First Record of Romanechite in the Apulian Karst (Southern Italy) Resulting from the Interaction of Limestones and Clay Minerals

Abstract

A new occurrence of the Mn-Ba ore mineral, romanechite, has been discovered in a small paleo-doline of the Apulian karst on Mesozoic carbonate rock successions, characterized by reddish incrustations and nodules made essentially by Fe-bearing calcite. The conditions under which Mn-Ba ore minerals form represent an intriguing area of research, as these minerals can act as scavengers for heavy elements, impacting soils, surface sediments, and even associated aquatic systems. The genesis of romanechite is linked to the progressive interaction of silicate aqueous solutions enriched in Al, Si, and Fe with the limestone substrate. The findings provide new insights into the genetic processes responsible for the formation of reddish Mn incrustations, supporting their polygenetic origin because of the chemical alteration of limestone and allochthonous siliciclastic muds.

1. Introduction

Manganese oxide/hydroxide minerals are important components of many soils and sediments and are commonly found as coatings on rocks and nodules. These phases can precipitate from solution as the result of macro- or microscale changes in redox conditions, pH, or composition. Mn oxide minerals are chemically active, readily participating in redox and cation-exchange reactions [1]. They also adsorb a large variety of heavy metals, arsenic, and other potentially toxic elements because they commonly occur as fine-grained coatings with large surface areas. Because of these properties, Mn oxides can control the metal concentrations in water associated with soils and sediments [2,3,4,5,6,7]. These characteristics of excellent selectivity and capacity as an adsorbent for specific cations are due to its layered and tunnel structure resulting from the linkage of the MnO₆ octahedra, characterized by extreme short-range order and long-range disorder [3,8]. In the Apulian karst, extensive areas are covered with red calcite incrustations [9], which could serve as an effective natural environmental control resource if Mn-Ba mineralization is found to be widespread over large areas. The purpose of this note is to report, for the first time, the discovery of a Mn and Ba phase found in the red encrustations of the Apulian Cretaceous limestone. The origin of Mn-Ba ores is related to supergene mobilization and oxidation by meteoric waters within carbonate and siliciclastic terrigenous sediments [10,11]. Supergene Mn-Ba ore minerals occur in various parts of the Mediterranean area, primarily on the African continent (e.g., Namibia in West Africa [12,13]). These minerals are predominantly stratiform but also occur as nodules and supergene/karst-hosted deposits, indicating an origin from the oxidation, leaching, and reworking of materials under environmental conditions [14,15]. Similar conditions can be described for the Mn-Ba minerals discovered on Cretaceous limestones of the Apulian platform (Southern Italy). The micro-petrographic, geochemical, and mineralogical compositions of Fe-Mn-Ba red incrustations and nodules constrain the evolution from anoxic to oxidizing conditions during the red incrustation formation. Clay minerals play a crucial role in the formation of Mn-Ba hydroxide, as they capture divalent cations, such as Fe²⁺, Mg²⁺ and Mn⁺², aiming to stabilize the octahedral layers and promote the growth of phyllosilicate structures like kaolinite [16,17] containing Fe-oxy-hydroxide and another mineral, which includes Mn³⁺ and Mn⁴⁺.

2. Materials and Methods

Twenty samples of reddish alteration products of Mesozoic limestones were collected around a doline occupied by a transient pond, named Vuotano Santiquando, a karst depression extending for 3000 m² formed during the late Quaternary phase of the Murge emersion on Cretaceous limestones near Cassano delle Murge village (Bari) (Figure 1). More details about field observations and sampling and textural–petrographic features of analyzed karst products are available in [9].

The samples were analyzed under optical microscopy and scanning electron microscopy equipped with energy-dispersive spectrometer and micro-Raman spectrometry. Powder X-ray diffraction was performed on the insoluble residue, obtained by hydrochloric acid attack of the sample.

The petrographic and textural characterization was performed with the following equipment: (i) a Zeiss Photomicroscope III Pol polarizing optical microscope (Carl Zeiss, Oberkochen Germany) equipped with a EUROMEX VC 3036 (Euromex, Arnhem, The Netherlands) digital camera (CCD Sony 1/2.8” and 6.0 Mpx; Sony Europe B.V., Weybridge, UK); (ii) an SEM from LEO, model EVO50XVP (Zeiss, Cambridge, Cambridgeshire, UK), equipped with an X-max (80 mm²) Silicon drift Oxford detector. Microanalytical data were obtained under the following operating conditions: 15 kV accelerating potential, 300 pA probe current, about 25,000 output cps as the average count rate on the whole spectrum, counting time of 50 s, and 8.5 mm working distance.

Powder X-ray diffraction (PXRD) was performed with a spectrometer from PANalytical, model X’Pert Pro (PANalytical B.V., Lelyweg 1, 7602 EA Almelo, The Netherlands), equipped with a Cu X-ray tube (power supply at 40 kV, 40 mA), whose CuKα radiation was partially monochromated with a Ni filter.

Micro-Raman investigations were carried out employing a LabRAMHR Evolution^® (Horiba^®, Kyoto, Japan) spectrometer, equipped with a Peltier-cooled charge-coupled device detector (CCD), He-Ne 633 nm lasers, and a BH2^® microscope (Olympus Corporation^®, Tokyo, Japan). The used parameters were adjusted after several tests, finding a compromise between intensity, noise abatement, and performance; the laser power was always kept below 0.3 mW at the sample to avoid sample degradation. The spectral resolution achieved with the 1800 g/mm grating was about 1 cm⁻¹. A linear baseline was subtracted from the raw spectra using the software LabSpec6^® (Version 6.5.2.11, Horiba^®, Kyoto, Japan). Substances were identified by the comparison with reference spectra of minerals.

The instruments described were supplied by the Department of Earth and Geo-environmental Sciences and to the Department of Chemistry of the University of Bari Aldo Moro (Bari, Italy).

3. Results

The carbonate incrustations consist of reddish calcite with a fibrous appearance, in which levels or pockets of opaque minerals are diffused (Figure 1). These opaque minerals could be oxides or hydroxides of Fe and/or Mn. A more detailed study using the combination of Raman microscopy and analytical SEM provides unparalleled insights into the micro-mineralogy and chemistry of samples [1,8] in which these phases are small and highly dispersed.

Micro-Raman analyses (Figure 2) reveal that these black agglomerates are composed of romanechite (Ba₂Mn₅O₁₀xH₂O), which has a typical tunnel structure that functions as an adsorbent for potentially polluting monovalent and divalent cations. The presence of romanechite is highlighted by the Raman bands at 148, 202, 287, 378, 509, 586, and 628 cm⁻¹ [1]. While the Raman spectra of hollandite-group minerals are generally alike, a detailed examination reveals subtle yet noticeable differences that could be utilized to identify specific species. In particular, the peak at 148 cm⁻¹, present in the spectrum of romanechite can be used as a key marker to differentiate this phase from minerals in the hollandite group and todorokite, as suggested by [1]. Additional spectral features helpful for identifying romanechite include the peak near 290 cm⁻¹.

The occurrence of romanechite was confirmed through numerous SEM-EDS analyses, which indicate the widespread presence of a hydrated phase containing Mn and Ba (Figure 3;

Table 1. SEM-EDS mean microanalyses of romanechite; the mean (in brackets, the number of microanalyses) is expressed in wt%.

Area (N. of Analyses)	MnO	BaO	FeO	MgO	SiO2	Al2O3	CaO	Na2O	K2O	Total
romanechite 1A (5)	60.27	5.79	0.68	1.32	2.01	1.58	2.81	0.30	0.52	74.76
romanechite 1B (4)	60.43	5.78	0.67	1.37	1.80	1.44	2.88	0.30	0.51	74.67
romanechite 2A (10)	79.42	6.33	0.27	1.40	0.41	0.39	3.10	0.35	0.51	92.18
romanechite 2B (10)	77.20	6.83	0.74	1.18	1.67	1.21	2.99	0.30	0.58	92.70
romanechite 2C (10)	77.33	10.00	0.04	0.59	0.21	0.37	2.99	0.21	0.24	91.98
romanechite 2D (10)	78.54	9.36	0.00	0.78	0.08	0.28	2.61	0.24	0.28	92.17

). The composition of romanechite varies in its Ba content: (i) white romanechite with about 9.0–10 wt% BaO shows low contents of Al₂O₃ and SiO₂ and (ii) romanechite with BaO = 5–6 wt% contains Al₂O₃ = 1.5 wt% and SiO₂ = 1.8–2.0 wt%, corresponding to greyer portions (Figure 3; Table 1). Low-Ba romanechite seems to be related to occurrences of a clay mineral, which were detected by powder X-ray diffraction as kaolinite, from insoluble residue. Romanechite is associated with kaolinite, which also occurs together with Fe-oxy-hydroxide, filling the pores (Figure 3).

The origin of the Mn-Ba phase from circulating solutions is supported by the growth of acicular, felt-like crystals along fractures, which are subsequently intersected by massive romanechite veins. These veins are later dislocated and welded by reprecipitated calcite (Figure 3). These sites have been filled with Mn-Ba hydroxides, displaying typical crystal growth from aqueous solutions circulating in karst environments. Regular crystals, 10–20 μm in size, coat the fractures, veins, and geodes, resulting in the spectacular free growth of minerals (Figure 3).

Romanechite displays spectacular morphologies with growth rings forming globular shapes (Figure 4). These growth rings result from alternating massive and porous romanechite, as shown by Mn X-ray maps, which highlight variation in Mn content. Some pores in this sample are occupied by kaolinite (Figure 4), which is connected to globular romanechite and to rims of calcite enriched in Al and Si. These structures suggest that romanechite grows from circulating solutions enriched in Mn, Ba, Fe, and Mg, along with the stabilization or neo-formation of kaolinite, which retains Al and Si.

4. Discussion and Conclusions

The main ore mineral is represented by romanechite in which cations such as Ca, Al, Si, Fe, Mg, Na, and K can be found. Mn-Ba hydroxide occurs predominantly as extended granular aggregates with more or less brightness, and as matrix replacing detrital grains. Late diagenetic features of the Mn-Ba phase are indicated by bright white romanechite veins, reflecting the precipitation of thin films lining the walls of the pore spaces near clay minerals. On the other hand, the presence of kaolinite was detected by powder X-ray diffraction from insoluble residue. The structure of romanechite can accommodate different proportions of Al, Si, and Fe depending on their availability. In addition, near the romanechite there are Al-Si-Fe-rich phases with large amounts of Mn that have clay-mineral-like compositions. The growth relationships between clay minerals and Mn-Ba phases can be envisaged to be related to Eh and pH environmental conditions [11,16,18].

A multidisciplinary approach to studying alteration products (such as red incrustations and calcite nodules) on karst limestones reveals mineralization patterns that indicate specific paleoenvironmental conditions [9]. A possible interaction between the limestone bedrock and stormwater runoff, which is enriched with siliciclastic mud and CO₂, plays a crucial role. These muddy waters containing clay minerals can accumulate on the carbonate surface, leading to the dissolution of CaCO₃. Under these conditions, Mn, Fe, and Ba from the mud become concentrated in the circulating solutions that infiltrate the karst limestone fractures. Subsequent water evaporation, accompanied by a rise in temperature, likely induces oxidizing conditions, resulting in CO₂ release and the re-precipitation of calcite at the edges of the pre-existing crystals. These newly formed calcite crystals may incorporate Al, Si, Fe (up to 1% wt), and Mn (around 0.6% wt). Romanechite can form within fractures and pockets when Mn and Ba reach supersaturated levels under these conditions.