Guyanaite
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Guyanaite (CrOOH) is a chromium oxide mineral that forms as an intergrowth with other chromium oxide minerals known as bracewellite (CrOOH) and grimaldiite (CrOOH) as well as
eskolaite Eskolaite is a rare chromium oxide mineral (chromium(III) oxide Cr2O3). Discovery and occurrence It was first described in 1958 for an occurrence in the Outokumpu ore deposit of eastern Finland. It occurs in chromium bearing tremolite skarns, me ...
(Cr2O3) which in early findings were nearly indistinguishable from one another. These oxides formed so closely as intergrowths with one another that they were initially, and erroneously, identified as a single definite mineral previously known as merumite.Milton, C. & Narain, S. (1969): Merumite occurrence in Guyana. Econ. Geol., 64, 910–914. Because of its complex history and the previously undiscovered nature of these chromium oxide polymorphs, the relevance of any information found in many early experiments involving the mineral formerly known as merumite in regard to guyanaite is unknownTombs, N.C., Croft, W.J., Carter, J.R., Fitzgerald, J.F. (1964): A new polymorph of CrOOH. Inorg. Chem., 3, 1791–1792. and it is implied that in any further reference of merumite it will have been composed of a mineral assemblage including guyanaite. The rare occurrence and complexity from intergrowth of naturally occurring guyanaite hinders experimental work, leading to laboratory synthesized samples which help to better experiment with the minerals.Milton, C., Appleman, D.E., Appleman, M.H., Chao, E.C.T., Cuttitta, F., Dinnin, E.J., Dwornik, B.L., Ingram, B.L., Rose Jr., H.J. (1976): Merumite, a complex assemblage of chromium minerals from Guyana. US Geol. Surv. Prof. Paper, 887, 1–29.


Composition

Guyanaite has a chemical formula of Cr3+O(OH), it was first identified primarily by means of
X-ray powder diffraction An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30  ...
and chemical data and has been confirmed in recent studies by means of X-ray diffraction, optical reflectance, and infrared absorption (IR) spectroscopy.Jahn, S., Wunder, B., Koch-Mueller, M., Tarrieu, L., Pohle, M.,Watenphul, A., Taran, M. (2012). "Pressure-induced hydrogen bond symmetrisation in guyanaite, β-CrOOH: evidence from spectroscopy and ab initio simulations." European Journal of Mineralogy 24(5): 839-850. It is both trimorphous with, and shares an exact chemical formula with both bracewellite and grimaldiite which are also chromium oxides, differing only in their mineralogical structure being orthorhombic with
space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it uncha ...
Pnnm, orthorhombic with space group Pbnm, and
hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A '' regular hexagon'' has ...
with space group R3m, respectively.Fleischer, M., Mandarino, J. A. (1977) New mineral names, American Mineralogist, 62, 173-176.John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, and Monte C. Nichols, Eds., (2003) Handbook of Mineralogy, Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org/. It is formed from the parent compound of CrO2 by means of one of two processes. The first process for the conversion of CrO2 into CrOOH occurs through a reduction of CrO2 in the presence of H2O and a reductant (
oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early inve ...
or steel) resulting in the chemical equation of (2CrO2 + H2O → 2CrO(OH) + O2). The second process is an oxidation of the chromium ion using a solution as a solvent. Such a reaction is represented by the chemical equation (3CrO2 + 2NaOH → Na2CrO4 + 2CrO(OH)).Shibasaki, Y. (1972) Synthesis of orthorhombic CrOOH and the reaction mechanism. Mat. Res. Bull. Vol. 7, pp. 1125-1134, 1972.


Structure

The identical chemical composition of guyanaite and other polymorphs of chromium oxide requires that the structure of the mineral become the primary characteristic in defining each mineral and differentiating them from one another, making it the single most significant attribute of guyanaite.Shpachenko, A. K., Sorokhtina, .N.V., Chukanov, N.V., Gorshkov, A.N., Sivtsov, A.V. (2006). "Genesis and compositional characteristics of natural γ-CrOOH." Geochemistry International 44(7): 681-689. Laboratory synthesized samples are identified by their separate crystal forms and denoted as α-CrOOH (grimaldiite), ß-CrOOH (guyanaite), and Γ-CrOOH (bracewellite).Fujihara, T., Ichikawa, M., Gustafsson, T., Olovsson, I., Tsuchida, T. (2002): Powder-neutron diffraction studies of geometric isotope and hydrogen-bonding effects in b-CrOOH. J. Phys. Chem. Solids, 63, 309–315. Guyanaite has an orthorhombic crystal structure, a space group of Pnnm, and has point group 2/m2/m2/m. Its cell dimensions are a = 4.857 Å , b = 4.295 Å , c = 2.958 Å and the structure is based upon a hexagonal closest packing of oxygen atoms parallel to (101) while edge-sharing CrO6 octahedra form along 01connected by oxygen-corners which forms layers of octahedral parallel to (101). Simply put, the Cr atoms are each surrounded by six oxygen atoms, and short hydrogen bonds are located in a mirror plane perpendicular to the c-axis. These bonds in neighboring planes are aligned in opposite directions to one another resulting in a lower level of symmetry than the parent compound. Studies done in order to clarify the
hydrogen bonding In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...
effect and determine if a hydrogen-centered model or hydrogen off centered model represented them best determined there to be no significant difference between either of the models due to the hydrogen-bond distance being so close to the critical distance.


Physical properties

Because of the high level of difficulty in obtaining a pure mineral sample of guyanaite, experimentation is carried out on samples of a known complex composition which is determined by x-ray and optical studies. The complex intergrowth of chromium oxide minerals results in poor samples for analyzing physical properties such as hardness, measured density, cleavage, habit, and luster giving incomplete data and an inability to determine values for each. A number of “merumite” grains shown to be almost entirely guyanaite by means of x-ray diffraction do however have a yellow-brown streak. The other known physical properties vary greatly based on which one of the two major locations this mineral is found. Samples from Guyana are distinguished by the brown, red, and sometimes green color of prismatic crystals as much as .1 mm long and a light-green to greenish-black variety is occasionally found forming in prismatic microcrystalline aggregates where samples from the Otokumpu mine in Finland occur as aggregates of golden-brown to greenish-brown fibers which replace smaller crystals of eskolaite that are less than 1.0 mm in size.


Geologic occurrence

Guyanaite as well as its polymorphs were discovered first in eskolaite from within alluvial shingle deposits of the Merume River in what was British Guiana where they occurred as fine-grained aggregates with one another. They were described as small rounded shingles in close association with quartz. Its presence with free
gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
,
pyrophylite Pyrophyllite is a phyllosilicate mineral composed of aluminium silicate hydroxide: Al2Si4O10(OH)2. It occurs in two forms ( habits): crystalline folia and compact masses; distinct crystals are not known. The folia have a pronounced pearly lu ...
rosettes, and double-terminated
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 ...
crystals also implies that these occurrences come from
hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
origins. It also occurs in
Finland Finland ( fi, Suomi ; sv, Finland ), officially the Republic of Finland (; ), is a Nordic country in Northern Europe. It shares land borders with Sweden to the northwest, Norway to the north, and Russia to the east, with the Gulf of B ...
in sulfide-rich veins cutting
skarn Skarns or tactites are hard, coarse-grained metamorphic rocks that form by a process called metasomatism. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate mineral ...
ified
quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...
s at the Outokumpu mine where it developed as fibrous
pseudomorph In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced b ...
s. Mineral associations include
carbonate mineral Carbonate minerals are those minerals containing the carbonate ion, . Carbonate divisions Anhydrous carbonates *Calcite group: trigonal **Calcite CaCO3 **Gaspéite (Ni,Mg,Fe2+)CO3 **Magnesite MgCO3 **Otavite CdCO3 **Rhodochrosite MnCO3 **Sider ...
s,
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
-bearing chromite,
rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer Polymorphism (materials science), polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest ...
,
uraninite Uraninite, formerly pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2 but because of oxidation typically contains variable proportions of U3O8. Radioactive decay of the uranium causes ...
, nolanite,
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
,
zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...
,
titanite Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals ...
, and
corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pres ...
as well as in chromium-rich
tremolite Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and fe ...
skarns, metaquartzites, and chlorite
veins Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...
. Due to its rare geologic occurrence much of experimental guyanaite is synthesized in a laboratory.


Special characteristics

Guyanaite has never played a significant historical or political role in any way primarily due to its relative isolation, rarity and abundance in very insignificant amounts. As an ore it had unfavorable views on its economic value and potential due to its low abundance and as a result has never held any significant role in industry or commerce. Although recently there have been experiments aimed at incorporating chromium oxides such as guyanaite for cathode materials in rechargeable lithium batteries, as cells created with chromium oxides may give a more efficient charge-discharge process compared with current technology, although no mention is made of the economic viability of using guyanaite and its polymorphs as opposed to current technology.Boldyrev, Y. I., Ivanova, N.D., Sokolsky, G.V., Ivanov, S.V., Stadnik, O.A. (2013). "Thin film nonstoichiometric chromium oxide-based cathode material for rechargeable and primary lithium batteries." Journal of Solid State Electrochemistry 17(8): 2213-2221.


Geographic location

Guyanaite as well as its polymorphs were discovered first in deposits of the Merume River in what was British Guiana and is currently known as Guyana. It is found most abundantly in the upper branches of Director Creek which is a small tributary of the Merume River flowing into the Mazaruni River. The nearest populated area is a government rest-house and hydroplane stop called Kamakusa of which the Merumite area is roughly southwest. The largest strip of merumite in Guyana along the base of the Robello Ridge consisting of Roraima-like
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...
s, conglomerates and
volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...
with the entire deposit located within a low-lying swampy and forested terrain which is contained between scarps of the Roraima formation and extensive talus slopes. The relatively small area in which the minerals are found indicates a local origin as well as indications of mild hydrothermal activity within the surrounding ridge rocks. The only other significant source of guyanaite occurs in the Outokumpu mine in Finland and despite the fact that it is a copper ore mine there was an estimated seven-million tons of metallic chromium alongside the copper, serpentine, and skarn rocks found within the mine.Kouvo, O. and Y. Vuorelainen (1958) Eskolaite, a new chromium mineral. Amer. Mineral., 43, 1098–1106.


References

{{reflist Chromium minerals Oxide minerals Orthorhombic minerals Minerals in space group 58