1. Introduction
Zn(NH
3)
2Cl
2, an anthropogenic mineral-like phase, was found by Chesnokov and co-authors in the burned coal dumps of the Chelyabinsk coal basin (ChCB) in the South Urals, Russia in 1984 [
1]. The Chelyabinsk burned coal dumps are well-known for their anthropogenic mineral-like phases. More than 240 different compounds have been found in this region and about 50 of them were unique at the time of their first description. Eight phases found in the Chelyabinsk dumps were approved as new mineral species, with the ChCB as a type locality [
2].
At least sixteen different ammonium-bearing phases from the ChCB are of special interest as they are formed through the contact of burning coal with organic matter at elevated temperatures. Their formation involves crystallization from gases, the phases are formed during the so-called “pseudofumarole” stage or as a result of supergene process, and they have much in common with the fumarolic ammonium minerals and similar minerals formed by the reactions of mineral components with organic substance, for example, guano [
3,
4,
5]. At the same time, only one phase containing ammine (NH
30) complexes, Zn(NH
3)
2Cl
2, has been described from the ChCB burned dumps. This work is devoted to the crystal chemical features of this phase found at the ChCB.
In general, minerals containing NH
30 complexes are very specific and extremely rare. Only five such compounds are IMA-approved mineral species which have completely natural origin. They were all described from the Pabellon de Pica guano deposit near Chanabaya, Iquique Province, Tarapacá, Chile, namely ammineite, CuCl
2(NH
3)
2 [
6]; chanabayaite, Cu
2Cl(N
3C
2H
2)
2(NH
3,Cl,H
2O,□)
4 [
7]; triazolite, NaCu
2(N
3C
2H
2)
2(NH
3)
2Cl
3·4H
2O [
8]; shilovite, Cu(NH
3)
4(NO
3)
2 [
9]; and joanneumite, Cu(C
3N
3O
3H
2)
2(NH
3)
2 [
10].
The synthetic compound Zn(NH
3)
2Cl
2 (zinc diammine chloride, or diamminedichlorozinc) is well known in the chemistry of complex compounds [
11] and belongs to the class of diamminechlorides (chlorides containing two NH
3 complexes) of various divalent metals with the general formula
M2+(NH
3)
2Cl
2, where
M2+ = Cu, Zn, Mg, Fe, Co, Ni, Cd, Hg, Ca and Pt. In nature, except for ammineite, CuCl
2(NH
3)
2 [
6], no such compounds have been described so far. Some synthetic metal chlorides with ammine complexes have applications in technologies and medicine: for example, the cis-[PtCl
2(NH
3)
2] complex (Cisplatin) is an important anticancer drug [
12] and the compound Mg(NH
3)
6Cl
2 is considered as a solid matrix for hydrogen storage [
13].
The purpose of this work is a detailed crystal chemical description of a mineral-like anthropogenic Zn(NH
3)
2Cl
2 from the ChCB burned dumps, including the determination of its crystal structure, spectroscopic studies and the correlation of the obtained results with the data known for its synthetic analogue. One of the aims of the current study is to investigate the thermal evolution of anthropogenic Zn(NH
3)
2Cl
2 by means of in situ single-crystal and powder X-ray diffraction studies at different temperatures. Such an investigation is of interest since Zn(NH
3)
2Cl
2 was found in discharged air–zinc batteries and may also be a byproduct of hydrocracking of heavy oil fractions (see [
14,
15] and references therein). The thermal stability and thermodynamic characteristics of synthetic Zn(NH
3)
2Cl
2 have been well studied (see [
15] and references therein). However, data from in situ X-ray diffraction studies at different temperatures are absent in the literature and would complement the characterization of the thermal evolution of Zn(NH
3)
2Cl
2, taking into account its formation under the conditions of the burning dumps of the ChCB.
4. Discussion
The crystal chemical studies demonstrate that anthropogenic Zn(NH
3)
2Cl
2 (“amminite”) from the ChCB burned dump is completely identical to its synthetic analogue. Although phases from burned dumps under some circumstances could now be considered as valid mineral species [
33], this zinc diammine chloride is definitely man-made (has anthropogenic or technogenic origin) due to the ad hoc introduction of the zinc plate. Thus, the possibility of the formation of Zn(NH
3)
2Cl
2 under completely natural conditions is still questionable.
In general, there are very few minerals containing ammine complexes (see Introduction). All these minerals contain copper and were found at the Pabellon de Pica guano deposit within the contact zone between bird guano and the surface of gabbro enriched with chalcopyrite, CuFeS
2. The latter, being exposed to air, oxidizes and serves as a source of Cu
2+ for numerous supergene minerals [
7,
8,
9]. From these five minerals, two are chlorides, and only ammineite, CuCl
2(NH
3)
2 (not to be confused with the anthropogenic “amminite” (Chesnokov et al.)), belongs to the class of diamminechlorides of the divalent metal cations with the general formula
M2+(NH
3)
2Cl
2 (see above), which also includes the studied Zn(NH
3)
2Cl
2. It is of interest that the crystal structures of CuCl
2(NH
3)
2 and Zn(NH
3)
2Cl
2 are not isotypic. In ammineite, CuCl
2(NH
3)
2, the coordination of Cu
2+ is distorted octahedral, due to the Jahn–Teller effect, and the structure is based upon zigzag chains of distorted octahedra [
6]. According to Bojar et al. [
6], the formation of ammineite in nature was due to the reaction of Cu minerals with guano.
Natural zinc chlorides containing ammine complex (NH
30) or ammonium cation (NH
4+) are currently unknown. Only one zinc mineral with ammonium has been described, katerinopoulosite, (NH
4)
2Zn(SO
4)
2·6H
2O, which was formed in the young oxidation zone of an ore enriched in sphalerite, ZnS, in Lavrion, Greece. The most probable source of ammonium was the leached soil saturated with organic residues [
34]. On the other hand, hydrogen-free zinc chlorides and Cl-bearing Zn oxysalts are only known to be in the oxidizing-type fumaroles of the Tolbachik volcano, Kamchatka, Russia: mellizinkalite, K
3Zn
2Cl
7 [
35], flinteite, K
2ZnCl
4 [
36], belousovite, KZn(SO
4)Cl [
37], chubarovite, KZn
2(BO
3)Cl
2 [
38], cryobostryxite, KZnCl
3·2H
2O [
39], prewittite, KPb
1.5ZnCu
6O
2(SeO
3)
2Cl
10 [
40] and sofiite, Zn
2(SeO
3)Cl
2 [
41]. Two natural hydrated zinc chlorides, simonkolleite, Zn
5Cl
2(OH)
8·H
2O [
42], and cryobostryxite, KZnCl
3·2H
2O [
39], are supergene minerals. Cryobostryxite forms as a product of the interaction of primary fumarolic Zn chlorides with atmospheric water or water vapor in the moderately hot (70–150 °C) zone of Tolbachik fumaroles, where ammonium minerals are also known, e.g., novograblenovite, (NH
4)MgCl
3·6H
2O [
43]. However, in fumarolic systems, the formation of minerals containing ammine (NH
30) complexes seems hardly probable, since it requires the formation of Zn–N bonds.
Thus, although the question remains open, under certain geochemical conditions, the formation of Zn(NH
3)
2Cl
2 may occur in nature, for instance, in the environments that involve reactions of Zn-bearing minerals (for example, sphalerite-rich rocks) with complicated organic matter, such as guano which contains organic compounds with ammine groups. Apparently, the source of ammonia for the formation of Zn(NH
3)
2Cl
2 in the ChCB burned coal dumps was a special organic substance contained in coal-bearing dump material; however, we cannot say whether this is directly related to coal. This fact explains the difference in the number of compounds with ammonia (NH
30) and ammonium (NH
4+) described from the burned coal dumps of the ChCB: one species
vs at least sixteen species, respectively. The presence of such special (specific) organic substances within the burning coal dumps of the ChCB is indirectly confirmed by the presence of other crystalline organic phases found there in sublimates of the “pseudofumaroles”, such as C
10H
12N
8O
8 and kladnoite, C
6H
4(CO)
2NH [
2].
Upon heating, the Zn(NH
3)
2Cl
2 phase is stable up to about 150 °C. This is in good agreement with the data of Chesnokov and co-authors, who suggested temperatures of Zn(NH
3)
2Cl
2 formation lower than 200 °C [
1]. The crystal structure of Zn(NH
3)
2Cl
2 expands anisotropically (
Table 8,
Figure 5); at the same time, the bond lengths within the ZnN
2Cl
2 tetrahedra practically do not change (within the errors; see
Table S1 for details), which indicates that the thermal expansion of the structure is controlled by the changes in the hydrogen bonding system (most obviously fixed by the increase in the distance D...A,
Table 5). The observed anisotropy is probably connected to a specific arrangement of hydrogen bonds relative to the crystallographic axes. Thus, the maximum expansion is observed along the
a axis, while the largest relative increase in the distance D...A (3.549(3) Å at −173 °C and 3.634(4) at 77 °C) is observed for the bond N-H1A...Cl
2 (
Table 6), which is oriented most closely to the
a axis (
Figure 5 and
Figure 6).
The role of H-bonds in the structural complexity values can be estimated as significant, using a calculation of
IG and
IG,total including all atoms and without localized H-atoms (
IG(no H),
IG,total(no H)) (
Table 10). Thus, the
IG,total/
IG,total(no H) value is 3.16, which is more than the similar value observed for halotrichite and related hydrates sulfates (
IG,total/
IG,total(no H)~2.33) [
44]. At the same time, the structural complexity of Zn(NH
3)
2Cl
2 is relatively low, which is typical for halides: the average values of structural complexity for all minerals are estimated as
IG = 3.54(0.02) bits/atom and
IG,total = 345(10) bits/cell, while, for halides, these values are
IG = 1.95(0.12) bits/atom and
IG,total = 62.91(11.57) bits/cell [
26].