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Ramazzoite, the First Mineral with a Polyoxometalate Cation


Recently e-Rocks enabled the sale of a very rare mineral - Ramazzoite. The specimen above, measuring 1 × 2.5 × 1.2 cm, sold by Ligurian Minerals for £681. Found so far only at the type locality, Mount Ramazzo Mine, Genoa, Italy, the specimen is part of the holotype for ramazzoite; i.e. part of the single specimen from which all the data for the original description were obtained. The mine in Liguria is ancient, copper being extracted as far back as the mid 1400s, with iron and magnesium sulphates taken in the 1800s. Now it’s a tip for Genoa’s rubbish.

Description -

“The only available specimen and part of the holotype of the exceedingly rare ramazzoite (IMA-2017-090).
This very new mineral species, first occurrence in nature of  polyoxometalate cation, was collected on a single occasion by myself in 2016 at the Monte Ramazzo mine, Genoa, Italy, that dates from the 14th century.
The specimen is very rich and shows several greenish blue cubic crystals of ramazzoite up to about 0,1 mm that lie on Fe oxides.”

 

 

Ramazzoite, [Mg8Cu12(PO4)(CO3)4(OH)24(H2O)20][(H0.33SO4)3(H2O)36] is the first mineral described with a polyoxometalate cation (a positively charged ion). The Mindat page describes the mineral as “The first polyoxometallate [sic] mineral.” This is erroneous. It should read "The first cationic polyoxometalate mineral" Several POM (polyoxometalate) minerals exist, for instance kegginite. However, these minerals have polyoxometalate anions (negatively charged ions).

 

 

Polyoxometalates have a beautiful structure at a molecular level, with groups of (usually) transition metal atoms forming closed 3-dimensional frameworks - caged structures. The clusters are polyatomic ions, predominantly anions. The chemistry of polyoxometalates is an active research subject, as the compounds have a multitude of uses, particularly in the field of catalytic organic synthesis. What makes ramazzoite special is that the polyoxometalate is magnesium and copper based (unlike the usual group 5 & 6 transition metals such as vanadium or tungsten) and that it is cationic.

Reference:
▪ Kampf, A.R., Rossman, G.R., Ma, C., Belmonte, D., Biagioni, C., Castellaro, F., Chiappino, L. (2018): Ramazzoite, [Mg8Cu12(PO4)(CO3)4(OH)24(H2O)20][(H0.33SO4)3(H2O)36], the first mineral with a polyoxometalate cation. European Journal of Mineralogy, 30, (in press).

Abstract:
Ramazzoite Q1 (IMA2017-090), [Mg8Cu12(PO4)(CO3)4(OH)24(H2O)20][(H0.33SO4)3(H2O)36], is a new polyoxometalate (POM) mineral from the Monte Ramazzo mine, near Genova, Liguria, Italy. It occurs on magnetite-rich matrix in association with chlorartinite, chrysotile, dypingite, goethite, lepidocrocite and nesquehonite, and is a late-stage, secondary mineral crystallizing from low-temperature, aqueous solutions. Ramazzoite forms as simple cubes up to about 0.15mm on edge. The mineral is blue to greenish-blue with a vitreous to oily lustre and pale blue streak. Crystals are very brittle with conchoidal fracture, and a perfect cleavage on {100}. The Mohs’ hardness is 2½. The measured density is 1.98(1) g cm-3. The mineral is soluble with mild effervescence in dilute HCl at room temperature. Optically, ramazzoite is isotropic with n = 1.491(1) (white light). Electron microprobe analyses (with CO2 from structural data and H2O from density) gave the empirical formula [(Mg8.00)(Cu8.00Mg3.78)(PO4)(CO3)4(OH)24(H2O)20] [(H0.65S1.01O4)3(H2O)36], based on 1 P atom per formula unit (apfu). Ramazzoite is cubic, P-43m, with the unit-cell parameters: a = 13.3887(10)Å, V = 2400.0(5)Å3 and Z = 1. The crystal structure, refined to R1 = 0.064 for 803 observed reflections [I > 2σI], contains a novel [Mg8Cu12(PO4)(CO3)4(OH)24(H2O)20]5+ POM cation.

 

And Now for Something Completely Different

 

 

 

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