite. A complex calcium
magnesium aluminous silicate occurring in many igneous rocks. (Fassaite,
Hypersthene. A rock-forming
greenish mineral of magnesium iron silicate, harder than hornblende.
Feldspar. (Aluminum silicates).
Ultrabasic to intermediate. (K,Na,Ca,Al) silicates in solid solution. Density
2.6-2.8, 60% of crust. Framework-tetrahedral structure. Hard. Glassy or
pearly looking, light colour. Moderately resistant to weathering and metamorphism.
Plagioclase calcium feldspar. Ultrabasic. White to medium gray with striations.
scolecite, thomsonite, mesolite, dachiardite, forestite, laumontite, mordenite,
arduinite, ferrierite, heulandite, stilbite, and many more.
Mica. Basic to acidic.
(K,Mg,Fe,Al) silicates. Black. Density 2.76-3.2. Common in igneous rocks.
4% of crust. Is a sheet silicate with properties of talcum/ formica. Any
of a group of silicate minerals with a layered structure.
Biotite. Basic. K(Mg,Fe)3(AlSi3O10)(OH)2.
a black, dark brown or green micaceous mineral occurring as a constituent
of metamorphic and igneous rocks.
Muscovite. Acidic. KAl3Si3O10(OH)2.
A silver-grey form of mica ('white mica') with a sheetlike crystalline
structure, giving a pearly lustre, used in the manufacture of electrical
equipment, etc. Also: fuchsite, alurgite
Chlorite group. Are similar
to the micas, lamellar, dark green or blue green.
A soft rock mainly of hydrated magnesium silicate, usually dark green (a
chlorite) and sometimed mottled or spotted like a serpent's skin, taking
a high polish and used as a decorative material.
serpentine with exceptionally long fibres. Colour white, greenish, silky
lustre. Used for buildings, insulation and high temperature applications.
Zircon. Zr(SiO4). Density
4.6-4.7. Hardness 7.5. Trigonal crystal., prismatic pyramid. Glassy or
milky in colour, but may be coloured (rock quartz, smoky quartz, morion
quartz, citrine, amethyst). Most abundant and widest disseminated mineral.
Essential constituent of volcanic, metamorphic and sedimentary rock.
Quartz. Acidic. SiO2.
Ionic co-valent bond. Density 2.6. Hard mineral, virtually insoluble in
water. Translucent. One of the most important components of granite. Opal,
jasper, agate, chalcedony, flint. Tetrahedral structure. Also: Chalcedony,
agate, onyx, jasper, tridymite, opal, hyalite, flint,
Density 3.3-3.5. Tetrahedral structure. A vitreous silicate mineral, especially
a transparent deep-red kind used as a gem.
Common, wine-red. Density 4.25
Clay minerals (hydrous
aluminium silicates). Are the end products of weathering. All clay minerals
are sheet silicates, each clay type owing its distinctive character to
the cations such as sodium (Na+), potassium (K+), magnesium (Mg++) or calcium
(Ca++), which occupy positions in and between the sheets.
Fibrous, open clays, palygorskite.
Open and well hydrated clays, not common in soils but are important in
lake deposits which have salt-lagoon characteristics.
Sepiolite and attapulgite.
Two-layer clays. (1:1
clay; Si-Al lattice) fixed distance hydrogen-bonded (-O-OH-) between the
silica and alumina sheets. Formed through rapid leaching of silica in volcanic
soils (allitic weathering). Consist of a tetrahedral silica and an octahedral
alumina layer. These are rich in alumina, have poor cation-exchange capacity,
stable structure and swell very little when moistened. Tropical climates,
high rainfall, acidic conditions.
Absorbs little water but is just right for pottery and ceramics. A fine
soft white clay produced by the decomposition of other clays of feldspar,
used especially for making porcelain (pipe clay), and in medicines. Also
called china clay. The word kao lin means high hill in Chinese.
Also: nacrite, dickite.
Hydrated 1:1 clays: Halloysite,
Al2Si2O5(OH)4.2H2O. A high alumina clay, which has slender tube crystals,
and is white coloured. Used for making high quality 'bone' china porcelain
and industrial ceramics.
Three-layer clays, smectite.
(2:1 clays; Si-Al-Si lattice) variable distance (-O-O-) bonded. Formed
by slow leaching of silica, consisting of one octahedral alumina layer
sandwiched between two tetrahedral silica layers. Have high cation-exchange
capacity (CEC), but lower than amorphous clays. Weak structure, low water
permeability and swells when moistened. Temperate climates, low rainfall,
neutral to alkaline soils, especially under grasslands.
Expanding lattice clays
or Al4(Si4O10)2(OH)4.xH2O. Holds and absorbs large amounts of water. 20%
in clay from volcanic weathering.
Bentonite: a soft plastic
light-coloured clay formed by chemical alteration of volcanic ash. It is
composed essentially of montmorillonite and related smectite minerals.
Used to bond moulding sands, oil well drilling and to remove colour from
Absorbs large amounts of water. A green flaky mineral, decomposition product
of dark micas and is found in many altered rocks such as schists. (Note
that this mineral bears no relationship to the element chloride Cl, but
its name relates to its green colour)
Illite (muscovite). K2Al4(Si6Al2)O20(OH)4.
Amorphous clays. Formed
from easily weatherable materials such as volcanic ashes, lava or
basalt. Have high cation-exchange capability. Low density, permeable to
water and air. Decay of organic matter is slow and humus content high.
Nontronite. Absorbs and
holds large amounts of water. 50% in clay from volcanic weathering.
Pelagic clays ('red clay').
Aluminium silicates. Any combination of montmorillonite, kaolinite, chlorite
with an admixture of silt- and clay-sized grains of quartz, feldspar and
other minerals. The chlorite crystal can be modified into an iron-rich
form. Deep sea sediments commonly contain this iron-rich form, from which
it derives its name 'red clay'.
A blue crystalline mineral of aluminium silicate.
Density 3.6, hardness 8.A transparent or translucent aluminium silicate
mineral, usually yellow, used as a gem. Many colours.
Density 3.65-3.77, hardness 7-7.5. Crystals cross shaped, dark grey to
An aluminium silicate occurring in orthorhombic crystals or fibrous masses.
Aillimanite, Andalusite, Kyanite or Dysthene.
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
Igneous rocks (Granites).
Igneous rocks are formed by the crystallisation of a magma. The difference
between granites and basalts is in silica content and their rates of cooling.
A basalt is about 53% SiO2, whereas granite is 73%.
Intrusive, slowly cooled
inside the crust. (Plutonic rock = formed in the earth). Large crystals.
crust) Density 2.7-2.8. High silica content (acidic). = quartz + mica +
K-feldspar in solid solution. 60% orthoclase and plagioclase fledspars
+ 25% quartz + 5% darker minerals (biotite, hornblende). Color from flesh
to black. Crystals intermingled. Hard, rigid, tough. Granitic rock is much
less common on the other terrestrial planets, a fact having to do with
the fractionation (where early crystallizing minerals separate fromt he
rest of a magma), a process that takes place uniquely on earth, due to
the prevalence of plate tectonics.
Granodiorite. An intermediate
form between granite and diorite.
Diorite. High silica
Gabbro. Density? Medium
silica content. (intermediate). Similar to granite = quartz + feldspar
+ pyroxene + amphibole + mica + olivene. A layer of gabbro is found in
the ocean crust, unerneath the basalt layer (0.5-2.5km), from 2.5 to 6.3
km deep. The lunar highlands have many gabbros (made largely of potassium
feldspar - also known as plagioclase)
Extrusive. cooled rapidly
at the surface. Small crystals.
Rhyolite. Medium silica
content (intermediate). A fine-grained volcanic rock of granitic composition.
Basalt. (Ocean crust)
Density 2.9. Low silica content. (basic). Dark, dense. = olivene + pyroxene
+ Ca-Feldspar in solid solution. Basaltic rocks (gabbro & basalt) are
made up of feldspars and other minerals common in planetary crusts. They
have been identified as major surface rocks on the dark lunar planes and
much of Mars, Venus and the asteroid Vesta.
rocks: debris ejected by volcanoes
Tuff is made of compacted
debris from old volcanic ash showers.
Volcanic breccia is composed
of angular mineral fragments embedded in a matrix, the product of explosive
Ignimbrites are sheets
of coalesced fine particles which once flowed at high speed, extremely
hot, fluid avalanches.
Notes: Density in kg/litre or
of igneous rocks
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.)
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
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).
process of forming a rock from a solid solution melt This
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
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.
Clastic sedimentary rocks
consist of rock and mineral grains derived from the chemical and mechanical
breakdown (weathering) of pre-existing rock. They contain rock fragments
and more commonly, particles of quartz and feldspar. Clastic rocks are
further classified on the basis of grain size. Underneath each rock
type, the Wentworth Scale of particle sizes is shown.
Conglomerates (> 2mm)
Cobble (65-256 mm)
Pebble (4-64 mm)
Granule (2-4 mm)
Sandstones (0.062-2 mm)
Very coarse (1.0 - 2.0 mm)
Coarse (0.5 - 1 mm)
Medium (0.25 - 0.5 mm)
Fine (0.125 - 0.25 mm)
Very fine (0.0625 - 0.125 mm)
Shales (<0.0062 mm)
consolidated mud, rich in organic matter.
Silt (0.0039 - 0.0625
Clay (0.0002 - 0.0039
Argillite. A sedimentary
rock, composed of clay particles which have been hardened and cemented.
Illite (muscovite). K2Al4(Si6Al2)O20(OH)4.
is a sedimentary fine-grained rock, equivalent to ordinari mica (muscovite).
Colloid (<0.0002 mm)
Chemical sedimentary rocks
are formed either from minerals that precipitate directly from aqeous (water)
solutions or from the accumulation of fossilised remains of organisms which
Halite (NaCl) salt
composition triangle The
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
Argillaceous (from French:
= 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.
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
Slates are foliated rocks
representing low-grade metamorphic alteration of shales (laminated clay).
Argillite is a mudstone,
much hardened by pressure.
Schists are foliated
medium-grade metamorphic rock with parallel layers, vertical to the direction
Gneiss are banded rocks
consisting of alternating layers of quartz and feldspar, of high metamorphic
Greywacke is a severely
hardened sandstone with mica and feldspar, sometimes containing fossils.
Chert is a siliceous
rock deposited chemically, often common among greywacke.
Marble is metamorphosed
limestone, just recrystallised.
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:
Pressure from sinking deeper
while overlaid by other sediments.
Pressure from continental collision
and consequent folding and overthrusting of the crust (dynamometamorphism).
Temperature from sinking deeper,
into warmer layers of the crust (metamorphism).
Temperature from igneous hot
lava running nearby, either overhead or from intrusions (contact or thermal