Grimaldiite (α-CrOOH) Structure: ABC2_hR4_160_a_a

Prototype	CrHO $_{2}$
AFLOW prototype label	ABC2_hR4_160_a_a_2a-002
Mineral name	grimaldiite
ICSD	64754
Pearson symbol	hR4
Space group number	160
Space group symbol	$R 3 m$
AFLOW prototype command	aflow --proto=ABC2_hR4_160_a_a_2a-002 --params= $a, c / a, x_{1}, x_{2}, x_{3}, x_{4}$

CrOOH is found naturally in three forms. They are usually found together with Cr $_{2}$ O $_{3}$ in a mineral known as merumite (Milton, 1976):
- Grimaldiite, rhombohedral $α$ –CrOOH (this structure),
- Guyanaite, orthorhombic $β$ –CrOOH, and
- Bracewellite, orthorhombic $γ$ –CrOOH is in the Lepidocrocite ( $γ$ –FeOOH, $E 0_{4}$ ) structure.
We use the $R 3 m$ data from Table IIIa (Christensen,1977), which locates the hydrogen atoms.
Space group $R 3 m$ #160 does not specify the origin of the $z$ -axis. Here we choose $z_{I} = 0$ for the position of the chromium atom.
$α$ –CrOOH and CrCuS $_{2}$ have the AFLOW prototype label, ABC2_hR4_160_a_a_2a. They are generated by the same symmetry operations with different sets of parameters (--params) specified in their corresponding CIF files.
Hexagonal settings of this structure can be obtained with the option --hex.

a_{1} a_{2} a_{3} = = = \frac{1}{2} a \hat{x} - \frac{3}{6} a \hat{y} + \frac{1}{3} c \hat{z} \frac{1}{3} a \hat{y} + \frac{1}{3} c \hat{z} - \frac{1}{2} a \hat{x} - \frac{3}{6} a \hat{y} + \frac{1}{3} c \hat{z}

Basis vectors

		Lattice coordinates		Cartesian coordinates	Wyckoff position	Atom type
$B_{1}$	=	$x_{1} a_{1} + x_{1} a_{2} + x_{1} a_{3}$	=	$c x_{1} \hat{z}$	(1a)	Cr I
$B_{2}$	=	$x_{2} a_{1} + x_{2} a_{2} + x_{2} a_{3}$	=	$c x_{2} \hat{z}$	(1a)	H I
$B_{3}$	=	$x_{3} a_{1} + x_{3} a_{2} + x_{3} a_{3}$	=	$c x_{3} \hat{z}$	(1a)	O I
$B_{4}$	=	$x_{4} a_{1} + x_{4} a_{2} + x_{4} a_{3}$	=	$c x_{4} \hat{z}$	(1a)	O II

A. N. Christensen, P. Hansen, and M. S. Lehmann, Isotope effects in the bonds of α-CrOOH and α-CrOOD, J. Solid State Chem. 21, 325–329 (1977), doi:10.1016/0022-4596(77)90130-X.
C. Milton, D. E. Appleman, M. H. Appleman, E. C. T. Chao, F. Cuttitta, J. I. Dinnin, E. J. Dwornik, B. L. Ingram, and J. H. J. Rose, Merumite – A Complex Assemblage of Chromium Minerals from Guyanna (1976). Geological Survey Professional Paper 887.

R. T. Downs and M. Hall-Wallace, The American Mineralogist Crystal Structure Database, Am. Mineral. 88, 247–250 (2003).

Encyclopedia of Crystallographic Prototypes