Parascorodite is a rare,

secondary Secondary may refer to: Science and nature * Secondary emission, of particles ** Secondary electrons, electrons generated as ionization products * The secondary winding, or the electrical or electronic circuit connected to the secondary winding i ...

iron-

arsenate mineral Arsenate minerals usually refer to the naturally occurring orthoarsenates, possessing the (AsO4)3− anion group and, more rarely, other arsenates with anions like AsO3(OH)2− (also written HAsO42−) (example: pharmacolite Ca(AsO3OH).2H2O) or (v ...

. It has a

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

of () and was discovered in 1967 using X-ray powder diffraction methods, when an unknown substance was found along with

scorodite Scorodite is a common hydrated iron arsenate mineral, with the chemical formula FeAsO4·2H2O. It is found in hydrothermal deposits and as a secondary mineral in gossans worldwide. Scorodite weathers to limonite. Scorodite was discovered in the S ...

on medieval ore dumps in the Czech Republic. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

of parascorodite can be found in the mineralogical collection of the National Museum, Prague, Czech Republic under acquisition number P1p25/98.

Occurrence

Parascorodite occurs at the Kank mine in the Kutna Hora ore district in Central Bohemia,

Czech Republic The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as Bohemia, it is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. The ...

. It is one of the rarest secondary minerals. Parascorodite is found in medieval ore dumps, that were most likely used for

silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

and polymetallic ore waste. The dumps contain

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...

rich ore, which in medieval times was considered waste.

Paragenesis

The medieval ore dumps are heavily weathered, but it is assumed that parascorodite, along with other secondary iron arsenates and arsenosulfates, actually formed much before the dumping of waste material on this area by natural weathering processes. Parascorodite formed as a product of

arsenopyrite Arsenopyrite ( IMA symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard ( Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases eleme ...

dissolution, followed by recrystallization of iron-arsenic bearing solutions, in near surface weathering conditions. Parascorodite is dimorphous with scorodite, and is also associated with pitticite,

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...

jarosite Jarosite is a basic hydrous sulfate of potassium and ferric iron (Fe-III) with a chemical formula of KFe3(SO4)2(OH)6. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides. Jarosite is often produced as a byproduct duri ...

, and amorphous

ferric hydroxide Iron(III) oxide-hydroxide or ferric oxyhydroxideA. L. Mackay (1960): "β-Ferric Oxyhydroxide". ''Mineralogical Magazine'' (''Journal of the Mineralogical Society''), volume 32, issue 250, pages 545-557. is the chemical compound of iron, oxygen, ...

Physical properties

Parascorodite occurs in aggregates of somewhat hemispherical shapes. The aggregates grow to be about 2 cm across, consisting of extremely small crystals that can be arranged in fan-like or irregular masses. Parascorodite is

cryptocrystalline Cryptocrystalline is a rock microstructure, rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert a ...

, and has a luster that can vary from earthy to vitreous. It is a soft mineral, falling between 1-2 on the Mohs hardness scale. Aggregates can be white to yellowish, or more rarely green-grey in color, and have a yellow-white streak. The measured density of earthy aggregates in ethyl alcohol is 3.212 g/cm³. The rare green-grey variety of parascorodite aggregates may exhibit

conchoidal fracture Conchoidal fracture describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows: "a fracture with smooth, curved surfaces, typically slig ...

. Individual crystal size varies between 0.1 μm and 0.5 μm, with some twinned crystals measuring 1.0 μm. Crystals occur as either prisms or thin flakes with a hexagonal outline. Parascorodite dissolves slowly in 10%

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

(HCl). In water, it will disintegrate rapidly into a powder. Under hydrothermal conditions parascorodite can re-crystallize back to scorodite.

Chemical composition

The chemical composition of parascorodite was determined using

qualitative spectral analysis Qualitative descriptions or distinctions are based on some quality or characteristic rather than on some quantity or measured value. Qualitative may also refer to: *Qualitative property, a property that can be observed but not measured numericall ...

. Two major elements were indicated:

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

and

. Quantitative analysis was also determined using two wet chemical analyses (results in the table below).

Crystallography

X-ray diffraction

The crystal structure of parascorodite was determined using X-ray powder diffraction. Using the X-ray diffraction data (in the table below), the parascorodite unit cell was determined as

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 ...

or trigonal. The unit cell parameters are

a_1,a_2

= 8.9327(5)Å,

c

= 9.9391(8)Å, with a cell volume of

V

= 686.83(8)Å³.

References

{{Reflist Arsenate minerals Hydrates Iron minerals Hexagonal minerals Minerals described in 1967