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Qualitative Analysis

Qualitative Analysis

For solutions containing mixtures of different cations, the identity of each cation can be determined by qualitative analysis. This technique involves a series of selective precipitations with different chemical reagents, each reaction producing a characteristic precipitate for a specific group of cations. Metal ions within a group are further separated by varying the pH, heating the mixture to redissolve a precipitate, or adding other reagents to form complex ions.
For instance, group IV...

Ionic Crystal Structures

Ionic Crystal Structures

Ionic crystals consist of two or more different kinds of ions that usually have different sizes. The packing of these ions into a crystal structure is more complex than the packing of metal atoms that are the same size.
Most monatomic ions behave as charged spheres, and their attraction for ions of opposite charge is the same in every direction. Consequently, stable structures for ionic compounds result (1) when ions of one charge are surrounded by as many ions as possible of the opposite...

Common Ion Effect

Common Ion Effect

Compared with pure water, the solubility of an ionic compound is less in aqueous solutions containing a common ion (one also produced by dissolution of the ionic compound). This is an example of a phenomenon known as the common ion effect, which is a consequence of the law of mass action that may be explained using Le Châtelier’s principle. Consider the dissolution of silver iodide:

Corrosion

Corrosion

The degradation of metals due to natural electrochemical processes is known as corrosion. Rust formation on iron, tarnishing of silver, and the blue-green patina that develops on copper are examples of corrosion. Corrosion involves the oxidation of metals. Sometimes it is protective, such as the oxidation of copper or aluminum, wherein a protective layer of metal oxide or its derivatives forms on the surface, protecting the underlying metal from further oxidation. In other cases, corrosion is...

Titration of Polyprotic Base with a Strong Acid

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The titration of a polyprotic base such as sodium carbonate with a strong acid such as hydrochloric acid results in two equivalence points on the titration curve. At the first equivalence point, the carbonate ions in the base are completely converted to bicarbonate ions. The second equivalence point corresponds to the complete conversion of bicarbonate ions to carbonic acid, which dissociates into carbon dioxide and water. The region before the first equivalence point corresponds to the...

Ionic Compounds: Formulas and Nomenclature

Ionic Compounds: Formulas and Nomenclature

An element composed of atoms that readily lose electrons (a metal) can react with an element composed of atoms that readily gain electrons (a nonmetal) to produce ions through complete electron transfer. The compound formed by this transfer is stabilized by the electrostatic attractions (ionic bonds) between the oppositely charged ions.

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Robertsite, Ca(2)Mn(III) (3)O(2)(PO(4))(3)·3H(2)O.

Marcelo B Andrade¹, Shaunna M Morrison, Adrien J Di Domizio

¹Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721-0077, USA.

Acta Crystallographica. Section E, Structure Reports Online

|November 6, 2012

Summary

Robertsite, a calcium manganese phosphate mineral, was structurally determined using X-ray diffraction. Its unique manganese-oxide sheets and calcium coordination provide insights into the arseniosiderite group.

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Area of Science:

Mineralogy
Crystallography
Inorganic Chemistry

Background:

Robertsite (Ca(2)Mn(3)O(2)(PO(4))(3)·3H(2)O) belongs to the arseniosiderite group (Ca(2)A(3)O(2)(TO(4))(3)·nH(2)O).
Previous studies reported structures of other arseniosiderite group members.

Purpose of the Study:

Determine the crystal structure of robertsite.
Refine anisotropic displacement parameters for all atoms.
Compare robertsite's structure to other arseniosiderite group members.

Main Methods:

Single-crystal X-ray diffraction was performed on a twinned crystal from the Tip Top mine, South Dakota.
Anisotropic displacement parameters were refined for all atoms.

Main Results:

The robertsite structure features sheets of [MnO(6)] octahedra arranged in nine-membered pseudo-trigonal rings.
Phosphate tetrahedra (PO(4)) are located at the center and sandwich the Mn-oxide sheets.
Six distinct Ca(2+) ions are seven-coordinated in distorted pentagonal bipyramids ([CaO(5)(H(2)O)(2)]).
Hydrogen bonding involving water molecules and calcium coordination stabilize the structure.
All [MnO(6)] octahedra exhibit distortion due to the Jahn-Teller effect.

Conclusions:

The determined structure of robertsite is consistent with the general features of the arseniosiderite group.
The Jahn-Teller distortion of MnO(6) octahedra is a key characteristic of robertsite.
Interactions between manganese-phosphate sheets, calcium ions, and water molecules dictate the overall structural stability.