Illite crystallinity

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Figure 1: Illite crystallinity classification chart showing the different metamorphic zones and facies corresponding to IC, adapted from Verdel 2012 Illite Crystallinity classification chart.svg
Figure 1: Illite crystallinity classification chart showing the different metamorphic zones and facies corresponding to IC, adapted from Verdel 2012
Figure 2: Illite structure Illstruc.jpg
Figure 2: Illite structure

Illite crystallinity is a technique used to classify low-grade metamorphic activity in pelitic rocks. [1] Determining the "illite crystallinity index" allows geologists to designate what metamorphic facies and metamorphic zone the rock was formed in and to infer what temperature the rock was formed. Several crystallinity indices have been proposed in recent years, but currently the Kübler index is being used due to its reproducibility and simplicity. [2] The Kübler index is experimentally determined by measuring the full width at half maximum for the X-ray diffraction reflection peak along the (001) crystallographic axis of the rock sample. [3] This value is an indirect measurement of the thickness of illite/muscovite packets which denote a change in metamorphic grade. The method can be used throughout the field of geology in areas such as the petroleum industry, plate tectonics.

Contents

Progression

As stated above, the Kübler index was not always the preferred index for illite crystallinity studies in the past. Prior to the introduction of the Kübler index, there were several other indices used to classify low grade metamorphic rocks. [2] Two of the more popular methods of the past are the Weaver index and the Weber index, introduced in 1960 [4] and 1972 [5] respectively. [2] These studies consist of mainly the same types of methods but vary in their expression of ratio measurements. [2] The Kübler index, introduced by Bernard Kübler in 1964 for petroleum exploration and improved on in later years, has come to be the go-to index for illite crystallinity based on its reproducibility and simplicity. [6] [7] [8] [2]

Applications

Illite crystallinity is useful when trying to determine what type of metamorphic conditions a rock was subjected to during its formation. Illite crystallinity can be used to trace the low grade metamorphic transition from zeolite facies to greenschist facies (diagenetic zone to epizone). [9] This change is flagged by the change of thin illite grains to thicker illite/muscovite grains. [9] This low grade metamorphic technique can also be put into use when there is an absence in change of mineral structure which applies to higher grade metamorphism. [2] Early use of illite crystallinity was in the petroleum industry to determine the transition from a dry gas phase to an unproductive rock. Recently, this technique has expanded in the field and now is used in areas such as palaeotectonics and geodynamic reconstructions. [2]

Rock preparation and methods

Rock sample preparation for illite crystallinity can vary slightly, but boils down to basically the same steps. Although for accurate returns in testing, consistency of sample preparation is a must. [1] General sample preparation for illite crystallinity is as follows: [1]

  1. Rinse and dry the field sample
  2. Crush the sample
  3. Stir sample into deionized water and let settle overnight to isolate clay sized particles (<2 μm)
  4. Dry supernatant containing <2 μm particles
  5. Mix with deionized water and centrifuge
  6. Collect supernatant containing <2 μm particles
  7. Centrifuge <2 μm particle solution and dry
  8. Mix with water and deposit on a glass slide

The sample is first broken down, using the steps above, and prepared for XRD analysis. Results from the XRD are then compared to pre-established values assigned to metamorphic zones/metamorphic facies. The targets of the results are the peaks on the XRD plots. Width of the illite XRD peak at one half of its height is collected and recorded with units of ∆ °2θ (XRD angle). [1] Comparison and classification of metamorphic facies is then determined for the sample.

Interpreting results

Illite (K0.65Al2.0[Al0.65Si3.35O10](OH)2) [10] and muscovite (KAl2(AlSi3O10)(OH)2) [10] are both phyllosilicates similar in structure and composition. [9] Illite is made up of thin, 1 nm, layers which are made up of tetrahedral-octahedral-tetrahedral (TOT) sheets. Illite contains more silicon, iron and magnesium than muscovite, as well as less tetrahedral aluminium and interlayered potassium. [1] [9]

X-Ray diffraction plots provide information on angles and intensities of refracted beams which allow scientists to construct a 3D model of the crystalline structure. The focus of an illite crystallinity XRD plot is the main peak. Width of the peak at one half of its height is measured and this angle (recorded with units of ∆ °2θ), [1] can be plotted on a chart with metamorphic zones and facies like the one in figure 1. If the illite crystallinity values fall in the 0-0.25 °2θ range, it corresponds with a metamorphic epizone or greenschist facies. If the illite crystallinity values fall in the 0.25-0.30 °2θ range, it corresponds with a metamorphic high anchizone or prehnite-pumpellyite facies. If the illite crystallinity values fall in the 0.30-0.42 °2θ range, it corresponds with a metamorphic low anchizone or prehnite-pumpellyite facies. If the illite crystallinity values fall in the 0.42-1.0 °2θ range, it corresponds with a metamorphic deep diagenetic zone or zeolite facies. If the illite crystallinity is > 1.0, it corresponds with a metamorphic shallow diagenetic zone or zeolite facies. [1]

Generally, width of the diffraction peak can be related to c-axis parallel thickness of the illite crystallites. [1] Thin packets produce broader peaks and thick packets return more narrow peaks. [11] This is based on the destructive interference of the thick packets or the lack of interference in the thin packets, which cause this difference.

Related Research Articles

<span class="mw-page-title-main">Metamorphic rock</span> Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.

<span class="mw-page-title-main">Metamorphism</span> Change of minerals in pre-existing rocks without melting into liquid magma

Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.

<span class="mw-page-title-main">Metasomatism</span> Chemical alteration of a rock by hydrothermal and other fluids

Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is traditionally defined as metamorphism which involves a change in the chemical composition, excluding volatile components. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.

<span class="mw-page-title-main">Clay mineral</span> Fine-grained aluminium phyllosilicates

Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2Si2O5(OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces.

<span class="mw-page-title-main">Granulite</span> Class of high-grade medium to coarse grained metamorphic rocks

Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.

<span class="mw-page-title-main">Hornfels</span>

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are caused by fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.

<span class="mw-page-title-main">Chlorite group</span> Type of mineral

The chlorites are the group of phyllosilicate minerals common in low-grade metamorphic rocks and in altered igneous rocks. Greenschist, formed by metamorphism of basalt or other low-silica volcanic rock, typically contains significant amounts of chlorite.

<span class="mw-page-title-main">Illite</span> Group of related non-expanding clay minerals

Illite, also called hydromica or hydromuscovite, is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina octahedron (O) – silica tetrahedron (T) layers. The space between this T-O-T sequence of layers is occupied by poorly hydrated potassium cations which are responsible for the absence of swelling. Structurally, illite is quite similar to muscovite with slightly more silicon, magnesium, iron, and water and slightly less tetrahedral aluminium and interlayer potassium. The chemical formula is given as (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2·(H2O)], but there is considerable ion (isomorphic) substitution. It occurs as aggregates of small monoclinic grey to white crystals. Due to the small size, positive identification usually requires x-ray diffraction or SEM-EDS analysis. Illite occurs as an altered product of muscovite and feldspar in weathering and hydrothermal environments; it may be a component of sericite. It is common in sediments, soils, and argillaceous sedimentary rocks as well as in some low grade metamorphic rocks. The iron-rich member of the illite group, glauconite, in sediments can be differentiated by x-ray analysis.

<span class="mw-page-title-main">Narryer Gneiss Terrane</span> Geological complex of ancient rocks in Western Australia

The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or granulite conditions, resulting in often complete destruction of original igneous or sedimentary (protolith) textures. Importantly, it contains the oldest known samples of the Earth's crust: samples of zircon from the Jack Hills portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon crystals are about 3.6-3.8 billion years old.

Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism.

The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres. It is a metamorphic grade transitional between zeolite facies and greenschist facies representing a temperature range of 250 to 350 °C and a pressure range of approximately two to seven kilobars. The mineral assemblage is dependent on host composition.

<span class="mw-page-title-main">Litchfieldite</span>

Litchfieldite is a rare igneous rock. It is a coarse-grained, foliated variety of nepheline syenite, sometimes called nepheline syenite gneiss or gneissic nepeheline syenite. Litchfieldite is composed of two varieties of feldspar, with nepheline, sodalite, cancrinite and calcite. The mafic minerals, when present, are magnetite and an iron-rich variety of biotite (lepidomelane).

<span class="mw-page-title-main">Index mineral</span>

An index mineral is used in geology to determine the degree of metamorphism a rock has experienced. Depending on the original composition of and the pressure and temperature experienced by the protolith, chemical reactions between minerals in the solid state produce new minerals. When an index mineral is found in a metamorphosed rock, it indicates the minimum pressure and temperature the protolith must have achieved in order for that mineral to form. The higher the pressure and temperature in which the rock formed, the higher the grade of the rock.

<span class="mw-page-title-main">Metamorphic facies</span> Set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

<span class="mw-page-title-main">Metamorphic zone</span>

In geology, a metamorphic zone is an area where, as a result of metamorphism, the same combination of minerals occur in the bedrock. These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure.

<span class="mw-page-title-main">Subduction zone metamorphism</span> Changes of rock due to pressure and heat near a subduction zone

A subduction zone is a region of the Earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets created by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process creates and destroys water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.

<span class="mw-page-title-main">Monazite geochronology</span> Dating technique to study geological history using nuclear decay of the mineral monazite

Monazite geochronology is a dating technique to study geological history using the mineral monazite. It is a powerful tool in studying the complex history of metamorphic rocks particularly, as well as igneous, sedimentary and hydrothermal rocks. The dating uses the radioactive processes in monazite as a clock.

<span class="mw-page-title-main">Geology of North Macedonia</span> Overview of geology in North Macedonia

The geology of North Macedonia includes the study of rocks dating to the Precambrian and a wide array of volcanic, sedimentary and metamorphic rocks formed in the last 539 million years.

References

  1. 1 2 3 4 5 6 7 8 Verdel, Charles; Niemi, Nathan; van der Pluijm, Ben A. (2011). "Variations in the Illite to Muscovite Transition Related to Metamorphic Conditions and Detrital Muscovite Content: Insight from the Paleozoic Passive Margin of the Southwestern United States" (PDF). The Journal of Geology. 119 (4): 419–437. Bibcode:2011JG....119..419V. doi:10.1086/660086. S2CID   129686326 . Retrieved 7 October 2013.
  2. 1 2 3 4 5 6 7 Abad, Isabel. "Physical meaning and applications of the illite Kübler index: measuring reaction progress in low-grade metamorphism" (PDF). In Nieto, Fernando; Jiménez-Millán, Juan (eds.). Diagenesis and low-temperature metamorphism. Theory, methods and regional aspects. ISSN   1698-5478 . Retrieved 7 October 2013.{{cite book}}: |journal= ignored (help)
  3. ↑ Eberl, D. D.; Velde, B. (December 1989). "Beyond the Kübler index" (PDF). Clay Minerals. 24 (4): 571–577. Bibcode:1989ClMin..24..571E. doi:10.1180/claymin.1989.024.4.01.
  4. ↑ Beaver, Charles E. (1959). "Possible Uses of Clay Minerals in Search for Oil". Clays and Clay Minerals. 8 (1): 214–227. Bibcode:1959CCM.....8..214W. doi:10.1346/CCMN.1959.0080120.
  5. ↑ Weber, K. (1972). "Notes on the determination of illite crystallinity". Neues Jahrbuch für Mineralogie - Monatshefte (9): 267–276.
  6. ↑ Kübler, Bernard (1964). "Les argiles, indicateurs de métamorphisme". Rev. Institut Français du Pétrole (in French). 19: 1093–1112. ISSN   1294-4475.
  7. ↑ Kübler, Bernard (1967). "La cristallinité de l'illite et les zones tout à fait supérieures du métamorphisme". Etages Tectoniques (Colloque de Neuchâtel) (in French): 105–121.
  8. ↑ Kübler, Bernard (1968). "Evaluation quantitative du métamorphisme par la cristallinité de l'illite". Bull. Centre Rech. Pau-SNPA 2 (in French): 385–397.
  9. 1 2 3 4 Verdel, Charles; van der Pluijm, Ben; Niemi, Nathan (2012). "Variation of illite/muscovite 40Ar/39Ar age spectra during progressive low-grade metamorphism: and example from the US Cordillera". Contrib Mineral Petrol. 164 (3): 521–536. Bibcode:2012CoMP..164..521V. doi:10.1007/s00410-012-0751-7. S2CID   73591976.
  10. 1 2 "Illite". Mindat.org. Retrieved 14 November 2013.
  11. ↑ Gharrabi, M.; Velde, B.; Sagon, J.-P. (1998). "The Transformation of Illite to Muscovite in Pelitic Rocks: Constraints from X-Ray Diffraction". Clays and Clay Minerals. 46 (1): 79–82. Bibcode:1998CCM....46...79G. doi:10.1346/CCMN.1998.0460109.
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