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Guyanaite
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 (Cr3O2) 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 unknown〔Tombs, 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 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 Pnnm, orthorhombic with space group Pbnm, and hexagonal 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 or steel) resulting in the chemical equation of (2CrO2 + H2O → 2CrO(OH) + ½O2). 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.〕
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