Normative mineralogy is a calculation of the composition of a rock sample that estimates the ''idealised mineralogy'' of a rock based on a quantitative
chemical analysis
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
according to the principles of
geochemistry
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
.
Normative mineral calculations can be achieved via either the CIPW Norm or the Barth-Niggli Norm (also known as the Cation Norm).
Normative calculations are used to produce an idealised mineralogy of a crystallized melt. First, a rock is chemically analysed to determine the
elemental
An elemental is a mythic being that is described in occult and alchemical works from around the time of the European Renaissance, and particularly elaborated in the 16th century works of Paracelsus. According to Paracelsus and his subsequent fo ...
constituents. Results of the chemical analysis traditionally are expressed as oxides (e.g., weight percent Mg is expressed as weight percent MgO). The normative mineralogy of the rock then is calculated, based upon assumptions about the order of mineral formation and known phase relationships of rocks and minerals, and using simplified mineral formulas. The calculated mineralogy can be used to assess concepts such as
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
saturation
Saturation, saturated, unsaturation or unsaturated may refer to:
Chemistry
* Saturation, a property of organic compounds referring to carbon-carbon bonds
**Saturated and unsaturated compounds
** Degree of unsaturation
**Saturated fat or fatty aci ...
of melts.
Because the normative calculation is essentially a computation, it can be achieved via computer programs.
CIPW Norm
The CIPW Norm was developed in the early 1900s by the petrologists
Cross
A cross is a geometrical figure consisting of two intersecting lines or bars, usually perpendicular to each other. The lines usually run vertically and horizontally. A cross of oblique lines, in the shape of the Latin letter X, is termed a sa ...
,
Iddings,
Pirsson and the geochemist
Washington
Washington commonly refers to:
* Washington (state), United States
* Washington, D.C., the capital of the United States
** A metonym for the federal government of the United States
** Washington metropolitan area, the metropolitan area centered o ...
. The CIPW normative mineralogy calculation is based on the typical minerals that may be precipitated from an
anhydrous
A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
melt at low pressure, and simplifies the typical igneous geochemistry seen in nature with the following four constraints:
# The magma crystallizes under anhydrous conditions so that no hydrous minerals (
hornblende
Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rocks ...
,
biotite
Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...
) are formed.
# The
ferromagnesian minerals are assumed to be free of Al
2O
3.
# The Fe/Mg ratio for all ferromagnesian minerals is assumed to be the same.
# Several minerals are assumed to be incompatible, thus
nepheline
Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites ...
and/or
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
never appear with
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
in the norm.
This is an artificial set of constraints, and therefore the results of the CIPW norm do not reflect the true course of igneous differentiation in nature.
The primary benefit of calculating a CIPW norm is determining what the ideal mineralogy of an
aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geologic ...
or
porphyritic
Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all ...
igneous rock is. Secondly, the degree of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
saturation of the melt that formed the rock can be assessed in the absence of diagnostic
feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are usually not found in rocks containing prim ...
species.
The silica saturation of a rock varies not only with silica content but the proportion of the various alkalis and metal species within the melt. The silica saturation
eutectic plane is thus different for various families of rocks and cannot be easily estimated, hence the requirement to calculate whether the rock is silica saturated or not.
This is achieved by assigning cations of the major elements within the rock to silica anions in modal proportion, to form
solid solution
A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word ...
minerals in the idealised mineral assemblage starting with phosphorus for
apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...
, chlorine and sodium for
halite
Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...
, sulfur and FeO into
pyrite
The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral.
Pyrite's metallic Luster (mineralogy), lust ...
, FeO and Cr
2O
3 is allocated for
chromite
Chromite is a crystalline mineral composed primarily of iron(II) oxide and chromium(III) oxide compounds. It can be represented by the chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can s ...
, FeO and equal molar amount of TiO
2 for
ilmenite
Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...
, CaO and CO
2 for
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
, to complete the most common non-silicate minerals.
From the remaining chemical constituents, Al
2O
3 and K
2O are allocated with silica for
orthoclase
Orthoclase, or orthoclase feldspar (endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles t ...
; sodium, aluminium and potassium for
albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. I ...
, and so on until either there is no silica left (in which case feldspathoids are calculated) or excess, in which case the rock contains normative quartz.
Normative and modal mineralogy
Normative mineralogy is an ''estimate'' of the mineralogy of the rock. It usually differs from the visually observable mineralogy, at least as much as the types of mineral species, especially amongst the ferromagnesian minerals and feldspars, where it is possible to have many solid solution series of minerals, or minerals with similar Fe and Mg ratios substituting, especially with water (e.g.; amphibole and biotite replacing pyroxene).
However, in aphanites, or rocks with
phenocryst
300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...
s clearly out of equilibrium with the
groundmass
The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.
The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, ca ...
, a normative mineral calculation is often the best to understand the evolution of the rock and its relationship to other igneous rocks in the region.
Cautions
The CIPW Norm or Cation Norm is a useful tool for assessing silica saturation or oversaturation; estimations of minerals in a mathematical model are based on many assumptions and the results must be balanced with the observable mineralogy. The following areas create the most errors in calculations;
*
Cumulate rocks
Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group o ...
. While a normative mineral calculation isn't necessarily improper for use on cumulate rocks, the information gleaned is of doubtful value because a cumulate rock does not represent the melt from which it was extracted. However, if the groundmass of a cumulate can be analysed, it is valid to use a normative calculation to gain information about the parental melt.
*
Oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
. Because the normative calculation divides Fe between oxide phases and availability for silicate phases, based on estimates of the ratio of Fe
2+/Fe
3+, miscalculating the ratio for the rock in question may produce erroneous amounts of magnetite or hematite, or alter the silicate mineralogy. If the Fe
2+/Fe
3+ ratio is known for the sample, the resulting calculation should match the observed mineralogy more closely.
* Pressure and temperature. Because the CIPW Norm is based on
anhydrous
A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...
melts and crystallisation at fairly low pressures, the resultant normative mineralogy does not reflect observed mineralogy for all rock types, especially those formed within the mantle. The normative mineralogy is not entirely accurate at reflecting the mineralogy of rocks formed at high pressures where, for instance, phlogopite may substitute for amphibole, amphibole for olivine and so forth. Altered normative calculations have been developed that more correctly reflect the particular pressure regimes of the deep crust and mantle.
* Carbon dioxide. The influence of CO
2 on most silicate melts is fairly minor but in some cases, especially
carbonatite
Carbonatite () is a type of intrusive rock, intrusive or extrusive rock, extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geoche ...
, but also certain
lamprophyre
Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxid ...
type rocks,
kimberlite
Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond called the Star of ...
and
lamproite
Lamproite is an ultrapotassic mantle-derived volcanic or subvolcanic rock. It has low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements.
Lamproites are geographically widespread ye ...
, the presence of carbon dioxide and calcite in the melt or accessory phases derives erroneous normative mineralogy. This is because if carbon is not analyzed, there is excess calcium, causing normative silica undersaturation, and increasing the calcium silicate mineral budget. Similarly, if graphite is present (as is the case with some kimberlites) this can produce excess C, and hence skew the calculation toward excess carbonate. Excess elemental C also, in nature, results in reduced oxygen fugacity and alters Fe
2+/Fe
3+ ratios.
*
Halide
In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...
s. The presence of some halides and non-metallic elements in the melt alter the resulting chemistry. For instance,
boron
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
forms
tourmaline
Tourmaline ( ) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is a gemstone and can be found in a wide variety of colors.
The ...
; excess
chlorine
Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...
may form
scapolite
The scapolites (Gr. σκάπος, rod, and λίθος, stone) are a group of rock-forming silicate minerals composed of aluminium, calcium, and sodium silicate with chlorine, carbonate and sulfate. The two endmembers are meionite () and marialit ...
instead of feldspar. This is generally rare, except in some A-type
granite
Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
s and related rocks.
* Mineral disequilibrium. Similar to cumulate rocks, a rock may contain extraneous minerals inherited from earlier melts, or may even contain xenoliths or
restite. It is improper to calculate normative mineralogy on an
igneous breccia, for instance.
For this reason it is not advised to utilise a CIPW norm on
kimberlite
Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond called the Star of ...
s,
lamproite
Lamproite is an ultrapotassic mantle-derived volcanic or subvolcanic rock. It has low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements.
Lamproites are geographically widespread ye ...
s,
lamprophyres
Lamprophyres () are uncommon, small-volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks, and small intrusions. They are alkaline silica- undersaturated mafic or ultramafic rocks with high magnesium o ...
and some silica-undersaturated igneous rocks. In the case of
carbonatite
Carbonatite () is a type of intrusive rock, intrusive or extrusive rock, extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geoche ...
, it is improper to use a CIPW norm upon a melt rich in carbonate.
It is possible to apply the CIPW norm to metamorphosed igneous rocks. The validity of the method holds as true for metamorphosed igneous rocks as any igneous rock, and in this case it is useful in deriving an assumed mineralogy from a rock that may have no remnant
protolith
A protolith () is the original, unmetamorphosed rock from which a given metamorphic rock is formed.
For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other kind of non-metamorphic rock and thu ...
mineralogy remaining.
See also
*
Geochemistry
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
*
Mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proces ...
*
Wüstite
Wüstite ( Fe O) is a mineral form of iron(II) oxide found with meteorites and native iron. It has a grey colour with a greenish tint in reflected light. Wüstite crystallizes in the isometric-hexoctahedral crystal system in opaque to transluc ...
References
* Hess, P. C. (1989), ''Origins of Igneous Rocks'', President and Fellows of Harvard College (pp. 276–285), .
* Blatt, Harvey and Robert Tracy (1996), ''Petrology'', 2nd ed., Freeman (pp. 196–7), {{ISBN, 0-7167-2438-3.
External links
Basic CIPW calculation excluding exotic minerals steps, with excel spreadsheet calculator
alternate resource, with explanations.
Geochemistry
Mineralogy
Igneous petrology