Zussmanite
Encyclopedia
Zussmanite is a hydrated iron-rich silicate mineral which was found in 1960 by Stuart Olof Agrell
, in the Laytonville quarry, Mendocino County, California
. Zussmanite is named in honor of Jack Zussman, Head of the University of Manchester
’s Department of Geology. Zussmanite is found as a pale green crystals with perfect cleavage. In the Laytonville quarry, Zussmanite occurs in metamorphosed shale
s, siliceous ironstone
s and impure limestone
s of the Franciscan Formation. It is a location of high pressure and low temperatures where blueschist facies metamorphic rock
s occur. This is also the locality in which Deerite and Howieite were also first discovered. This type of locality also produces mica
s, which have a similar structure as Zussmanite.
and forms glaucophane schists (Miyashiro et al.,1973). Glaucophane schists, commonly referred to as blueschist-facies, result from metamorphism of basalt
ic rocks and are usually located in folded geosynclinal
terrane
s (Deer, Howie, Zussman et al.,1992). Glaucophane schists are characterized by low temperature (100-250 °C) high pressure (4-9 kbar) metamorphism (Deer, Howie, Zussman et al.,1992). Zussmanite is commonly found with stilpnomelane
and quartz
, usually forming abundant porphyroblast
s up to 1 mm in size, in the newly discovered locality in Southern Central Chile ( Massonne et al., 1998).
of oceanic crust
al rocks and oceanic-continental margin sediments along convergent plate boundaries. The ideal formula for zussmanite is KFe13Si17AlO42(OH)14 with possible substitutions of sodium (Na) for potassium (K), in extremely small amounts (Lopes et al.,1969). The possible iron (Fe2+) substitutes are mainly magnesium (Mg) with trace amounts that could include: manganese (Mn), aluminium (Al), iron3+ (Fe3+) and titanium (Ti) (Lopes et al.,1969). Zussmanite was discovered in combination with deerite and howieite, two new minerals discovered in the Franciscan formation, Mendocino County, California. Deerite and howieite have been found at other locations while zussmanite has only been found at this type locality, making it a rare occurring mineral. Experiments have revealed that zussmanite is stable up to 600 °C at pressures between 10 kb and 30 kb and that the end members of zussmanite are orthoferrosilite, biotite and quartz. The example of the reaction is KFe13[AlSi17042](OH)14 (zussmanite) yields 10FeSiO3 (orthoferrosilite) + ½ K2Fe6Si6Al2O20(OH)4 (biotite) + 4SiO2 (quartz) + 6H20 (water) (Dempsey et al.,1981). The manganese analogue of zussmanite, coombsite, has been found in manganese-rich siliceous rocks in the Otago Schist in New Zealand
.
The perfect cleavage is a result of the continuous sheets of (Fe,Mg)-(O,OH) octahedra parallel to (0001). The optical properties result from virtually pure zussmanite that was separated from thin sections, approximately 200 micrometres thick, under a polarizing microscope by means of a microdrill. The indices of refraction compare well with those determined be Agrell et al.,1965 for the chemically different Zussmanite from the Laytonville quarry (Massonne et al., 1998).
Stuart Olof Agrell
Stuart Olof Agrell was an outstanding optical mineralogist and pioneer collaborator applying the electron microprobe to petrology. His involvement in the Apollo program brought him to the attention of the British media and public.Agrell was born in Ruislip, Middlesex to a Scandinavian father and...
, in the Laytonville quarry, Mendocino County, California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
. Zussmanite is named in honor of Jack Zussman, Head of the University of Manchester
University of Manchester
The University of Manchester is a public research university located in Manchester, United Kingdom. It is a "red brick" university and a member of the Russell Group of research-intensive British universities and the N8 Group...
’s Department of Geology. Zussmanite is found as a pale green crystals with perfect cleavage. In the Laytonville quarry, Zussmanite occurs in metamorphosed shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s, siliceous ironstone
Ironstone
Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical repacement, that contains a substantial proportion of an iron compound from which iron either can be or once was smelted commercially. This term is customarily restricted to hard coarsely...
s and impure limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s of the Franciscan Formation. It is a location of high pressure and low temperatures where blueschist facies metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s occur. This is also the locality in which Deerite and Howieite were also first discovered. This type of locality also produces mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
s, which have a similar structure as Zussmanite.
Introduction
The locality in which zussmanite occurs is one of ultra high to high pressure and low temperatures. This baric type of metamorphism is usually distinguished by the P/T range rather than the ranges in pressure and temperatures (Miyashiro et al.,1973). The three principal baric types are low P/T type, medium P/T type, and high P/T type. The high P/T type, referred to as glaucophanic metamorphism, is characterized by the presence of glaucophaneGlaucophane
Glaucophane is a mineral belonging to the amphibole group, chemical formula []Na2]Mg3Al2][2|Si8O22].-Name:It is named from its typical blue color . Glaucophane crystallizes in the monoclinic system....
and forms glaucophane schists (Miyashiro et al.,1973). Glaucophane schists, commonly referred to as blueschist-facies, result from metamorphism of 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...
ic rocks and are usually located in folded geosynclinal
Geosyncline
In geology, geosyncline is a term still occasionally used for a subsiding linear trough that was caused by the accumulation of sedimentary rock strata deposited in a basin and subsequently compressed, deformed, and uplifted into a mountain range, with attendant volcanism and plutonism...
terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
s (Deer, Howie, Zussman et al.,1992). Glaucophane schists are characterized by low temperature (100-250 °C) high pressure (4-9 kbar) metamorphism (Deer, Howie, Zussman et al.,1992). Zussmanite is commonly found with stilpnomelane
Stilpnomelane
Stilpnomelane is a phyllosilicate mineral of the mica group. It has the formula K81227.Stilpnomelane occurs associated with banded iron formations. It is a metamorphic mineral associated with the blueschist and greenschist facies....
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,...
, usually forming abundant porphyroblast
Porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....
s up to 1 mm in size, in the newly discovered locality in Southern Central Chile ( Massonne et al., 1998).
Composition
The blueschist-facies phyllosilicate mineral occurs as a result of subductionSubduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
of 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...
al rocks and oceanic-continental margin sediments along convergent plate boundaries. The ideal formula for zussmanite is KFe13Si17AlO42(OH)14 with possible substitutions of sodium (Na) for potassium (K), in extremely small amounts (Lopes et al.,1969). The possible iron (Fe2+) substitutes are mainly magnesium (Mg) with trace amounts that could include: manganese (Mn), aluminium (Al), iron3+ (Fe3+) and titanium (Ti) (Lopes et al.,1969). Zussmanite was discovered in combination with deerite and howieite, two new minerals discovered in the Franciscan formation, Mendocino County, California. Deerite and howieite have been found at other locations while zussmanite has only been found at this type locality, making it a rare occurring mineral. Experiments have revealed that zussmanite is stable up to 600 °C at pressures between 10 kb and 30 kb and that the end members of zussmanite are orthoferrosilite, biotite and quartz. The example of the reaction is KFe13[AlSi17042](OH)14 (zussmanite) yields 10FeSiO3 (orthoferrosilite) + ½ K2Fe6Si6Al2O20(OH)4 (biotite) + 4SiO2 (quartz) + 6H20 (water) (Dempsey et al.,1981). The manganese analogue of zussmanite, coombsite, has been found in manganese-rich siliceous rocks in the Otago Schist in New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
.
Structure
The space group and cell of Zussmanite are R*3, ahex 11.66 and chex28.69 Angstroms (Agrell et al., 1965). The structure of Zussmanite contains continuous sheets of rhombohedrally stacked layers of Fe-O octahedral parallel to (0001) (Lopes et al.,1967) and to either side of these are attached (Si,Al)-O tetrahedral in a way to produce a rhombohedral unit cell(Lopes et al.,1969). These layers are linked to one another by Potassium (K) atoms and also by three-member rings of tetrahedra that share oxygens with the six-members; displayed in figure 2 (Lopes et al.,1967). Zussmanite’s structure has a close affinity to that of the trioctahedral micas which have a layer of Fe-O octahedral sandwiched between inward pointing tetrahedral. It differs from the micas because its Si-O ratio is 9:21 which results in a sharing coefficient 1.83, as compared with 2.5 and 1.75 for micas, and 1.2 and 2.0 for framework silicates (Lopes et al.,1969). The Fe-(O,OH) mean distance in the first octahedron is 2.1 Angstroms, the second octahedron is 2.14 Angstroms, and in the third Octahedron is 2.17 Angstroms. The mean distance in the Si-O bonds in Zussmanite are 1.61 Angstroms for the first tetrahedron, 1.61 Angstroms for the second tetrahedron, and 1.65 Angstroms for the third tetrahedron; data given in table I (Lopes et al.,1969). The six-member rings are not directly linked to one another which allows for adjustment by tilting outwards of all tetrahedral, as opposed to many micas where rotations and tilts are used to achieve the larger dimensions of the octahedral layer. The flattening of the octahedral layer perpendicular to the layer is pronounced in Zussmanite due to shared and unshared edges. This flattening could be due to the tendency for shared oxygens to come closer and shields iron (Fe) atoms from other neighboring iron (Fe) atoms.Physical properties
Zussmanite occurs in pale green tabular crystals with perfect cleavage. It tends to be uniaxial, weakly pleochroic and a specific gravity of 3.146 (Agrell et al.,1965). Other types of zussmanite found in Laytonville, which are of fine-grained samples are assumed to be late-stage metamorphic products.The perfect cleavage is a result of the continuous sheets of (Fe,Mg)-(O,OH) octahedra parallel to (0001). The optical properties result from virtually pure zussmanite that was separated from thin sections, approximately 200 micrometres thick, under a polarizing microscope by means of a microdrill. The indices of refraction compare well with those determined be Agrell et al.,1965 for the chemically different Zussmanite from the Laytonville quarry (Massonne et al., 1998).