Review on Mackinawite and Valleriite: Formulae, Localities, Associations and Intergrowths of the Minerals, Mode of Formation and Optical Features in Reflected Light

Investigations of the two minerals concern their chemical compositions and formulae, their intergrowths with other minerals, typical minerals that occur in association with them, their mode of formation within defined types of ore deposits/occurrences and finally also their optical characteristics under reflected light. Mackinawite was observed at 16 localities distributed all over the world from which more than 50 polished sections were available and more than 70 analyses were carried out. Valleriite was found at six localities. Microscopic work was based on more than 40 polished sections and more than 30 analyses. In the literature, mackinawite is reported as a metal excess phase with the formula (Fe, Ni)1+xS and (x ≤ 0.07). Deduced from analyses of this study, mackinawite has a wider compositional range and the formula: (Fe, Ni, Co)1-xS to (Fe, Ni, Co)1+y S, where x=0.00–0.10 and y=0.00–0.10. Therefore, mackinawite may not only occur as a metal excess phase, but also as a metal deficiency one. The high-temperature (max. about 500°C) mineral mackinawite occurs in various types of sulphidic deposits, and is characteristically accompanied with chalcopyrite, pyrrhotite, sometimes pentlandite, pyrite, sphalerite and cubanite and occurs often in oriented intergrowth within chalcopyrite which is always twinned due to inversion. At lower temperature, mackinawite was also formed by the replacement of chalcopyrite (not twinned) and rarely by that of pentlandite and linneite. The formula of valleriite is (Fe, Cu)2S2 • 1.5 [(Mg, Al)1(OH)2]. The analyses of this study show, that valleriite has a wider compositional range and the proposed formula is: (Fe, Cu)2S2 • [(Mg, Fe)1(OH)2] • y [Al(OH)3] with x=1.20 – 2.10 and y=0 – 0.50. Identical with the latter formula, namely that Al and vacancies are constituents of the OH-layer, the formula inverts to: (Fe, Cu)2S2 • [(Mg2+, Fe2+, Aly, □y/2)1(OH)2] where x=1.24–2.25 and y=0.00–0.26. Valleriite, formed under mesothermal up to low katathermal conditions, is always a newly-formed mineral. It occurs together with chromite and/or magnetite, mostly along their cracks and fissures, in ultramafic rocks that are serpentinized, but only in the presence of chalcopyrite. Valleriite was also observed as replacer of chalcopyrite. Valleriite decomposes under high-grade metamorphic conditions (e. g. in Outokumpu, Finland).


Introduction
During their history, only a few minerals may have created so much confusion as scientific investigations about the two minerals mackinawite and valleriite. Up to the end of the 60th years of the last century, mackinawite and valleriite were considered as minerals which could not be clearly differentiated from each other. This was in particular emphasized by Ramdohr [1] in his book "The ore minerals and their intergrowths", where he stated, that the optical properties of the two minerals often intersect to such a degree so that they are barely distinguishable.
optical similar mineral was discovered, having a composition not identical with valleriite [13]. Berner [14] synthesized mackinawite by giving metallic iron into a H 2 S-saturated solution. He characterized this product as tetragonal Fe-sulphide. This mineral, attributed as tetragonal Fe-sulphide, was found by Kuovo et al. [15] in various Finnish deposits. Finally, it were Evans et al. [6] which characterised this Fe-sulphide as the new mineral mackinawite from the type-deposit of Mackinaw.

Analytical technique
The electron microprobe analyses were carried out at the Geochemical Department, Centre of Geosciences of the Georg-August-University of Göttingen with an ARL-SEMQ-II equipped with six spectrometers and four different crystals (LiF, PET, ADP, TAP). It was operated at 15 kV accelerating voltage and a 15nA current on brass. As standards were used: "Kuki" (= chalcopyrite) for Fe, Cu and S, "Mill" (= millerite) for Ni, "Cobi" (= cobaltite) for Co and "Kaer" (= kaersutite) for Ca, Mg and Al.
The analyzed positions were carefully selected under the microscope and nearly only restricted to grains which were large enough to represent data of only single phases. Using too small grains (restricted to mackinawite), the neighbouring mineral was also analyzed and based on these results, corrections were undertaken. The detailed calculation procedures of mackinawite and valleriite can be obtained from the author.

Origin, Mineral Associations and Analyses of Mackinawite and Valleriite Mackinawite
The investigated samples originated from 16 localities distributed all over the world. From each locality, at least three samples were available and in total 71 analyses were carried out. Material of the following localities was used: Olympias/Greece: The Pb-Zn deposit contains mainly pyrite, but also galena, sphalerite and rarely graphite. Sphalerite contains inclusions of twinned chalcopyrite which carries inclusions of myrmekitic mackinawite. The composition of mackinawite is uniform ( Table 1, Columns I -III).
Sjögruvan, Grythyttan, Västmanland/Sweden: Main minerals of the deposit are hematite, hausmannite and braunite. However, the investigated polished sections consist of sulphides only. These are pyrrhotite, pyrite and subordinate twinned chalcopyrite which contains cubanite lamellae and tiny needle-like crystals of mackinawite which is Co-bearing and homogeneously composed ( Table 1, columns VI and VII).
Tibschi/Nigeria: The sulphide mineralization occurs in the so-called "Younger Granites" of the Jos Plateau/Nigeria [16]. The following ore minerals were detected: sphalerite, twinned chalcopyrite, stannite, cassiterite, pyrrhotite, mackinawite, covellite, galena, matildite and native Bi. Myrmekites of homogeneously composed mackinawite ( Table 2, columns I -III) are restricted to exsolutions of chalcopyrite from sphalerite. Panasqueira/Portugal: The deposit is a tin-tungsten deposit associated with collisional granites. Observed minerals are: sphalerite, chalcopyrite, pyrrhotite, cubanite, mackinawite, arsenopyrite, native Bi, pyrite, wolframite and stannite. Sphalerite contains inclusion of pyrrhotite and twinned chalcopyrite. The latter contains oriented intergrown inclusions of star-like sphalerite, cubanite lamellae and flame-like and elongated mackinawite which is homogeneously composed ( Figure 1;    The hill consists of folded limestones that are horizontally and vertically intersected by gangue rocks consisting of lamprophyre and aplite [17]. The samples were collected from a vertical arranged lamprophyre. Apart from chromite, pyrrhotite (partially replaced by pyrite), and untwinned chalcopyrite (partially replaced by mackinawite; Figure 2) occur. The latter is Ni-bearing and homogeneously composed ( Table 2, Columns X and XI). Owyhee County, Idaho/USA: Within the pegmatite, the following ore minerals were detected: twinned chalcopyrite, pyrrhotite, sphalerite, pyrite, native Bi, bismutinite and mackinawite. Mackinawite occurs in the form of abundant tiny and needle-like crystals homogeneously distributed within chalcopyrite or along grain boundaries between chalcopyrite and pyrrhotite. Mackinawite is Nibearing and inhomogeneously composed (Table 3, Columns I -II).
Outokumpu/Finland: The Cu-Co-Zn-Ni deposit is related to submarine volcanism and lies in Eastern Finland within the Archian Basement. Predominating minerals are twinned chalcopyrite, pyrrhotite and pyrite. Chalcopyrite contains inclusions of sphalerite, cubanite and mackinawite (Figure 3 and 4). Pyrrhotite, partially replaced by pyrite, carries inclusions of pentlandite. Mackinawite may also occur in the form of replacements of chalcopyrite and pentlandite. In homogeneously composed mackinawite is Co-and Ni-bearing (Table 3, columns IV -IX).
Singbhum/India: The deposit belongs to the massive sulphide type in high-grade metamorphic terranes. Detected minerals are: twinned chalcopyrite, cubanite, mackinawite, sphalerite and bornite. Chalcopyrite, the main mineral, contains inclusions of cubanite and elongated, twin-like mackinawite which is either Co-and Ni-bearing (Table 5, columns VI and VII) or Co-bearing (columns IX and X). Broken Hill, NSW/Australia: Broken Hill belongs to the massive sulphide deposits occurring in high-grade metamorphic terranes. The following ore minerals were detected: sphalerite, galena, twinned chalcopyrite, pyrrhotite, stannite, pyrite, mackinawite and magnetite. Chalcopyrite is the host of tiny mackinawite crystals. These are arranged in the form of strings of pearls along two directions ( Figure  6). Mackinawite is uniform composed and Ni-bearing (Table 6, columns I -II).

Valleriite
The investigated samples originated from 6 world-wide distributed localities. From Palabora and Gole Gohar about 30 samples were available and from the other localities, at least always more than two. 32 analyses were carried out, from which 17 were used in this paper only, because various results are either nearly identical or the total of the analyses is too low caused by the bad polishing behaviour of valleriite. Material of the following localities was used:  (Figure 10), but also coarse-grained and polysynthetic twinned crystals can be observed ( Figure 11). Chalcopyrite is often replaced by valleriite. The composition of valleriite is inhomogeneous (Table 8, columns I -VI). Chromite, replaced along grain boundaries by magnetite and spinell, is mechanically broken. Along the cracks and small fissures, valleriite was newly-formed. The chemical composition is shown in Table 8 (columns VII -IX).  Table 9, columns I -III.
Domokos/Greece: The locality was already described under mackinawite. The oxide-dominated mineralization contains valleriite which is included in pyrrhotite ( Figure 12) and more abundant along cracks and fissures in magnetite ( Figure 13). In Table 9 (columns IV -VI) the analyses are summarized.
Gole Gohar/Iran: The iron deposit lies in the Province of Kerman and belongs to the Sanandaj-Sirjan Zone and is hosted by ultramfic rocks [21,22]. The ore mineralization contains magnetite, pyrrhotite which contains flame-like exsolutions of pentlandite, pyrite,  Figure 14). Valleriite may also occur as replacer of chalcopyrite ( Figure 15). The composition of valleriite is summarized in Table 9 (columns VII and VIII). Outokumpu/Finland: Within the Outokumpu deposit, which was already described, abundant valleriite is distributed within chromiterich and serpentinized ultramafic rocks. Other minerals are magnetite and subordinate pyrite which replaces pyrrhotite. Valleriite occurs in relatively big, platy crystals. As a sign of their decomposition, they are optical inhomogeneous (Figure 16). The analytical data are summarized in Table 10.

Mackinawite
According to Evans et al. [6] mackinawite has the formula (Fe, Ni) 1+x S (with x ≤ 0.07), is tetragonal, has the space group P4 2 /m and the lattice constants of a=3.68 and c=5.03 Å with Z=2. Refining the structure, Lennie et al. [23] presented new data, namely a=3.67 Å and the space group P4/nmm. The structure of mackinawite possesses a layer similar to that found in PbO (litharge with the same space group) with Fe and S occupying the sites of O and Pb, respectively, in litharge. Explained in terms of a distorted cubic close-packed arrangement of S atoms, the structure contains slightly distorted tetrahedron formed by S-atoms, whereas Fe fills the tetrahedral interstices which occur in the centre [23,24]. The FeS4 tetrahedra share edges and thus, form sheets of tetrahedra that are stacked normal to the c-axis. These sheets are hold together by weak van der Waals forces [25].     Table 9: Valleriite analyses of Hitura/Finland, Domokos/Greece and Gole Gohar/Iran. A: analytical data including the analytical total (= Σ 1 ) in wt. %. B 1 : Fe total in atom %. B 2 : sulfidic layer: Fe I + Cu (= Σ 1 ) and S; brucitic layer: amount of (1) Fe II + (2) Mg in relation to the sulfidic layer [= x (= Σ 2 ) in the formula], ratio of Fe II + Mg on the basis that their total is = 1.000 (=Σ 3 ), amount of (3) Al (= y of the formula) and the total of (OH=Σ 4 ) calculated from Fe II (OH) 2 , Mg(OH) 2 and Al(OH) 3 (all data in apfu). C: data of A. including the distribution of Fe total into Fe I + Fe II and the calculated OH-amount by transformation of Σ 4 (= mol. %) into wt. % (Σ 1+2+3 :OH) including the calculated total (=Σ 2 ).  The calculated data of the mackinawite analyses into apfu are summarized under B of the Tables 1-7. In cases that mackinawite is homogeneously composed, only the averaged analyses were used for calculation. The apfu-data reveal two characteristic features:

Rustenburg/South Africa
Apart from compositions consisting of Fe and S only (Olympias, Tibschi, Panasqueira, Hagendorf, Rajpura-Dariba and Zungeru), others contain additional Ni (Frerone, Owyhee County, Vihanti, Shingbhum, Broken Hill and Rustenburg) or Co (Sjögruvan) or both Ni and Co (Outokumpu, Domokos, Otterstope, Shingbhum and Rustenburg); and Apart from the Italian occurrence Mte. Frerone, it was observed that chalcopyrite is twinned showing oleander-leaf like polysynthetic twinning ( Figure 17). Twinning results from the fact that chalcopyrite, formed at high temperature (higher than 550°C), has a cubic structure in which Fe and Cu are statistically distributed and thus, has the formula of (Fe,Cu) 2 S 2 . With decreasing temperature, chalcopyrite inverts into the tetragonal modification in which Cu and Fe are in ordered arrangement with the formula of FeCuS 2 . The inversion is accompanied with the formation of twins. The temperature of inversion decreases in the presence of CuFeS 2 -ZnS solid solutions (which were often observed in this study) to a temperature of about 500°C [30]. This temperature is the upper limit for the formation of mackinawite, insofar as it occurs in oriented intergrowth with twinned chalcopyrite. Some of the chalcopyrite crystals that host mackinawite in oriented intergrowth were also analysed. The calculated formulae are summarized in Table 12 showing that Fe is always greater than 1 and varies between 1.005 and 1.030. It is inferred that unexsolved high-temperature chalcopyrite had originally a distinctly higher Feconcentration in relation to Cu. It is also inferred that the removal of Fe from primary Fe-rich chalcopyrite or the tendency to convert into the ordered and ideal FeCuS 2 -composition, was initiated by the formation of the iron-sulphide mineral mackinawite.

Valleriite
Valleriite consists of two parallel arranged trigonal layers (S-and OH-or brucite-layer) and has the ideal formula (Fe, Cu)S • 0.75 [(Mg, Al, Fe)(OH) 2 ]. The S-layer (space group: R3̅ m) has lattice constants of a=3.79 and c=34.10 Å with Z=3 and the OH-layer (space group P3̅ m1) those of a=3.07 and c=11.37 Å with Z=2 [31]. The formula and the lattice constants are attributed from the data of Evans and Allmann [7] who established firstly the formula of valleriite with (Fe 1  The data of the valleriite analyses, summarized in Tables 8 and 9, reveal that the data of the Outokumpu analyses (Table 10) differ distinctly from the other analyses. Supporting the microscopic observation of the valleriite decay at Outokumpu, the data of Table 10 could not be transformed into valleriite or valleriite-related formulae, The calculated analytical data are summarized in Tables 8 and 9. The resulting general valleriite formula is: (Fe, Cu) 2 S 2 • x [(Mg, Fe) 1 (OH) 2 ] • y [Al(OH) 3 ] with x=1.20 -2.10 and y=0 -0.50. The results of the presented formula clearly demonstrate that the ratio between the Sand brucite-layer is not constant as it varies in a relatively wide range. In respect of the proposed formula, a paper must be mentioned, comparable to mackinawite, which obviously did not make its way into the relevant literature. This concerns the paper of Schot et al. [27] which proposed the following formula: CuFeS 2 • n [(Mg, Ca, Fe, Ni, Co, Mn) 1 (OH) 2 ] • m [Al(OH) 3 ] with n=1.14 -2.33 and m=0 -0.50. These authors obviously inferred, that Al(OH) 3 forms a third layer (= gibbsite-layer) within the valleriite structure.
Writing the formula of this study in the sense of Evans and Allmann [7] by substitution of Mg by Al, it transforms into (Fe, Cu) 2 S 2 • x [(Mg 2+ , Fe 2+ , Al 3+ )(OH) 2 ] with x=1.20 -2.12. However, this substitution is not supported by the calculation of the analyses, because in that case, the amount of OH became too low, and as a consequence, also the analytical total. This can be deduced from Tables 8 and 9 under B2, where the Al: OH-ratio was calculated with 1: 3 [= Al(OH) 3 ]. However, in the Evans-formula, the ratio of Al: OH is 1: 2 and therefore, the OH-layer has to be positively charged [= Al(OH) 2 ] + . In their discussion, Evans and Allmann [7] argued that valleriite belongs to the well-established group of minerals, where the positively charged brucite-layers are interleaved by negatively charged layers of another kind. However, their argumentation cannot be applied to valleriite, because the sulphide-layer is electroneutral. However, their assumption may be correct, if the Al-substitution is limited to only small quantities. This was not observed in this study. In order to eliminate this problem, the following exchange is proposed: Mg 3 (OH) 6 =(Al 2 □ 1 ) 3 (OH) 6 (□ = vacancy) and therefore the introduction of vacancies into the brucite-layer. Based on this interpretation, the analytical data of B 2 were newly calculated (Tables  13 and 14) and the corresponding formulae were summarized in Table  15. The newly deduced general formula is:   Among the two formulae discussed in this paper, the vacancybearing version should be preferred. Otherwise it must be postulated, that valleriite is a three-layer mineral (sulphide-, brucite-and gibbsitelayer) and that Evans and Allmann [7] disregarded existing gibbsite during their structural investigations.
According to this study, valleriite is a product of secondary hydrothermal conditions (ranging from mesothermal to low katathermal) and occurs within deposits/occurrences that are, apart from the carbonatite deposit of Palabora, always associated with more or less serpentinized ultramafic rocks. Valleriite is the youngest mineral in the association and restricted to mineral associations that are dominated by chromite and/or magnetite, but only in the presence of chalcopyrite.
Under high-grade metamorphic conditions, valleriite is not stable and decomposes e.g. in Outokumpu (Figure 16). Thermal decomposition studies of valleriite carried out in an inert atmosphere by Iglesias et al. [34] show that the dehydroxylation of the brucite layers starts at about 350°C, followed by the lattice breakdown. At 450°C, valleriite disappears completely and new phases appear. In an oxidizing atmosphere, the same authors observed between 250°C and 650°C a long lasting oxidation process which obviously masked the valleriite breakdown.

Optical features of mackinawite and valleriite
Mackinawite and valleriite belong, similar to graphite, to those minerals which are characterized by striking optical features. Mackinawite is strongly bireflecting showing a reflectivity in the range between 44% and 37% (18% and 6%) corresponding with uncoloured effects from pure white to light grey (grey to dark grey) (in brackets: data in oil). Identical data for valleriite are: 21% and 11% (10% and 3%) corresponding with coloured effects from light crème-yellow to grey (dark crème-yellow to dark grey). Valleriite reveals therefore strong reflection pleochroism. Under crossed polars, strong anisotric effects become obvious. The extinction positions of mackinawite and valleriite are black in air and oil (in valleriite darker). The 45° position (brightest position) is pure white in air and oil for mackinawite and yellowish for valleriite.
These data reveal that confusion between mackinawite and valleriite during microscopic observations should normally be excluded, in particular in cases when the two minerals occur together ( Figure 18). However, it should be noted that valleriite and graphite are not easily distinguishable from one another e. g. valleriite of Gole Gohar/Iran was wrongly identified as graphite by Mücke and Golestaneh [21] because the colour, the reflectivity in the bright position and the anisotropic effects in the 45° position of the two minerals are similar. Under crossed polars, the extinction position, often not clearly visible due to undulatory extinction, is darker in graphite, especially in air. Under particular circumstances, the macroscopic recognition of valleriite may be possible. Mackinawite, on the other hand, can only be observed in reflected light. Also by the support of a pocket-lens, mackinawite is always by far too small as to be recognized.