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A carbonate fluoride, fluoride carbonate, fluorocarbonate or fluocarbonate is a double salt containing both
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
and
fluoride
Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts typ ...
. The salts are usually insoluble in water, and can have more than one kind of metal cation to make more complex compounds.
Rare-earth
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
fluorocarbonates are particularly important as ore minerals for the light rare-earth elements
lanthanum
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lantha ...
,
cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 o ...
and
neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes i ...
.
Bastnäsite
The mineral bastnäsite (or bastnaesite) is one of a family of three carbonate-fluoride minerals, which includes bastnäsite-( Ce) with a formula of (Ce, La)CO3F, bastnäsite-( La) with a formula of (La, Ce)CO3F, and bastnäsite-( Y) with a formul ...
is the most important source of these elements. Other artificial compounds are under investigation as
non-linear optical materials and for transparency in the
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
, with effects over a dozen times greater than
Potassium dideuterium phosphate.
Related to this there are also
chlorocarbonates and
bromocarbonates. Along with these fluorocarbonates form the larger family of halocarbonates. In turn halocarbonates are a part of
mixed anion materials. Compounds where fluorine connects to carbon making acids are unstable,
fluoroformic acid decomposes to carbon dioxide and hydrogen fluoride, and
trifluoromethyl alcohol also breaks up at room temperature.
Trifluoromethoxide compounds exist but react with water to yield
carbonyl fluoride
Carbonyl fluoride is a chemical compound with the formula COF2. It is a carbon oxohalide. This gas, like its analog phosgene, is colourless and highly toxic. The molecule is planar with ''C''2v symmetry, bond lengths of 1.174 Å (C=O) and 1.312 ...
.
Structures
The structure of the carbonate fluorides is mainly determined by the carbonate anion, as it is the biggest component. The overall structure depends on the ratio of carbonate to everything else, i.e. number (metals and fluorides)/number of carbonates. For ratios from 1.2 to 1.5 the carbonates are in a flat dense arrangement. From 1.5 to 2.3 the orientation is edge on. From 2.5 to 3.3 the arrangement is flat open. With a ratio from 4 to 11, the carbonate arrangement is flat-lacunar.
The simplest formula is LnCO
3F, where Ln has a 3+ charge.
For monocations there is A
3CO
3F, where A is a large ion such as K, Rb or Tl.
For M =
alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
, and Ln = lanthanide: MLnCO
3F
2 1:1:1:2; M
3Ln(CO
3)
2F
2 3:1:2:2; M
2Ln(CO
3)
2F 2:1:2:1; M
4Ln(CO
3)
2F
3·H
2O 4:1:2:3; M
4Ln
2(CO
3)
3F
4 2:3:3:4.
M
2Ln(CO
3)F
2 2:1:1:3.
For B =
alkaline earth
The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are all s ...
and Ln = lanthanide (a triple-charged ion) BLn(CO
3)
2F 1:1:2:1; BLn
2(CO
3)
3F
2 1:2:3:2 B
2Ln
3(CO
3)
5F
3 2:3:5:3; B
2Ln(CO
3)
2F
3 2:1:2:3; B
2Ln(CO
3)F
5 2:1:1:5 B
2Ln(CO
3)
3F 2:1:3:1; B
3Ln(CO
3)F
7 3:1:1:7; B
3Ln
2(CO
3)
5F
2 3:2:5:2.
For alkali with dication combinations: MB: MBCO
3F MB
3(CO
3)
2F
3·H
2O.
For dications A and B there is ABCO
3F
2 with a degenerate case of A = B.
KPb
2(CO
3)
2F is layered. Each layer is like a sandwich, with lead and carbonate in the outer sublayers, and potassium and fluoride in the inner layer. K
2.70Pb
5.15(CO
3)
5F
3 extends this structure with some of the layers also being a double-decker sandwich of carbonate, fluoride, carbonate, fluoride, carbonate.
In the rare-earth fluorocarbonates the environment for the rare-earth atoms is 9-coordinated. Six oxygen atoms from carbonate are at the apices of a trigonal prism, and fluoride ions cap the rectangular faces of the prism.
Formation
Carbonate fluoride compounds can be formed by a variety of related methods involving heating the precursor ingredients with or without water. Thallous fluoride carbonate was made simply by evaporating a fluoride thallium solution in ethanol and water in air. It absorbed sufficient carbon dioxide to yield the product. Most other carbonate fluorides are very insoluble and need high-temperature water to crystallise from. Supercritical water heated between 350 and 750 °C under pressures around 200 bars can be used. A sealed platinum tube can withstand the heat and pressure. Crystallisation takes about a day. With subcritical water around 200 °C, crystallisation takes about 2 days. This can happen in a teflon-coated pressure autoclave. The starting ingredients can be rare-earth fluorides and alkali carbonates. The high pressure is needed to keep the water liquid and the carbon dioxide under control, otherwise it would escape. If the fluoride levels are low, hydroxide can substitute for the fluoride. Solid-state reactions require even higher temperatures.
Bastnäsite along with lukechangite (and
petersenite) can be precipitated from a mixed solution of CeCl
3, NaF, and NaOH with carbon dioxide.
Another way to make the simple rare-earth fluorocarbonates is to precipitate a rare-earth carbonate from a nitrate solution with
ammonium bicarbonate
Ammonium bicarbonate is an inorganic compound with formula (NH4)HCO3. The compound has many names, reflecting its long history. Chemically speaking, it is the bicarbonate salt of the ammonium ion. It is a colourless solid that degrades readily to ...
and then add fluoride ions with
hydrofluoric acid
Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...
(HF).
Pb
2(CO
3)F
2 can be made by boiling a water solution of
lead nitrate,
sodium fluoride
Sodium fluoride (NaF) is an inorganic compound with the formula . It is used in trace amounts in the fluoridation of drinking water, in toothpaste, in metallurgy, and as a flux. It is a colorless or white solid that is readily soluble in water. I ...
and
potassium carbonate
Potassium carbonate is the inorganic compound with the formula K2 CO3. It is a white salt, which is soluble in water. It is deliquescent, often appearing as a damp or wet solid. Potassium carbonate is mainly used in the production of soap and gl ...
in a 2:2:1 molar ratio.
Properties
The visible spectrum of fluorocarbonates is determined mainly by the cations contained. Different structures only have slight effect on the absorption spectrum of rare-earth elements.
The visible spectrum of the rare-earth fluorocarbonates is almost entirely due to narrow absorption bands from
neodymium
Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes i ...
.
In the
near infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
around 1000 nm there are some absorption lines due to
samarium
Samarium is a chemical element with symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. Being a typical member of the lanthanide series, samarium usually has the oxidation state +3. Compounds of samar ...
and around 1547 nm are some absorption features due to
praseodymium
Praseodymium is a chemical element with the Symbol (chemistry), symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered to be one of the rare-earth metals. It is a soft, silvery, malleable and ductile ...
. Deeper into the infrared, bastnäsite has carbonate absorption lines at 2243, 2312 and 2324 nm. Parisite only has a very weak carbonate absorption at 2324 nm, and synchysite absorbs at 2337 nm.
The infrared spectrum due to vibration of carbon–oxygen bonds in carbonate is affected by how many kinds of position there are for the carbonate ions.
Reactions
An important chemical reaction used to prepare rare-earth elements from their ores, is to roast concentrated rare-earth fluorocarbonates with sulfuric acid at about 200 °C. This is then leached with water. This process liberates carbon dioxide and hydrofluoric acid and yields rare-earth sulfates:
: 2 LnCO
3F + 3 H
2SO
4 → Ln
2(SO
4)
3 + 2 HF + 2 H
2O + 2 CO
2.
Subsequent processing precipitates a double sulfate with sodium sulfate at about 50 °C. The aim is to separate out the rare-earth elements from calcium, aluminium, iron and thorium.
At high enough temperatures the carbonate fluorides lose carbon dioxide, e.g.
: KCu(CO
3)F → CuO + KF + CO
2
at 340 °C.
The processing of bastnäsite is important, as it is the most commonly mined
cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 o ...
mineral. When heated in air or oxygen at over 500 °C, bastnäsite oxidises and loses volatiles to form
ceria
Cerium(IV) oxide, also known as ceric oxide, ceric dioxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare-earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO2. It is an important commercial produ ...
(CeO
2). Lukechangite also oxidises to ceria and sodium fluoride (NaF). Ce
7O
12 results when heated to over 1000 °C.
: 2 Ce(CO
3F) + O
2 → 2 CeO
2 + 2 CO
2 + F
2
: Na
3Ce
2(CO
3F)
4F + O
2 → 2 CeO
2 + 3 CO
2 + NaF + Na
2CO
3
At 1300 °C Na
2CO
3 loses CO
2, and between 1300 and 1600 °C NaF and Na
2O boil off.
When other rare-earth carbonate fluorides are heated, they lose carbon dioxide and form an oxyfluoride:
: LaCO
3F → LaOF + CO
2
In some rare-earth extraction processes, the roasted ore is then extracted with hydrochloric acid to dissolve rare earths apart from cerium. Cerium is dissolved if the pH is under 0, and thorium is dissolved if it is under 2.
KCdCO
3F when heated yields
cadmium oxide
Cadmium oxide is an inorganic compound with the formula CdO. It is one of the main precursors to other cadmium compounds. It crystallizes in a cubic rocksalt lattice like sodium chloride, with octahedral cation and anion centers. It occurs na ...
(CdO) and
potassium fluoride
Potassium fluoride is the chemical compound with the formula KF. After hydrogen fluoride, KF is the primary source of the fluoride ion for applications in manufacturing and in chemistry. It is an alkali halide and occurs naturally as the rare m ...
(KF).
When lanthanum fluorocarbonate is heated in a hydrogen sulfide, or carbon disulfide vapour around 500 °C, lanthanum fluorosulfide forms:
: LaCO
3F + CO
2 → LaSF + 1.5 CO
2
Note that this also works for other lanthanides apart from cerium.
When lanthanum carbonate fluoride is heated at 1000 °C with alumina,
lanthanum aluminate
Lanthanum aluminate is an inorganic compound with the formula LaAlO3, often abbreviated as LAO. It is an optically transparent ceramic oxide with a distorted perovskite structure.
Properties
Crystalline LaAlO3 has a relatively high relative diel ...
is produced:
: LaCO
3F + 2 Al
2O
3 → LaAlO
3 + CO
2 + equiv AlOF
Within the hot part of the Earth's crust, rare-earth fluorocarbonates should react with
apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...
to form
monazite
Monazite is a primarily reddish-brown phosphate mineral that contains rare-earth elements. Due to variability in composition, monazite is considered a group of minerals. The most common species of the group is monazite-(Ce), that is, the cerium- ...
.
Minerals
Some
rare-earth
The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
fluorocarbonate minerals exist. They make up most of the economic ores for light rare-earth elements (LREE). These probably result from hydrothermal liquids from granite that contained fluoride. Rare-earth fluorocarbonate minerals can form in
bauxite
Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO(O ...
on
carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolosto ...
s, as rare-earth fluoride complexes react with carbonate. Carbonate fluoride compounds of rare-earth elements also occur in
carbonatite
Carbonatite () is a type of intrusive rock, intrusive or extrusive rock, extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geoche ...
s.
Artificial
These are non-linear optical crystals in the AMCO
3F family
KSrCO
3F
KCaCO
3F
RbSrCO
3F
KCdCO
3F
CsPbCO
3F
RbPbCO
3F
RbMgCO
3F
KMgCO
3F
RbCdCO
3F
CsSrCO
3F
RbCaCO
3F
KZnCO
3F
CsCaCO
3F
RbZnCO
3F
References
{{Reflist
Fluorides
Carbonates
Mixed anion compounds
Double salts