**2.1.1. Hedyphane**

**Fig. 1** Distribution of described minerals from the supergroup of apatite (discredited species are also included) among individual groups (a) and distribution of kind of XO4 tetrahedra (b), crystal system (c) and space group (d) among

Other minerals from the supergroup of apatite include 65%, i.e. 28 described mineral species **Fig. 1**(**a**), which predominantly crystallize in hexagonal system (**c**) and in the space group P63/M

(**b**), but its content in individual groups varies strongly (**Fig. 2**).

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

**Fig. 2** Frequency of XO4 ions and point groups for individual groups from the supergroup of apatite.

3−unitis themostfrequent ortho-oxyanionforthe supergroupof apatite ingeneral

these species.

(**d**).The [PO4]

Hedyphane (Ca2Pb3(AsO4)3Cl), calcium-lead chloroarsenate [1],[2],[3],[4] is a mineral that was originally described from Langban, Sweden. It also occurs at the Harstig Mine, Pajsberg, Sweden and was moderately abundant at the Franklin Mine, Franklin, Sussex County, New Jersey. The mineral occurs in the localities introduced in **Fig. 3**. The mineral was named in 1830 by German mineralogist JOHANN FRIEDRICH AUGUST BREITHAUPT [5] and its Greek name is usually translated as "pleasant appearance or beautifully bright."


**Fig. 3** Known localities for the mineral hedyphane.

**Fig. 4** The structure (perspective view according to the *c*-axis; a) and the crystal habit of hedyphane crystals (b).

The neotype (refer to **Footnote 48** in **Chapter 1**) hedyphane occurs as light yellow or white1 euhedral crystal2 approximately 1 mm in maximum dimension. Hedyphane is translucent with white streaks and has greasy to vitreous or (sub)resinous luster on crystal faces and fracture surfaces. The cleavage is indiscernible, the fracture is even, and the mineral is moderately brittle. Hedyphane is isostructural (arsenate analogue) with phosphohedyphane (**Sec‐ tion 2.1.3)** [3]. The structure and the crystal habit of hedyphane are shown in **Fig. 4**.

The unit cell parameters of hedyphane are *a* = 10.14, *c* = 7.185 Å, *Z* = 2 and *V* = 639.78 Å3 . The average density of mineral is 5.81 g·cm−1. Hedyphane is optically uniaxial and positive. The hardness of the mineral on the Mohs scale is in the range from 4 to 5. Despite the fact that the formal charges of Ca2+ and Pb2+ are the same, the Pb2+ ion predominantly occupies M(2) sites in the structure of hedyphane. The exclusive presence of Pb in the M(2) site is probably due to the presence of stereoactive lone pair of electrons (please see **Fig. 7** and **Fig. 9**), which is characteristic for the Pb2+ ion in many compounds [6],[3],[7].

The average length of Ca-O(*n*), bond where *n* = 1, 2 and 3, in CaO9 polyhedron is 2.60 Å. The structure of PbO5Cl2 polyhedron with interatomic distances is shown in **Fig. 5**. The average value of O(*n*)-Pb-O(3) angle, where *n* = 2 and 3, is 99.3°. The average interatomic distance and O-As-O angle in AsO4 tetrahedron are 1.69 Å and 109°, respectively [3].

<sup>1</sup> Also described as "white variety" of "green lead ore" [4], i.e. the mineral mimetite (**Section 1.6.7)**. It should be noted that "white lead ore" (grayish-white color, glassy luster and crystallized in small prisms) is related to the mineral cerussite (PbCO3).

<sup>2</sup> Crystal habit shows well-formed and easily recognized faces. On the contrary, crystal faces that are not well formed are termed as anhedral. The intermediate texture between euhedral and anhedral is called subhedral.

**Fig. 5** The coordination polyhedron around the Pb site in hedyphane (projection on 001). The number within the circle denotes *x* coordinates of atoms [3].

#### **2.1.2. Fluorphosphohedyphane**

**Fig. 4** The structure (perspective view according to the *c*-axis; a) and the crystal habit of hedyphane crystals (b).

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

**tion 2.1.3)** [3]. The structure and the crystal habit of hedyphane are shown in **Fig. 4**.

characteristic for the Pb2+ ion in many compounds [6],[3],[7].

O-As-O angle in AsO4 tetrahedron are 1.69 Å and 109°, respectively [3].

termed as anhedral. The intermediate texture between euhedral and anhedral is called subhedral.

The unit cell parameters of hedyphane are *a* = 10.14, *c* = 7.185 Å, *Z* = 2 and *V* = 639.78 Å3

average density of mineral is 5.81 g·cm−1. Hedyphane is optically uniaxial and positive. The hardness of the mineral on the Mohs scale is in the range from 4 to 5. Despite the fact that the formal charges of Ca2+ and Pb2+ are the same, the Pb2+ ion predominantly occupies M(2) sites in the structure of hedyphane. The exclusive presence of Pb in the M(2) site is probably due to the presence of stereoactive lone pair of electrons (please see **Fig. 7** and **Fig. 9**), which is

The average length of Ca-O(*n*), bond where *n* = 1, 2 and 3, in CaO9 polyhedron is 2.60 Å. The structure of PbO5Cl2 polyhedron with interatomic distances is shown in **Fig. 5**. The average value of O(*n*)-Pb-O(3) angle, where *n* = 2 and 3, is 99.3°. The average interatomic distance and

 Also described as "white variety" of "green lead ore" [4], i.e. the mineral mimetite (**Section 1.6.7)**. It should be noted that "white lead ore" (grayish-white color, glassy luster and crystallized in small prisms) is related to the mineral cerussite

Crystal habit shows well-formed and easily recognized faces. On the contrary, crystal faces that are not well formed are

euhedral crystal2

1

(PbCO3). 2

The neotype (refer to **Footnote 48** in **Chapter 1**) hedyphane occurs as light yellow or white1

white streaks and has greasy to vitreous or (sub)resinous luster on crystal faces and fracture surfaces. The cleavage is indiscernible, the fracture is even, and the mineral is moderately brittle. Hedyphane is isostructural (arsenate analogue) with phosphohedyphane (**Sec‐**

approximately 1 mm in maximum dimension. Hedyphane is translucent with

. The

Fluorphosphohedyphane (Ca2Pb3(PO4)3F, [1],[8]): occurs in the oxidation zone of a small Pb-Cu-Zn-Ag deposit in the Blue Bell claims, about 11 km west from Baker, San Bernardino County, California (**Fig. 6**). Fluorphosphohedyphane is a fluor-analogue of phosphohedy‐ phane, forms subparallel intergrowths and irregular clusters of transparent, colorless, highly lustrous, hexagonal prisms with pyramidal terminations.

**Fig. 6** The locality for the mineral phosphohedyphane.

Fluorphosphohedyphane is found in cracks and narrow veins in highly siliceous hornfels3 [9] in association with cerussite1, chrysocolla4 ((Cu2−*<sup>x</sup>*Al*x*)H2−*<sup>x</sup>*Si2O5(OH)4·*n*H2O [10]), fluorite, fluorapatite, goethite, gypsum, mimetite, opal (SiO2·*n*H2O [11],[12]), phosphohedyphane, plumbogummite (PbAl3(PO4)(PO3OH)(OH)6 [13],[14],[15]), plumbophyllite (Pb2Si4O10·H2O [16]), plumbotsumite (Pb5Si4O8(OH)10 [17],[18],[19], pyromorphite (**Section 1.6.4**), quartz and wulfenite (PbMoO4 [20]). The streak of the new mineral is white; the luster is subadamantine [8]. The structure and the crystal habit of fluorphosphohedyphane hedyphane are shown in **Fig. 7**.

**Fig. 7** The structure (perspective view along the *c*-axis), the coordination of Pb with likely approximate location of lone-electron pair and the crystal habit of fluorphosphohedyphane in clinographic projection5 [8].

Fluorphosphohedyphane has the apatite structure with the ordering of Ca and Pb in two cation sites, as in hedyphane and phosphohedyphane. The Pb2+ cation exhibits a stereoactive 6*s*<sup>2</sup> lone electron pair6 [21] (**Fig. 7**). The *Z* anion site at (0, 0,½) is fully occupied by *F* forming six bonds of 2.867 Å to Pb atoms, in contrast to six Pb-Cl bonds of 3.068 Å in phosphohedyphane. For

<sup>3</sup> The name for this type of contact metamorphic rock was given by K. VON LEONHARDT. The name originates from the designation of the highest peaks in the Alps but it can be also derived from ancient mining term from Saxony (Germany) which was used to describe hard, compact metamorphic rock developed at the margin of an igneous body. These rocks possess outstanding toughness due to fine-grained nonaligned crystals of platy or prismatic habit. Hornfels are sometimes banded, but their texture can be also porphyroblastic, i.e. they occur as large crystals within fine ground groundmass of metamorphic rock [9].

<sup>4</sup> The name of the mineral comes from the Greek words *chyrosos* (gold) and *kolla* (glue). The mineral is also named as bisbeeite (blue mineral of the composition of (Cu,Mg)SiO3·nH2O named after Bisbee Cochise County, Arizona).

<sup>5</sup> In the clinographic projection the crystal is turned by angle *Θ* around a vertical axis in order to make the front- and the right-hand faces visible. Other forms are orthographic projection and perspective projection.

<sup>6</sup> The electronic configuration for Pb is [Xe] 4f14 5d10 6s2 6p2 . Cations with formal ns2 np6 electronic configuration usually display novel properties and it is widely believed that the so-called n*s*<sup>2</sup> lone pair is responsible for the stereochemical activity that causes the Jahn–Teller geometry distortion, specific optical properties, and ferroelectricity. Lone electron pair is also used for the explanation of anisotropies of thermal expansion coefficient, piezoelectric and elastic properties, and optoelectronic properties [21].

fluorphosphorhedyphane, phosphohedyphane and hedyphane in which Ca2+ occupies the M(1) site and Pb2+ occupies the M(2) site, the M(1) metaprism twist angles are notably smaller, 10.0°, 8.6° and 5.2°, respectively [8].

The mineral is brittle with subconchoidal fracture and no cleavage. Based on the empirical formula, the calculated density is 5.45 g·cm−3. Fluorphosphohedyphane is hexagonal with the space group P63/M and the cell parameters *a* = 9.6402, *c* = 7.0121 Å, *a*:*c* = 1:0.727, *V* = 564.4 Å<sup>3</sup> and *Z* = 2. The hardness of the mineral on the Mohs scale is 4.
