Zeolite facies
Encyclopedia
Zeolite facies describes the mineral assemblage resulting from the pressure and temperature conditions of low-grade metamorphism
.
The zeolite facies is generally considered to be transitional between diagenetic
processes which turn sediments into sedimentary rock
s, and prehnite-pumpellyite facies
, which is a hallmark of subseafloor alteration of the oceanic crust
around mid-ocean ridge
spreading centres. The zeolite and prehnite-pumpellyite facies are considered burial metamorphism as the processes of orogenic
regional metamorphism are not required.
Zeolite facies is most often experienced by pelitic sediments; rocks rich in aluminium, silica, potassium and sodium, but generally low in iron, magnesium and calcium. Zeolite facies metamorphism usually results in the production of low temperature clay
minerals into higher temperature polymorphs such as kaolinite
and vermiculite
.
Mineral assemblages include kaolinite and montmorillonite
with laumontite
, wairakite
, prehnite
, calcite
and chlorite
. Phengite
and adularia occur in potassium rich rocks. Minerals in this series include zeolite
s, albite
, and quartz
.
This occurs by dehydration of the clays during compaction, and heating due to blanketing of the sediments by continued deposition of sediments atop it. Zeolote facies is considered to start with temperatures of approximately 50 - 150 °C and some burial is required, usually 1 - 5 km.
Zeolite facies tends to correlate in clay-rich sediments with the onset of a bedding plane foliation
, parallel with the bedding of the rocks, caused by alignment of platy clay minerals in a horizontal orientation which reduces their free energy state.
Generally plutonic and volcanic rocks are not affected overly much by zeolite facies metamorphism, although vesicular
basalt
s and the like will have their vesicles flled with zeolite
minerals, forming amygdaloidal
texture. Tuff
can also become zeolitized, as is seen in the Obispo formation on the California coast.
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
.
The zeolite facies is generally considered to be transitional between diagenetic
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
processes which turn sediments into sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
s, and prehnite-pumpellyite facies
Prehnite-pumpellyite facies
The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres....
, which is a hallmark of subseafloor alteration of the oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
around mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
spreading centres. The zeolite and prehnite-pumpellyite facies are considered burial metamorphism as the processes of orogenic
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
regional metamorphism are not required.
Zeolite facies is most often experienced by pelitic sediments; rocks rich in aluminium, silica, potassium and sodium, but generally low in iron, magnesium and calcium. Zeolite facies metamorphism usually results in the production of low temperature clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
minerals into higher temperature polymorphs such as kaolinite
Kaolinite
Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...
and vermiculite
Vermiculite
Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite...
.
Mineral assemblages include kaolinite and montmorillonite
Montmorillonite
Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central...
with laumontite
Laumontite
Laumontite is a mineral, one of the zeolite group. Its molecular formula is 2·4H2O, a hydrated calcium-aluminium silicate. Potassium or sodium may substitute for the calcium but only in very small amounts....
, wairakite
Wairakite
Wairakite is a zeolite mineral with an analcime structure but containing a calcium ion. The chemical composition is Ca8*16H2O. It is named for the location of its discovery in North Island, New Zealand, by Alfred Steiner in 1955. The mineral has since been found in metamorphic rocks and in...
, prehnite
Prehnite
Prehnite is a phyllosilicate of calcium and aluminium with the formula: Ca2Al2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most oftens forms as stalactitic or botryoidal aggregates, with only just the crests of small...
, calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
and chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
. Phengite
Phengite
Phengite is a series name for dioctahedral micas of composition K224O10, similar to muscovite but with addition of magnesium. It is a non-IMA recognized mineral name representing the series between muscovite and celadonite....
and adularia occur in potassium rich rocks. Minerals in this series include zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
s, albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
, and quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
.
This occurs by dehydration of the clays during compaction, and heating due to blanketing of the sediments by continued deposition of sediments atop it. Zeolote facies is considered to start with temperatures of approximately 50 - 150 °C and some burial is required, usually 1 - 5 km.
Zeolite facies tends to correlate in clay-rich sediments with the onset of a bedding plane foliation
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
, parallel with the bedding of the rocks, caused by alignment of platy clay minerals in a horizontal orientation which reduces their free energy state.
Generally plutonic and volcanic rocks are not affected overly much by zeolite facies metamorphism, although vesicular
Vesicular texture
Vesicular texture is a volcanic rock texture characterised by a rock being pitted with many cavities at its surface and inside. The texture is often found in extrusive aphanitic, or glassy, igneous rock...
basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
s and the like will have their vesicles flled with zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
minerals, forming amygdaloidal
Amygdule
Amygdules or amygdales form when the gas bubbles or vesicles in volcanic lava are infilled with a secondary mineral such as calcite, quartz, chlorite or one of the zeolites. Amygdules usually form after the rock has been emplaced, and are often associated with low-temperature alteration. Amygdules...
texture. Tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
can also become zeolitized, as is seen in the Obispo formation on the California coast.