**1.6.8. Vanadinite**

The mineral hardness on the Mohs scale is equal to 5. Calculated and measured density of

Mimetite (arsenopyromorphite, lead chloroarsenate, mimetite-H, Pb5(AsO4)3Cl ) [45],[55], [181],[182]), is the end memberin the ternary system pyromorphite–vanadinite–mimetite. This is also the reason why the name of mineral is derived from Greek word "*mimethes*," i.e. the imitator. Mimetite is also Pb5 analogue of hedyphane (**Section 2.1**). Mimetite (**Fig. 33**) is an arsenate mineral; it usually forms as a secondary mineral in lead deposits through the

) and arsenates [184].

Mimetite usually crystallizes in oxidized zones of lead deposits as small hexagonal prism with colors ranging from pale to bright yellow, orange, yellowish-brown, white, translucent, to opaque [184]. In accordance with other hexagonal apatite-group minerals, it crystallizes in the space group P63/M. The unit cell parameters are *a* = 10.46, *c* = 7.44 Å, *a*:*c* = 1:0.71 *Z* = 2 and *V* =

is in the range from 3½ to 4. The structure and the shape of mimetite crystal is shown in **Fig. 34**.

**Fig. 34.** The structure (perspective view along the *c*-axis) and the shape of mimetite crystal of hexagonal (a) and mono‐

. The calculated density is 7.10 g·cm−3. The hardness of the mineral on the Mohs scale

mineral are 4.81 and 4.77 g·cm−3, respectively [53],[54],[180].

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

**1.6.7. Mimetite and clinomimetite**

oxidation of galena (PbS [183], Gn50

**Fig. 33.** Mimetite (Příbram, Czech Republic).

704.96 Å3

clinic polymorph (b).

<sup>50</sup> Symbol of mineral for rock- and ore-forming minerals.

Vanadinite was named with regard to the content of vanadium52 (Pb5(VO4)3Cl or 3Pb3(VO4)2·PbCl2 [103],[187],[188],[189]). Since simple vanadates53 incorporate other ions in theirlattices, several series are known, including descloizite [190],[191]–mottramite [192],[193] and mounanaite54 [194],[195]–krettnichite55 [196],[197].

**Fig. 35.** Vanadinite (Mibladén, Morocco).

<sup>51</sup> Both monoclinic polymorphs were discredited from the IMA list of minerals in 2010.

<sup>52</sup> The most common vanadinite minerals are vanadate, descloizite (PbZnVO4(OH), orthorhombic [191]), and mottramite (PbCuVO4(OH), orthorhombic [193]) [204]. Vanadium was discovered in 1891 within lead vanadate ore by a Mexican mineralogist ANDRES MANUEL DEL RIO (and named as erythronium), but it was mistaken as a form of chromium. The new element was recognized and named in 1830 by Swedish chemist NILS GABRIEL SEFTSTRÖM. Vanadium was named after the Norse goddess VANADIS, who represented beauty and fertility [199],[205]. About 80% of vanadium is used for the production of special steel and alloys. Other applications include catalysts (e.g. in the production of sulfuric acid), pigments, and the manufacture of batteries [210]. Vanadium is used as alloy with a number of metals, e.g. ferrovanadium (40–80% of vanadium), nickel-vanadium, alumino-vanadium, etc. [189].

<sup>53</sup> There are about 65 vanadium minerals, the most important sources of vanadium are titaniferous magnetites, carnotite (potassium uranyl vanadate used for the extraction of uranium, vanadium and radium), vanadinite, roscoelite (K(V,Al,Mg)2AlSi3O10(OH)2), patronite (VS4), sulvanite (Cu3V2+S4), uvanite (U2V5+6O21·15H2O), bravoite, and davidite [210]. Vanadium is also found in clays, crude oil [189], and vanadium-rich variety of lignite (quisqueite).

<sup>54</sup> Monoclinic mineral (space group C2/M) of the composition PbFe3+2(VO4)2(OH)2.

<sup>55</sup> Monoclinic mineral (space group C2/M) of the composition PbMn2+2(VO4)2(OH)2.

Vanadinite mineral is the lead chlorovanadate (lead chloro orthovanadate) analogue of minerals mimetite (**Section 1.6.7**) and pyromorphite (**Section 1.6.4** [52]) and is considered as the end member in the ternary system pyromorphite–vanadinite–mimetite [198],[199]. The arseniferous variety of vanadinite was named as endlichite (arsenian vanadinite, PbCl2·3Pb3(V,As)2O8 [200],[201],[202]).

Vanadinite is formed as the secondary product in oxidized zone of lead-bearing deposits56 [203],[204],[205],[206] associated usually with galena, cerussite, or limonite [207]. It is known to form typically well-developed hexagonal prismatic crystals with smooth faces and sharp edges along [0001] [203]. It occurs frequently as acicular, hairlike, fibrous, rounded, globular, or hollow prismatic crystals. Synthetic vanadinite was first presented in 1957 by DURAND [208].

The structure of hexagonal vanadinite belongs to the space group P63/M. The unit cell parameters are *a* = 10.33, *c* = 7.34 Å, *a*:*c* = 1: 0.71, *Z* = 2 and*V* = 678.72 Å3 . Calculated and measured average densities are 6.93 and 6.94 g·cm−3, respectively. It is brittle mineral of brown, brown‐ ish yellow, brown red, orange or yellow color, and some varieties can be colorless. The hardness of vanadinite on the Mohs scale varies in the range from 3½ to 4 [207],[209]. The structure of vanadinite and the example of crystal habit is shown in **Fig. 36**.

Vanadinite and pyromorphite (**Section 1.6.4**) possess similar structure where Pb(1) bonds to six oxygen atoms (3 × O(1) and 3 × O(2)) in the form of an approximate trigonal prism, with three longer bonds to oxygen atoms (3 × O(3)) through the prism faces. Adjacent Pb(1)-O9 "prisms" share pinacoidal faces at the mirror planes (*z* = ¼ and ¾) to form Pb(1)-O9 polyhe‐ dral chains parallel to the *c*-axis. Pb(2) lies in the mirror planes *z* = ¼ and ¾ and bonds to two oxygen atoms within the plane (O(1) and O(2)), four oxygen atoms (4 × O(3)) and two Cl atoms located on the hexad at 0,0,0 and 0,0,½ positions [188].

The major structural difference between vanadinite and pyromorphite occurs in XO4 tetrahe‐ dra, which are occupied by V5+ (radius 0.59 Å) in vanadinite and by P5+ (0.35 Å) in pyromor‐

**Fig. 36.** The structure (perspective view along the *c*-axis) and the example of crystals habit of vanadinite.

<sup>56</sup> Almost all base-metal vanadate deposits occur in oxidized zones of the base-metal vein and replacement deposits. They also occur in other vanadium minerals in sediments. Vanadate deposits are largely restricted to tropical and temperature zones and to regions of dry climate [204],[206].

phite. The O-V-O and O-P-O angle in vanadate (VO4 3−) and phosphate ion (**Section 1.2**) tetrahedron varies from 105.4 to 113.1° and from 107.1 to 111.8°, respectively. The most important structural features are the octahedral coordination of Pb(2) around Cl<sup>−</sup> ions and the tetrahedral coordination of oxygen atoms around the vanadium atom. Each Cl<sup>−</sup> ion is surrounded by six Pb(2) at the corner of a regular octahedron in which the Pb(2)-Cl distance is 3.17 Å and the shortest Pb(2)-Pb(2) distance (along an edge of the octahedron) is 4.48 Å [204], [210].

Vanadinite mineral is the lead chlorovanadate (lead chloro orthovanadate) analogue of minerals mimetite (**Section 1.6.7**) and pyromorphite (**Section 1.6.4** [52]) and is considered as the end member in the ternary system pyromorphite–vanadinite–mimetite [198],[199]. The arseniferous variety of vanadinite was named as endlichite (arsenian vanadinite,

Vanadinite is formed as the secondary product in oxidized zone of lead-bearing deposits56 [203],[204],[205],[206] associated usually with galena, cerussite, or limonite [207]. It is known to form typically well-developed hexagonal prismatic crystals with smooth faces and sharp edges along [0001] [203]. It occurs frequently as acicular, hairlike, fibrous, rounded, globular, or hollow prismatic crystals. Synthetic vanadinite was first presented in 1957 by DURAND [208].

The structure of hexagonal vanadinite belongs to the space group P63/M. The unit cell

average densities are 6.93 and 6.94 g·cm−3, respectively. It is brittle mineral of brown, brown‐ ish yellow, brown red, orange or yellow color, and some varieties can be colorless. The hardness of vanadinite on the Mohs scale varies in the range from 3½ to 4 [207],[209]. The

Vanadinite and pyromorphite (**Section 1.6.4**) possess similar structure where Pb(1) bonds to six oxygen atoms (3 × O(1) and 3 × O(2)) in the form of an approximate trigonal prism, with three longer bonds to oxygen atoms (3 × O(3)) through the prism faces. Adjacent Pb(1)-O9 "prisms" share pinacoidal faces at the mirror planes (*z* = ¼ and ¾) to form Pb(1)-O9 polyhe‐ dral chains parallel to the *c*-axis. Pb(2) lies in the mirror planes *z* = ¼ and ¾ and bonds to two oxygen atoms within the plane (O(1) and O(2)), four oxygen atoms (4 × O(3)) and two Cl atoms

The major structural difference between vanadinite and pyromorphite occurs in XO4 tetrahe‐ dra, which are occupied by V5+ (radius 0.59 Å) in vanadinite and by P5+ (0.35 Å) in pyromor‐

**Fig. 36.** The structure (perspective view along the *c*-axis) and the example of crystals habit of vanadinite.

56 Almost all base-metal vanadate deposits occur in oxidized zones of the base-metal vein and replacement deposits. They also occur in other vanadium minerals in sediments. Vanadate deposits are largely restricted to tropical and temperature

. Calculated and measured

parameters are *a* = 10.33, *c* = 7.34 Å, *a*:*c* = 1: 0.71, *Z* = 2 and*V* = 678.72 Å3

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

located on the hexad at 0,0,0 and 0,0,½ positions [188].

zones and to regions of dry climate [204],[206].

structure of vanadinite and the example of crystal habit is shown in **Fig. 36**.

PbCl2·3Pb3(V,As)2O8 [200],[201],[202]).

The production of vanadium metal from vanadinite ore consists of the following steps [188], [212]:

**1. Preparation of ammonium metavanadate** (NH4VO3) from powdered vanadinite ore, which is treated by concentrated HCl. This leads to the precipitation of PbCl2 and to the solution of complex salt of [V(OH)4]Cl (**Eq. 6**). Ammonium metavanadate precipitates when [V(OH)4]Cl solution is boiled with NH4Cl (**Eq. 7**).

$$\text{Pb}\_3\text{(VO}\_4\text{)}\_3\text{Cl} + 12\text{ HCl} \rightarrow 5\text{ PbCl}\_2\text{,} \sqrt{+} 3\left[\text{V}\,\text{(OH)}\_4\right] \text{Cl} \tag{6}$$

$$\begin{aligned} \left[\text{V(OH)}\_{4}\right] \text{Cl} + \text{NH}\_{4}\text{Cl} &\rightarrow \text{NH}\_{4}\text{VO}\_{3} \stackrel{\downarrow}{\text{-} + 2 \text{ HCl} + \text{H}\_{2}\text{O}} \end{aligned} \tag{7}$$

**2. Conversion of ammonium metavanadate into V2O5** is reached by ignition. The process can be described by the following equation:

$$2\text{ NH}\_4\text{VO}\_3 \rightarrow \text{V}\_2\text{O}\_5 + 2\text{ NH}\_3 + \text{H}\_2\text{O}\tag{8}$$

**3. Reduction of V2O5to vanadium metal** via fluxing with Ca and CaCl2 (**Eq. 9**) or by the aluminothermic process (**Eq. 10**):

$$\text{CaV}\_2\text{O}\_5 + \text{S Ca} + \text{5 CaCl}\_2 \rightarrow 2\text{ V} + \text{5(CaO} \cdot \text{CaCl}\_2) \tag{9}$$

$$\text{13 V}\_2\text{O}\_3 + 10\text{ Al} \to 6\text{ V} + \text{5 Al}\_2\text{O}\_3\tag{10}$$

**4. Purification of vanadium metal** by *Van Arkel-de Boer method* [211],[216]57 in which the impure vanadium metal is heated with a limited amount of I2 under vacuum.58 Formed

<sup>57</sup> The method of reactive distillation of metal compounds (or also the chemical vapor transport reactions [214] developed by Dutch chemists ANTON EDUARD VAN ARKEL AND JAN HENDRIK DE BOER in 1925. The process can be applied if volatile iodides (from that they are also termed as the iodine process) of metal were formed and if the metal has higher melting point than the dissociation temperature of formed iodide. This technique has practical importance for Ti, Zr, Hf, Th, Cr, and V. The method which uses CO reactive gas instead of I2 is known as the carbonyl or **Mond**–**Langer process** [213], e.g. Ni(s) + 4 CO(g) ↔ Ni(CO)4(g).

<sup>58</sup> Chemical vapor transport (CVT) or vaporization reaction with iodine [214]. The investigation of the vaporization processes for several vanadium halide systems shown the existence of mixed halides VX4−*<sup>n</sup>*Y*<sup>n</sup>* (*X* = Cl, Br; *Y* = Br, I) formed in reaction of Br2 or I2 with VX2 or VX3 solid phases at elevated temperature [215].

volatile iodide (VI4, vanadium tetraiodide or vanadium(IV) iodide) decomposes at a higher temperature on the wolfram filament into pure vanadium and I2, which becomes available to react with the impure vanadium, thus sustaining the process [189],[213],[214], [215],[216]:

$$\begin{array}{ll} \text{2 } \text{V(s, impure metal)} + \text{2 } \text{I}\_2(\text{g}) \rightarrow \text{VI}\_4(\text{g})\\ \text{2 } \xrightarrow{\text{W = flamest}} \text{V(s, pure)} + \text{2 } \text{I}\_2(\text{g}) \end{array} \tag{11}$$
