The Halotrichites and the Extent of Solid Solution in the Group of Minerals Report

The minerals encompassed in the halotrichite group of minerals have been in existence for a long period. Also known as pseudo-alums, these minerals are found in many environmental systems as post-mining phases. Minerals in this cluster are monoclinic sulfates in nature and have a chemical formula of AB₂(SO₄)₄.22H₂O. In most occasions, [A] in the chemical formula represents either one or a combination of Mg²⁺, Zn²⁺, Fe²⁺, Ni²⁺, and Mn²⁺ cations, while [B] represents either one or a combination of Al³⁺, Cr³⁺, or Fe³⁺ cations (Frost et al 2009). As with other alums founded on monovalent cations, minerals in this group can be readily synthesized in a laboratory setting. When synthesized, the double sulfates arising from ions of metals such as manganese, ferrous iron, zinc, and magnesium are related to the halotrichite series of minerals. The double sulfates often form solid solutions. This report sets out to critically analyze the extent of solid solutions in the halotrichite group of minerals.

The halotrichite group is comprised of several minerals with seemingly similar characteristics. One member goes by the group name itself – halotrichite. This mineral, also identified as feather alum due to its common hair-like characteristics, is a highly hydrated sulfate mineral composed of aluminum and Iron. It has a chemical composition of Fe²⁺SO₄.Al₂(SO₄)₃.22H₂O. The other members making up the group include apjohnite Mn²⁺SO₄.Al₂(SO₄)₃.22H₂0 and dietrichite ZnSO₄.Al₂(SO₄)₃.22H₂O (Frost et al, 2009). According to Ulery & Drees (2008) and Frost et al (2005), the minerals can be analyzed and verified using the latest technologies such as Energy Dispersive Spectrometer (EDS), Raman Spectroscopy, X-ray Diffraction (XRD), and Infrared Spectroscopy for quantitative chemical composition.

Members of the Halotrichite Group of Minerals

As observed above, the halotrichite group is made up of several minerals that bear strikingly similar characteristics in their physical outlook as well as chemical composition. This paper will limit itself to the discussion of halotrichite, apjohnite, pickeringite, dietrichite, and wupatkiite. Other members of the group that will not be discussed include bilinite and redingtonite.

Halotrichite

The mineral derives its name from halotrichum, the Latin word for salt hair. As already mentioned above, the chemical composition is Fe²⁺SO₄.Al₂(SO₄)₃.22H₂O. Geologists believe the mineral is generally formed as an efflorescence in weathering and decomposing sedimentary rocks containing aluminum and metallic sulfide deposits that precipitate around or inside volcanic vents and hot springs (Farkas et al 2009). The mineral is widespread, though in characteristically small amounts, and mostly accumulate in arid climates. Some of the classic localities where the mineral can be found include Atacama Desert, Chile; Solfatara di Pozzuoli, Naples; Mont Saint-Hilaire and Grace Bay coal deposits, Canada; Falun, Sweden; Dresden, Germany; Kamchatka Peninsula, Russia; and San Juan County, Utah, US (Schumann 2008).

According to Ballirano, Bellatreccia, & Grobessi (2003), the halotrichite mineral is closely associated with melanterite Fe++So₄.7(H₂O); gypsum CaSo₄.2(H₂O); copiapite Fe+++4(So₄)6(OH)₂.20(H₂O); Epsomite MgSo₄.7(H₂0), and Alunogen Al₂(So₄)₃.17(H₂O). The mineral is known for its pungent taste and easy solubility in water. Physically, it has poor cleavage, conchoidal fracture, and somewhat brittle tenacity. According to Schumann (2008), the mineral is mostly found in “brown coal, ore mines, and slate containing pyrite, [a] by-product in fumaroles” (p.36). It is transparent and translucent and is found in varying colors – from colorless to white, pale grey, yellow, and pale green. However, the mineral becomes colorless in transmitted light. Halotrichite has a silky and vitreous luster and monoclinic crystallization, which is acicular and fibrous.

Schumann (2008), says the halotrichite mineral of FeSO₄.Al₂(SO₄)₃.22H₂O has a Mohs’ hardness of one-and-half and a specific gravity of between 1.73 and 1.79. According to Frost et al (2005), the halotrichite mineral is “characterized by four infrared bands at 3569.5, 3485.7, 3371.4 and 3239.0 cm^-1” (p.1). According to the standardized chemical investigation, the jarosite contains around 6% Mg²⁺, and forms an extensive continuous solid solution with other members of the halotrichite group such as pickingerite MgSO₄.Al₂(SO₄)₃.22H₂O. However, these mineral is “rarely isomorphous with other univalent alums” (Schumann, p.32).

Apjohnite

The single-crystal data that exists for apjohnite reveals that AB₂ in the chemical composition of the halotrichite group of minerals [AB₂(SO₄)₄.22H₂O] represents MnAl₂. According to Lane (2007), the mineral has a molecular weight of 889.49 gm. The following elements are included in its chemical composition: manganese Mn, 6.18%; aluminum Al, 6.07%; hydrogen H, 4.99%; sulfur S, 14.42%; and oxygen O, 68.35%. Various powder diffraction studies done on the mineral have also found the following oxides: magnesium oxide MnO, 7.98%; aluminum oxide Al₂O₃, 11.46%; and water H₂0, 44.56% (Lane). The mineral is found as efflorescences, mainly in Delagoa Bay, Maputo, Mozambique; Terlano, Italy; Alum Cave, Tennessee, US; Alum point, Salt Lake, Utah, US; and Old Placers District, New Mexico. It is named after its founder, James Apjohn (1796-1886).

The physical properties of Apjohnite are somewhat similar to those of halotrichite, gunningite, and epsomite. Its color ranges from a colorless ambiance to white, pale yellow, rose green or pale pink. The mineral has a density of around 1.8 g/cm³ and a hardness of 1.5 to 2. It has a transparent diaphaneity, with a non-fluorescent luminescence and a silky luster. The mineral is highly soluble in water and has nonmagnetic properties. It has fibrous to acicular monoclinic crystals with a point group of 2/m (Palmer & Frost 2006). The apjohnite mineral is illustrated below.

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Pickeringite

Pickeringite, also known as magnesia alum or magnesium aluminum sulfate hydrate, is a member of the halotrichite group of minerals. The mineral is named after John Pickering, an American lawyer, and philologist who lived between 1777 and 1846. It has a molecular weight of 858.86 gm and a chemical formula of MgAl2(SO4)4•22(H2O). According to Quartieri, Triscari, & Vieni (2000), the exact composition of the alum is as follows: magnesium Mg, 2.83%; aluminum Al, 6.28%; hydrogen H, 5.1%; sulfur S, 14.93%; and oxygen O, 70.79%. The oxides consist of magnesium oxide MgO, 4.69%; aluminum oxide Al₂O₃, 11.87%; water H₂O, 46.15%; and sulfur oxide SO₃, 37.29%. Although pickeringite is found in very many localities, only a few have been listed and documented by geologists. They include Cerros Pintados, Chile; Tucumcari, the Geysers, and Alum Point, USA; Newport, Nova Scotia and near Smoky River, Alberta, Canada, and Wetzelstein in Germany. According to Cotterell (2009), verification of this mineral can be done by both XRD and EDS analysis.

Pickeringite has an indistinct cleavage and comes in a variety of colors ranging from colorless to yellowish-white, yellow-green, reddish-white, red, greenish-white, and plain white. No matter its original color, the mineral turns colorless in transmitted light. The mineral bears a transparent or translucent diaphaneity and has an average density of 1.82 g/cm³ and a hardness of between 1.5 and 2. A comprehensive analysis of the mineral reveals that its fracture has conchoidal jagged flat surfaces, and its tenacity is somewhat brittle.

According to Quartieri, Trinscari, & Vieni (2000), the mineral is fibrous, resembling a kidney in shape, and forms encrustations on matrix. The mineral is also known to be non-fluorescent in luminescence, and silky to vitreous in luster. Available data reveals that pickeringite has acicular to hair like monoclinic crystals of point group 2. Pickeringite is closely associated with other minerals such as kaolinite, epsomite, copiapite, and gypsum. It is also closely linked to alunogen and melanterite minerals.

Pickeringite has a high solubility in water and is known to form a solid-solution series with other members of the halotrichite group such as apjohnite and halotrichite. According to Palma & Frost (2006), the halotrichite mineral [FeAl₂(SO₄)₄·22H₂O] becomes pickeringite if more than 50 % of the ferrous iron [Fe] is replaced by magnesium [Mg].

Dietrichite

Dietrichite, having a chemical formula of (Zn, Fe++, Mn)Al₂(SO₄)₄.22(H₂O), is yet another member of the halotrichite group of minerals. The mineral derives its name from the individual who analyzed and tested the first specimens – Gustav Heinrich Dietrich of the Czech Republic. The mineral has a molecular weight of 896.04 gm, and it is composed of the following: manganese Mg, 0.61%; aluminum Al, 6.02%; zinc Zn, 4.38%; iron Fe, 1.87%; hydrogen H, 4.95%; sulfur S, 14.31%; and oxygen O, 67.85%. The oxides include 0.79% MnO; 11.38% Al₂0₃; 5.45% ZnO; 2.41% FeO; 44.23% H₂O; and 35.74% SO₃ (Ballirano, Ballatreccia, & Grobessi 2003). The empirical formula is Zn_0.6Fe²⁺_0.3Mn²⁺_0.1Al₂(SO₄)₄•22(H₂O). The mineral has been sighted in Baia Sprie, Romania, and it’s usually found as post-mining efflorescences in unused or discarded minefields.

Dietrichite mineral has a density of 1.8 g/cm³ and a hardness characterization of 2 – Gypsum. It is found in different colors such as brownish-yellow, dirty white, and gray-white. Like most other members of the halotrochite group, the mineral turns colorless in transmitted light. Also, dietrichite has transparent diaphaneity and a glossy vitreous luster with a white streak. The crystal data on the mineral reveals that it posses fibrous, tufted monoclinic aggregates of the 2/m point group. Dietrichite is highly soluble in water.

Wupatkiite

This particular mineral is one of the newest members of the halotrichite group and has a chemical composition of (Co,Mg,Ni)Al2(SO4)4•22(H2O) and a molecular weight of 883.07 gm. Mostly found in the Gray Mountain, Arizona USA and Lorena gold deposit, Queensland, Australia, wupatkiite has an empirical formula of Co_0.6Mg_0.3Ni_0.1Al₂(SO₄)₄•22(H₂O). In the US site, the mineral occurs as a rare post-mine oxidation by-product (Williams & Cesbron 1995). The mineral has an average density of 1.89 g/cm², and a hardness of between 1.2 and 2 Talc –Gypsum. The mineral is mostly associated with pickeringite, nickel-boussingaultite, and clay. As is with the other minerals in the group, wupatkiite has a transparent diaphaneity, monoclinic crystallization, and a silky lustre. It has a distinct cleavage and a white streak. Its color ranges from light pink to plain pink, pink-red and pinkish yellow. Williams and Cesbron say the mineral “occurs as cross-fiber veinlets with fibers up to 8mm long” (p. 553).

The extent of Solid Solution in the Halotrichite Group of Minerals

In a layman’s language, a solid solution can be described as a solid-state of a liquid solution. As is the case with liquids, a propensity for mutual solubility also exists between two or more coexisting solids. However, the solubility is dependent on the nature and composition of the chemicals and molecules in the solids (Nelson 2008). According to the Hume-Rothery rules, solid solutions are bound to form if both the solute and the solvent have analogous atomic radii, same crystallization, similar electronegative, and an analogous valence (Plieth 2008).

Mutual solubility of two or more substances may either become a reality or impossibility depending on the chemical and molecular similarities of the solids (Nelson 2008). For example, the mutual solubility between silver and gold is almost 100% due to their chemical compositions, while solubility between copper and bismuth is a near impossibility. In deeper terms, a solid solution can be described as a compositional discrepancy in the crystallization of a solid substance due to the replacement or omission of a variety of atomic constituents making up the crystal structure. This scenario is often observed in the halotrichite group of minerals. For instance, pickeringite, which is highly soluble in water, is known to form a solid solution series with other members of the halotrichite group such as apjohnite and halotrichite

In general terms, solid solutions can be classified under four broad categories – substitutional, interstitial, thermodynamic, and missional (Nelson 2008). Both substitutional and thermodynamic factors play a significant role in occasioning the extensive solid solution witnessed in the halotrichite group of minerals. According to Frost et al (2009), minerals in this group are monoclinic sulfates in nature and have a general formula of AB₂(SO₄)₄.22H₂O. In most occasions, [A] in the chemical formula represents either one or a combination of Mg²⁺, Zn²⁺, Fe²⁺, Ni²⁺, and Mn²⁺ cations, while [B] represents either one or a combination of Al³⁺, Cr³⁺, or Fe³⁺ cations (Frost et al 2009).

Due to the monovalent nature of the cations, minerals in this group can be readily synthesized in a laboratory. The double sulfates that result from such analysis and synthesis have been found to form extensive solid solutions. Minerals in this group have an extensive solid solution series because they share a similar crystal structure, though it may be constituted differently (Lane 2007). All of the minerals in the group have been found with monoclinic crystallization which is rather acicular and fibrous (Schumann 2008). Minerals in the halotrichite group are therefore isostructural due to the virtue of their similar crystals.

The concept of a solid solution is common among many known minerals. As a matter of principle, only a small number of naturally occurring mineral deposits subsist as wholesome end-member substances (Nelson 2008). All minerals within the halotrichite group demonstrate trace to a widespread solid solution. As already mentioned, minerals in the group are known to form an extensive solid solution with other members of the group. For instance, pickeringite is known to form a solid solution series with apjohnite and halotrichite (Schumann 2008). The vice versa has also been found to be true. The extent and nature of solid solution in the mineral group is dependent on the following factors

The Atoms and Ions Involved

All minerals, including the halotrichites, are known to possess a definite chemical composition. However, this composition is not necessarily fixed. Four crystallographically autonomous sulfate ions are always present in all pseudo-alums that comprise the halotrichite group of minerals – halotrichite, pickeringite, apjohnite, wupatkiite, and dietrichite (Frost et al 2005). According to the researchers, one sulfate ion performs the duties of a unidentate ligand to the M²⁺ ion, while the others are actively involved in sophisticated hydrogen bond arrays entailing synchronized H₂O molecules to both the cations and lattice H₂O molecules

In the halotrichite group of minerals, the extent and nature of solid solutions largely depend on the relative sizes, numbers, and charges of atoms or ions involved. As already mentioned, the chemical formula of the monoclinic sulfates that make up most of the minerals in the group is AB₂(SO₄)₄.22H₂O. The letter [A] represents Mg²⁺, Zn²⁺, Fe²⁺, Ni²⁺, and Mn²⁺ cations, which can combine either individually or in combination with [B], representing Al³⁺, Cr³⁺, or Fe³⁺ cations to bring forth minerals such as apjohnite MnAl₂(SO₄)₄.22H₂O, halotrichite Fe²⁺SO₄.Al₂(SO₄)₃.22H₂O, pickeringite MgAl2(SO4)4•22(H2O), dietrichite (Zn,Fe++,Mnn)Al₂(SO₄)₄.22(H₂O), and wupatkiite (Frost et al 2009).

According to Nelson (2008), most of the associations above undergo either simple substitution or coupled substitution to form the minerals mentioned above. The solid solution is termed as simple when ions of similar charge and almost equal size undergo a process of substituting each other. In general, terms, if the ions have nearly the same size as is the case with most minerals within the halotrichite group, then the solid solution can take place over the absolute range of probable compositions to occasion a complete solid solution series (Nelson). In principle, ions of equal size have been known to substitute one another. This explains why a mineral such as a pickeringite can form a solid solution series with other end members of the group. It should be noted that partial solid solution may occur if the sizes of the ions are analogous, but still very different. A partial solid solution may also occur if the environment in which the mineral deposit appears has inadequate concentrations of the substituting ion (Bruno 2007). Halotrichites do not form partial or limited solid solution series.

Apart from the simple substitution, the ions in the halotrichite are known to use coupled substitution that results in extensive solid solution series. According to Nelson (2008), a coupled substitution is observed when an ion of dissimilar charge is substituted, triggering another substitution aimed at maintaining the charge balance. For example, halotrichite FeAl₂(SO₄)₄·22H₂O changes to become pickeringite in the group when over 50% of the ferrous iron ions (Fe²⁺) are substituted by magnesium ions (Mg²⁺). In such a circumstance, the extent of the solid solution is termed as extensive. It is therefore imperative to note that an extensive solution occurs within the halotrichite group of minerals as ions substitute one another in the monoclinic crystal structure (Frost et al 2005).

A study commissioned by Ballirano (2006) to investigate the crystallization chemistry of the halotrichite group of minerals [XAl₂(SO₄)₄· 22H₂O] surprised many geologists since it revealed a complete solid solution along the joints. The joints for the experiment had been set at x=Fe-Mg-Mn-Zn to represent the major minerals that make up the group. The experiment was analyzed using the latest technologies such as the Rietveld technique and X-ray powder diffraction. First, the study reported a complete solid solution along the joints of Fe, Mg, Mn, and Zn. This scenario, though not clearly understood scientifically, could be attributed to the attraction or repulsion of the five solitary pairs of H₂O molecules about the XO(H₂O)₅ octahedron.

According to a Raman Spectroscopy done on the halotrichite group of minerals by Ross, halotrichite was found to develop “infrared bands at v1, 1000cm^-1; v2, 435cm^-1; v3, 1085, 1025cm^-1; v4, 645, 600 cm^-1. Pickeringite, the Mg end-member of the halotrichitepickeringite series had infrared bands at v1, 1000 cm^-1; v2, 435 cm^-1; v3, 1085, 1025 cm^-1; v4, 638, 600 cm^-1” (Frost et al 2005, p. 2). This group of alums demonstrated infrared water bands in OH stretching between the region of 3400 and 3000 cm^-1. The infrared water bands in OH deformation and OH libration were displayed at 1650 cm^-1 region and 725 cm^-1 regions respectively. Most ions in the halotrichite group can favorably combine and substitute each other within the stated infrared bands to bring about complete or extensive chemical compositional changes in the end products.

In scientific terms, an extensive solid solution is possible if the size difference between two or more substituting elements is less than 15%. This, therefore, means that various members within the halotrichite group, especially those with Mg²⁺ and Fe²⁺ cations, can successfully be able to achieve extensive solid solutions. According to Palma & Frost (2006), Mg²⁺ and Fe²⁺ have a crystal size disparity of only about 7%. This explains why halotrichite and pickeringite can form a continuous solid solution series. It also explains why halotrichite has been found to match closely with wupatkiite in successive X-ray diffraction studies undertaken by geologists over time (Williams & Cesbron 1995). In the latter instance, ferocious iron and cobalt elements substitute one another in an extensive solid solution. It should be noted that wupatkiite is the first mineral within the group to have cobalt as the predominant cation.

Temperature and Pressure variations

Temperature and pressure variations have been known to occasion complete or extensive solid solutions within the minerals that make up the halotrichite group. Most profoundly, the solid solutions for these minerals are influenced by the temperature at which the substitution occurs. As a matter of principle, the greater amount of atoms and ions substitution within this group takes place at extreme temperatures, bringing about a complete or extensive solid solution series. According to Nelson (2008), this is mainly because atoms within the monoclinic crystal vibrate at a much higher rate. Intense pressure on the chemical composition of the mineral AB₂(SO₄)₄.22H₂O can occasion a change in both the monoclinic crystallization of the mineral and the cations (A= Mg²⁺, Zn²⁺, Fe²⁺, Ni²⁺, and Mn²⁺; B= Al³⁺, Cr³⁺, or Fe³⁺), ultimately bringing about diverse substitutions that might take place either at high or low pressure. This is made possible due to extensive solid solutions that characterize constituent members of the halotrichite mineral group.

The halotrichite group of minerals has a proportionately extensive solid solution if an experiment done by Frost et al (2009) is anything to go by. The researchers in this particular study analyzed three halotrichite minerals – halotrichite, apjohnite, and dietrichite – using dynamic, controlled rate thermogravimetric analysis as well as differential thermogravimetric analysis. Due to limitations of time, the researchers undertook two experiments in the controlled rate thermogravimetric experiment of 900 minutes; one lasting from ambiance to 430 °C, and the other lasting from 430 °C to around 980 °C. Huge mass losses occasioned by dehydration were observed in the halotrichite mineral under the dynamic experiment. These immense mass losses are particularly attributed to the high level of solid solution that characterizes these series of minerals (Nelson 2008).

In the experiment, the mass losses for halotrichite were especially noted at 80 °C, 102 °C, 319 °C, and 343 °C. Three other extensive mass losses occasioned by dehydration of the H2O molecules in the halotrichite were observed at 621 °C, 750 °C, and 805 °C. Mass losses for the apjohnite occasioned by substantial dehydration of tetrahedron water molecules occurred at 99 °C, 116 °C, 256 °C, 271 °C, and 304 °C. Extreme high-temperature mass losses for the apjohnite were observed at 781 °C and 992 °C. Mass losses for the dietrichite due to dehydration were noted at 115 °C, 173 °Cm, 251 °C, 276 °C, and 342 °C. One extreme temperature mass loss for the dietrichite was observed at 746 °C (Frost et al 2009). Although this experiment was specifically concerned with the thermal analysis of the halotrichite group of minerals, it can be actively used to show how the minerals within this group characteristically form extensive solid solution series.

High temperatures have been found to favor the formation of extensive solid solutions in halotrichites. According to the experiment described above, some end members which may be immiscible at low temperatures such as apjohnite and pickeringite form complete or continuous extensive solid solutions with one another and with other members of the group at high temperature. Apart from increasing atomic vibration for members of this group, high temperature is also known to open structures, making it easier to twist them locally to house differently-sized elements as is the case with Fe²⁺ and Mg²⁺ (Frost et al 2009). This enables individual members to form extensive solid solution series with each other. Subjecting the halotrichites to high temperatures also influences the structural flexibility of individual minerals, making it possible to bond bends and accommodate some local strains. This is a significant pointer of the extent of solid solution between or among several end members (Nelson 2008). The monoclinic crystallization in all the members of the group makes it possible for the elements to be structurally flexible, occasioning a complete or extensive solid solution series among the minerals within the group. Under extreme temperatures, halotrichites are known to form extensive solid solution series with other members such as pickingerite (Frost et al 2005).

Coexistence in Environments with Other Mineral Deposits

Members within the halotrichite group of minerals have been known to exist as efflorescences in deserted minefields and other areas such as volcanic vents. This is made possible due to their extensive solid solution state. For instance, halotrichite Fe²⁺SO₄.Al₂(SO₄)₃.22H₂O is formed as an efflorescence in weathering and decomposing sedimentary rocks containing aluminum and metallic sulfide deposits precipitating around or inside volcanic vents and hot springs (Farkas et al 2009). The extensive solid solution within the group causes weathering sulfide deposits in an abandoned mine to mix and combine well with oxidizing pyritic coals to produce the halotrichite mineral. In this transaction, the ions represented in both the sulfide deposits within the mine and the oxidizing pyritic coals must substitute one another to bring out the end product – halotrichite. The extensive solid solution may also occur if the environment in which the mineral deposit appears has adequate concentrations of the substituting ion.

According to Ballirano (2006), precise chemical composition and characterization of the halotrichite group of minerals is yet to be known due to challenges faced in the process of collecting samples. Presently, it is impossible to acquire unadulterated natural samples of the minerals because of the minute dimension and aggregation of the crystals. Ballirano argues that the acicular crystals making up the minerals are generally sub-millimetric. This makes it extremely difficult to obtain a pure sample of the minerals. Also, they are often intertwined with very complex mixtures of other sulfides such as gypsum, alunogen, and epsomite in their natural occurrence, making the process of obtaining a pure form of the alums an almost impossibility.

However, one thing is certain; the minerals within this group form complete and extensive solid solution series with other minerals, most of the same group. As such, minerals within this group can illustrate a good outlook of isomorphic substitutions (Frost et al 2005). Successive studies on the minerals have revealed the existence of complete and extensive solid solution series between the various end-members, key among them the halotrichite (Fe²⁺), pickeringite (Mg²⁺), and apjohnite (Mn²⁺). Future research should be directed at addressing the issue of precise chemical composition and characterization of the halotrichite group of minerals. The problem of impurities within the group must be addressed forthwith.

List of references

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13. Williams, S.A., & Cesbron, F.P 1995. “Wupatkiite from the Cameroon Uranium District, Arizona, a New Member of the Halotrichite group.” Mineralogical Magazine, Volume 59, pp. 553-556