**7.5. Non-apatitic phosphate minerals**

Apart from those in the supergroup of apatite minerals, the well-known phosphate minerals include [25]:

**1. Autunite** [110],[111]: is orthorhombic mineral (space group I4/MMM) of the composition of (Ca[(UO2)(PO4)]2(H2O)11, **Fig. 16**), which crystallizes in the space group Pnma with the cell parameters: *a* = 14.0135 Å, *b* = 20.7121 Å, *c* = 6.9959 Å, *V* = 2030.55 Å3 and *Z* = 4. It belongs to the most abundant and widely distributed uranyl phosphate minerals. The structure contains the well-known autunite-type sheet with the composition [(UO2)(PO4)] <sup>−</sup> resulting from the sharing of equatorial vertices of uranyl square bipyramids with phosphate tetrahedra. Calcium atom in the interlayeris coordinated by seven H2O groups and two longer distances to uranyl apical O atoms. Two symmetrically independent H2O groups are held in the structure only by hydrogen bonding. The bond-length-constrain‐ ed refinement provides a crystal-chemically reasonable description of the hydrogen bonding.

**Fig. 16.** The examples of forms and the structure of mineral atunite [110] viewed along the b-axis.

The mineral was named by HENRY J. BROOKE and WILLIAM H. MILLER in 1854 afterthe typical locality at Saint Symphorien, Autun District, Saône-et-Loire, France. Autunite dehy‐ drates rapidly in air (except for high relative humidity) to tetragonal meta-autunite (P4/ NMM, *a* = 6.96 Å, *c* = 8.40 Å and *c*:*a* = 1.21) [112]. The loss of O12 and O13 from autunite results in the formula Ca[(UO2)(PO4)]2(H2O)7 (**Fig. 17**) [110].

projects will become viable and the production will be stimulated. In the future, fuel and fuelrelated transportation costs may become even more important components in the world phosphate rock production scenario. The political disruption is always an unknown factor, and it can profoundly influence the supply and demand for fertilizer raw materials on a

366 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

Apart from those in the supergroup of apatite minerals, the well-known phosphate minerals

**1. Autunite** [110],[111]: is orthorhombic mineral (space group I4/MMM) of the composition of (Ca[(UO2)(PO4)]2(H2O)11, **Fig. 16**), which crystallizes in the space group Pnma with the cell parameters: *a* = 14.0135 Å, *b* = 20.7121 Å, *c* = 6.9959 Å, *V* = 2030.55 Å3 and *Z* = 4. It belongs to the most abundant and widely distributed uranyl phosphate minerals. The structure contains the well-known autunite-type sheet with the composition [(UO2)(PO4)] <sup>−</sup> resulting from the sharing of equatorial vertices of uranyl square bipyramids with phosphate tetrahedra. Calcium atom in the interlayeris coordinated by seven H2O groups and two longer distances to uranyl apical O atoms. Two symmetrically independent H2O groups are held in the structure only by hydrogen bonding. The bond-length-constrain‐ ed refinement provides a crystal-chemically reasonable description of the hydrogen

**Fig. 16.** The examples of forms and the structure of mineral atunite [110] viewed along the b-axis.

results in the formula Ca[(UO2)(PO4)]2(H2O)7 (**Fig. 17**) [110].

The mineral was named by HENRY J. BROOKE and WILLIAM H. MILLER in 1854 afterthe typical locality at Saint Symphorien, Autun District, Saône-et-Loire, France. Autunite dehy‐ drates rapidly in air (except for high relative humidity) to tetragonal meta-autunite (P4/ NMM, *a* = 6.96 Å, *c* = 8.40 Å and *c*:*a* = 1.21) [112]. The loss of O12 and O13 from autunite

worldwide basis [22].

include [25]:

bonding.

**7.5. Non-apatitic phosphate minerals**

**Fig. 17.** The examples of forms and the structure of mineral meta-atunite [112] viewed along the b-axis.

**2. Crandallite** [113]: CaAl3(OH)6[PO3(O1/2(OH)1/2]2, has hexagonal structure (*a* = 7.005 Å and *c* = 16.192 Å), which is analogous with alunite. The structure of mineral (**Fig. 18**) con‐ sists of corner-sharing Al octahedra, which are linked into trigonal and hexagonal rings to form the sheets perpendicular to the c-axis. Ca ions, surrounded by 12 oxygen and hydroxyl ions, lie in large cavities between the sheets. Each phosphate tetrahedron shares three corners with three Al octahedra from a trigonal ring in the sheet. The unshared corner is turned away from the trigonal hole towards the adjacent sheet to which it is hydrogen bonded. The mineral "deltaite" was found to be identical to crandallite, within the accuracy of the structural results.

**Fig. 18.** The structure of mineral crandallite [113] viewed along the c-axis.

**3. Lazulite** [114],[115]: monoclinic mineral of the composition of MgAl2(PO4)2(OH)2, which crystallizes in the P21/C space group and has the cell parameters: *a* = 7.16 Å, *b* = 7.26 Å and *c* = 7.24 Å and *β* = 120.67°. The mineral is the magnesium analogue of scorzalite (Fe2+Al2(PO4)2(OH)2). Named in 1795 by MARTEN H. KLAPROTH from the Arabic word meaning "heaven," in allusion to its color.

**Fig. 19.** The form and the structure of lazulite [114] viewed along the c-axis.


**Fig. 20.** The structure of monazite-(Ce): *a* = 6.7902 Å, *b* = 7.0203 Å, *c* = 6.4674 Å and *β* = 103.38° (a) and monazite-(Sm) (b): *a* = 6.6818 Å, *b* = 6.8877 Å, *c* = 6.3653 Å and *β* = 103.386° (b) [118] viewed along the b-axis.

Monazite (**Fig. 20**) and xenotime dimorphs31 [119] are the most ubiquitous rare-earth (REE) minerals. Monazite incorporates preferentially larger, light rare-earth elements (LREEs, here, La-Gd), whereas xenotime tends to incorporate smaller, heavy rare-earth elements (HREEs, here, Tb-Lu, and Y).

<sup>31</sup> One chemical compound is capable of crystallizing in two different systems, e.g. CaCO3 can occur as calcite or aragonite [119].

**Fig. 19.** The form and the structure of lazulite [114] viewed along the c-axis.

368 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

collected the first specimens.

Monazite (**Fig. 20**) and xenotime dimorphs31

(HREEs, here, Tb-Lu, and Y).

= 4).

[119].

**4. Millisite** [116]: tetragonal mineral of the composition of NaCaAl6(PO4)4(OH)9·3H2O (the space group P41212, *a* = 7.00 Å and *c* = 18.99Å, *a:c* = 1:2.713 and *Z* = 2), which was named in 1942 by ESPER SIGNIUS LARSEN III in the honor of MR. F.T. MILLIS of Lehi, Utah, who had

**5. Monazite** [117],[118]: is natural lightrare-earth element phosphate that generally contains large amounts of uranium and thorium. The monazite-type compounds (AXO4, **Fig. 21**) form an extended family that is described in this review in the terms of field of stability versus composition. They crystallize in a monoclinic lattice with the space group P21/N (*Z*

**Fig. 20.** The structure of monazite-(Ce): *a* = 6.7902 Å, *b* = 7.0203 Å, *c* = 6.4674 Å and *β* = 103.38° (a) and monazite-(Sm) (b): *a* = 6.6818 Å, *b* = 6.8877 Å, *c* = 6.3653 Å and *β* = 103.386° (b) [118] viewed along the b-axis.

minerals. Monazite incorporates preferentially larger, light rare-earth elements (LREEs, here, La-Gd), whereas xenotime tends to incorporate smaller, heavy rare-earth elements

31 One chemical compound is capable of crystallizing in two different systems, e.g. CaCO3 can occur as calcite or aragonite

[119] are the most ubiquitous rare-earth (REE)

**Fig. 21.** The classification diagram of AXO4-type compounds. The monazite stability domain is colored by gray [117].

**Fig. 22.** The structure of tobernite and isostructural zeunerite [120] viewed along the c-axis.

**6. Tobernite** [120],[121]: tetragonaltorbernite (P4/NNC, *a* = 7.0267 Å, *c* = 20.8070 Å, *c*:*a* = 2.9611, Cu[(UO2)(PO4)]2(H2O)12, **Fig. 22**(**a**)) and zeunerite (Cu[(UO2)(AsO4)]2(H2O)12, **Fig. 22**(**b**)), as well as metatorbernite (Cu[(UO2)(PO4)]2(H2O)8) and metazeunerite (Cu[(UO2) (AsO4)]2(H2O)8), belong to the autunite and meta-autunite groups, respectively, which make up together approximately 40 mineral species of hydrated uranyl phosphates and arsenates. The structures, compositions and stabilities of minerals of the autunite and meta-autunite groups are of high interest because of their environmental significance. They are widespread and abundant and exert an impact on the mobility of uranium in phosphate-bearing systems and soils contaminated by actinides.

These minerals contain the autunite-type sheet, of the composition of [(UO2)(PO4)]<sup>−</sup> , which involves the sharing of equatorial vertices of uranyl square bipyramids with phosphate tetrahedra. In each of these structures, Cu2+ cations are located between the sheets in Jahn-Teller32 [122] distorted (4 + 2) octahedra, with short bonds to four H2O groups in a squareplanar arrangement and two longer distances to oxygen atoms of uranyl ions. A symmetrically independent H2O group is held in each structure only by H-bonding, and in torbernite (and in zeunerite), it forms the square-planar sets of interstitial H2O groups both above and below the planes of Cu2+ cations. In metatorbernite (and in metazeuner‐ ite), the square-planar sets of interstitial H2O groups are either above or below the planes of Cu2+ cations. The bond-length-constrained refinement provides the crystal-chemically reasonable descriptions of H-bonding in those four structures [120].

**7. Turquoise** [123],[124]: CuAl6(PO4)4(OH)8·4H2O is a copper analogue of triclinic mineral fausite ZnAl6(PO4)4(OH)8·4H2O (the space group P1 with the cell parameters *a* = 7.410 Å, *b* = 7.633 Å, *c* = 9.904 Å, *α* = 68.4°, *β* = 69.65° and *γ* = 65.05°). The structure (**Fig. 23**) consists of distorted MO6 polyhedra (M= Zn, Cu), AIO6 octahedra and PO4 tetrahedra. By the edgeand corner-sharing of these polyhedra, a fairly dense three-dimensional framework is formed, which is further strengthened by a system of hydrogen bonds. The metal atoms in the unique MO6 (M = Zn or Cu) polyhedron show a distorted [2 + 2 + 2] coordination, the distortion being more pronounced in turquoise. About 10% of the M site is vacant in both minerals. In turquoise, a previously undetected structural site with very low occupancy of (possibly) Cu is present at the position (1/2,0,1/2).

**Fig. 23.** The structure of turquoise viewed along the b-axis [124].

<sup>32</sup> According to the Jahn-Teller theorem, any nonlinear molecule in a degenerate electronic state will be unstable and will undergo some kind of distortion that will lower its symmetry so as to remove the degeneracy of the electronic state and also to attain lower energy. The Jahn-Teller effect is termed as static when there is permanent distortion in the structure of molecule [122].

**8. Vivianite** [125],[126],[127]: is monoclinic mineral of the composition of Fe3(PO4)3·8H2O, which crystallizes in the space group of C1 2 /M with the cell parameters *a* = 10.08 Å, *b* = 13.43 Å, *c* = 4.70 Å and *β* = 104.50° (**Fig. 24**). The mineral was named by ABRAHAM GOTTLOB WERNER in 1817 after JOHN HENRY VIVIAN. Vivianite belongs to the simplest group of minerals with the composition given by general formula: A3(XO4)2·8H2O, where A = Mg, Zn, Ni, Co or Fe and X is P or As [128].

**Fig. 24.** The structure of vivanite viewed along the b-axis [125].

make up together approximately 40 mineral species of hydrated uranyl phosphates and arsenates. The structures, compositions and stabilities of minerals of the autunite and meta-autunite groups are of high interest because of their environmental significance. They are widespread and abundant and exert an impact on the mobility of uranium in

involves the sharing of equatorial vertices of uranyl square bipyramids with phosphate tetrahedra. In each of these structures, Cu2+ cations are located between the sheets in Jahn-

 [122] distorted (4 + 2) octahedra, with short bonds to four H2O groups in a squareplanar arrangement and two longer distances to oxygen atoms of uranyl ions. A symmetrically independent H2O group is held in each structure only by H-bonding, and in torbernite (and in zeunerite), it forms the square-planar sets of interstitial H2O groups both above and below the planes of Cu2+ cations. In metatorbernite (and in metazeuner‐ ite), the square-planar sets of interstitial H2O groups are either above or below the planes of Cu2+ cations. The bond-length-constrained refinement provides the crystal-chemically

, which

These minerals contain the autunite-type sheet, of the composition of [(UO2)(PO4)]<sup>−</sup>

**7. Turquoise** [123],[124]: CuAl6(PO4)4(OH)8·4H2O is a copper analogue of triclinic mineral fausite ZnAl6(PO4)4(OH)8·4H2O (the space group P1 with the cell parameters *a* = 7.410 Å, *b* = 7.633 Å, *c* = 9.904 Å, *α* = 68.4°, *β* = 69.65° and *γ* = 65.05°). The structure (**Fig. 23**) consists of distorted MO6 polyhedra (M= Zn, Cu), AIO6 octahedra and PO4 tetrahedra. By the edgeand corner-sharing of these polyhedra, a fairly dense three-dimensional framework is formed, which is further strengthened by a system of hydrogen bonds. The metal atoms in the unique MO6 (M = Zn or Cu) polyhedron show a distorted [2 + 2 + 2] coordination, the distortion being more pronounced in turquoise. About 10% of the M site is vacant in both minerals. In turquoise, a previously undetected structural site with very low

phosphate-bearing systems and soils contaminated by actinides.

370 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

reasonable descriptions of H-bonding in those four structures [120].

occupancy of (possibly) Cu is present at the position (1/2,0,1/2).

**Fig. 23.** The structure of turquoise viewed along the b-axis [124].

<sup>32</sup> According to the Jahn-Teller theorem, any nonlinear molecule in a degenerate electronic state will be unstable and will undergo some kind of distortion that will lower its symmetry so as to remove the degeneracy of the electronic state and also to attain lower energy. The Jahn-Teller effect is termed as static when there is permanent distortion in the structure

Teller32

of molecule [122].

**9. Wavellite** [129]: the mineral of the composition of Al3(PO4)2(OH)3·4.5-5H2O (the space group *Pcmn*, *a* = 9.62 Å, *b* = 17.36 Å and *c* = 6.99 Å). The two aluminum atoms in the structure are octahedrally coordinated (**Fig. 25**): one is bonded to two O atoms, two –OH groups and two H2O molecules and the other to three O, two (–OH) and one H2O. Phosphorus is in tetrahedral coordination with oxygen. Al octahedra, linked through (OH) corners, form chains parallel to the c-axis, and P tetrahedra are attached to this chain by sharing O atoms of subsequent octahedra. An extra H2O molecule occupies the large cavity between the chains, and as indicated by a high temperature factor, it has a statistical distribution within this cavity.

**Fig. 25.** The structure of wavellite viewed along the b-axis (a) and stereoscopic view of the wavellite structure (b) [129].

**10. Xenotime** [118]: monazite (**Fig. 26**) is isostructural with zircon (I4 <sup>1</sup> /AMD). Monazite atomic arrangement as well as that of xenotime is based on [001] chains of intervening phos‐ phate tetrahedra and RE polyhedra, with REO, polyhedron in xenotime that accommo‐ dates heavy lanthanides (Tb-Lu in the synthetic phases) and REO polyhedron in monazite that preferentially incorporates larger light rare-earth elements (La-Gd). As the struc‐ ture "transforms" from xenotime to monazite, the crystallographic properties are comparable along the [001] chains, with the structural adjustments to the different sizes of REE atoms occurring principally in (001).

**Fig. 26.** The structure of xenotime-(Y): *a* = 6.8947 Å, *b* = 6.8947 Å and *c* = 6.0276 Å (a) and xenotime-(Dy) (b) *a* = 6.9052 Å, *b* = 6.9053 Å and *c* = 6.0384 Å (b) [118] viewed along the b-axis.

Isostructural arsenate analogues of many phosphate minerals are known, and in some cases, vanadates too. Some orthophosphates capable of forming complete ranges of solid solutions with the corresponding orthoarsenates are [25]:


Phosphate minerals, like silicate minerals, are found with a great variety of cations. Unlike the latter group that contains numerous types of condensed silicate anions, almost all phosphate minerals are orthophosphates that contain PO4 3− anion. Non-phosphorus anions, such as O2−, OH<sup>−</sup> , F<sup>−</sup> , Cl<sup>−</sup> , SO4 2−, SiO4 4− and AsO4 3−, may also be present in these stoichiometric (or as occluded) materials [25].
