Reflectance Spectral Characteristics of Minerals in the Mboukoumassi Sylvite Deposit, Kouilou Province, Congo

This study presents reflectance spectra, determined with an ASD Inc. TerraSpec® spectrometer, of five types of ore and gangue minerals from the Mboukoumassi sylvite deposit, Democratic Republic of the Congo. The spectral absorption features, with peaks at 999, 1077, 1206, 1237, 1524, and 1765 nm, of the ore mineral carnallite were found to be different from those of gangue minerals. Spectral comparison among carnallite samples from different sylvite deposits suggests that, in contrast to spectral shapes, the absorption features of carnallite are highly reproducible. Heating of carnallite to 400 and 750 ̋C, and comparing the spectra of heated and non-heated samples, indicates that spectral absorption is related to lattice hydration or addition of hydroxyl. Since carnallite undergoes deliquescence easily, the absorption features of carnallite in the 350–2500 nm spectrum could serve as a robust tool for carnallite identification and separation.


Introduction
Potassium, a common soil fertilizer, is important for modern agriculture as it improves water retention, nutrient value, and disease resistance of food crops. Often referred to as potash, most industrial potassium is derived from sylvite deposits, including those in Canada, Russia, China, Belarus, Israel, Germany, Chile, the United States, Jordan, Spain, the United Kingdom, and Brazil. Potash ores are typically rich in potassium chloride (KCl) and sodium chloride (NaCl), and are often buried deep below the Earth's surface. Extraction of potash ore is typically by conventional shaft mining with the extracted ore ground into grains.
Carnallite (KCl¨MgCl 2¨6 H 2 O), a key potassium-bearing mineral used for potassium production, is usually spatially associated with halite, a common gangue mineral in carnallite ore. The occurrence of halite may reduce the economic benefit of sylvite and increase difficulties during potassium purification. Therefore, to improve processing efficiency, it is necessary to determine halite content and distinguish carnallite from gangue minerals. Traditional geochemical methods for halite content estimation are limited because analyzed samples are typically collected discontinuously from drill cores, and the dissolution method for separating carnallite from halite is time-consuming and uneconomic.
Hyperspectral remote sensing, which has been successfully used in lithological classification [1][2][3][4][5] and alteration mineral identification [6][7][8], provides a potential alternative for halite estimation and carnallite separation in the solid state. Materials that interact with the electromagnetic waves emitted from the sun can be recorded by hyperspectral sensors in the form of a spectrum. A spectrum can essentially fingerprint a material; it can be used to search for material with a similar spectrum and to investigate the composition of the material [9].
In this study, the reflectance spectra (350-2500 nm) of several mineral samples (anhydrite, glauberite, halite, carnallite, and tachydrite) were recorded from the Mboukoumassi sylvite deposit, Kouilou Province, Democratic Republic of the Congo. This was done using an ASD Inc. (Boulder, CO, USA) TerraSpec ® spectrometer in an attempt to understand the spectral features of these minerals and evaluate the capability of hyperspectral remote sensing technology in identification and classification of minerals in the sylvite deposit.

Geological Backgrounds
The Mboukoumassi deposit in the southwest of Kouilou Province in the Democratic Republic of the Congo is an important potash and halite deposit in the Congo Basin ( Figure 1). The deposit is hosted in Cretaceous strata and covered by Quaternary sediments. The Cretaceous sedimentary strata consist of basal continental sedimentary rocks, a middle stratum of rock salt, and a capping of marine sedimentary rocks. The continental sedimentary rocks were formed before or coeval with the formation of the African-American continent rift. The rock salt layer bears light pink, salmon-colored and dark salmon colored carnallite, bluish white to steel gray or light brown halite, bischofite, and tachydrite. This unit is composed of four sedimentary cyclothems, with bischofite and tachydrite in the top layer, and bituminous shale (or asphaltenes-halite) in the bottom layer. The rock salt layer thickens from the edges to the center of the basin, with thickness in the range of 170-600 m. The marine sedimentary rocks were deposited at the edge of the then African continental margin and differ from most other marine deposits because it lacks sulfates and carbonates.
Minerals 2016, 6, 55 2 of 9 discontinuously from drill cores, and the dissolution method for separating carnallite from halite is time-consuming and uneconomic. Hyperspectral remote sensing, which has been successfully used in lithological classification [1][2][3][4][5] and alteration mineral identification [6][7][8], provides a potential alternative for halite estimation and carnallite separation in the solid state. Materials that interact with the electromagnetic waves emitted from the sun can be recorded by hyperspectral sensors in the form of a spectrum. A spectrum can essentially fingerprint a material; it can be used to search for material with a similar spectrum and to investigate the composition of the material [9].
In this study, the reflectance spectra (350-2500 nm) of several mineral samples (anhydrite, glauberite, halite, carnallite, and tachydrite) were recorded from the Mboukoumassi sylvite deposit, Kouilou Province, Democratic Republic of the Congo. This was done using an ASD Inc. (Boulder, CO, USA) TerraSpec ® spectrometer in an attempt to understand the spectral features of these minerals and evaluate the capability of hyperspectral remote sensing technology in identification and classification of minerals in the sylvite deposit.

Geological Backgrounds
The Mboukoumassi deposit in the southwest of Kouilou Province in the Democratic Republic of the Congo is an important potash and halite deposit in the Congo Basin ( Figure 1). The deposit is hosted in Cretaceous strata and covered by Quaternary sediments. The Cretaceous sedimentary strata consist of basal continental sedimentary rocks, a middle stratum of rock salt, and a capping of marine sedimentary rocks. The continental sedimentary rocks were formed before or coeval with the formation of the African-American continent rift. The rock salt layer bears light pink, salmon-colored and dark salmon colored carnallite, bluish white to steel gray or light brown halite, bischofite, and tachydrite. This unit is composed of four sedimentary cyclothems, with bischofite and tachydrite in the top layer, and bituminous shale (or asphaltenes-halite) in the bottom layer. The rock salt layer thickens from the edges to the center of the basin, with thickness in the range of 170-600 m. The marine sedimentary rocks were deposited at the edge of the then African continental margin and differ from most other marine deposits because it lacks sulfates and carbonates.  During the Cretaceous Period, the Mboukoumassi deposit was located in a rift basin where tectonic movements were lacking, resulting in negligible deformation. This is evident from the horizontal and highly continuous deposit, and its uniform thickness. Only minor small faults and some caverns can be found in the strata.
The dominant ore minerals in the Mboukoumassi deposit are carnallite and potassium-halite. The potassium-halite ore bodies are discontinuous and are lenticular in shape, as opposed to the stratiform and continuous carnallite ore bodies. The gangue minerals include halite, anhydrite, bischofite, tachydrite, plus minor amounts of dolomite, calcite, and clay. The ore-bearing stratum is mainly composed of carnallite and halite interbeds, where bituminous shale and bischofite are developed. The total average thickness of strata that hosts ore bodies is 259.61 m.

Samples and Analytical Methods
The mineral samples analyzed in this study were collected from drill cores BS-3 and ZK143. The photographs of individual representative samples are shown in Figure 2, and the location and physical properties of the samples are given in Table 1. The elemental compositions of these samples ( Table 2) were determined using atomic absorption at the Chinese Academy of Geological Sciences. During the Cretaceous Period, the Mboukoumassi deposit was located in a rift basin where tectonic movements were lacking, resulting in negligible deformation. This is evident from the horizontal and highly continuous deposit, and its uniform thickness. Only minor small faults and some caverns can be found in the strata.
The dominant ore minerals in the Mboukoumassi deposit are carnallite and potassium-halite. The potassium-halite ore bodies are discontinuous and are lenticular in shape, as opposed to the stratiform and continuous carnallite ore bodies. The gangue minerals include halite, anhydrite, bischofite, tachydrite, plus minor amounts of dolomite, calcite, and clay. The ore-bearing stratum is mainly composed of carnallite and halite interbeds, where bituminous shale and bischofite are developed. The total average thickness of strata that hosts ore bodies is 259.61 m.

Samples and Analytical Methods
The mineral samples analyzed in this study were collected from drill cores BS-3 and ZK143. The photographs of individual representative samples are shown in Figure 2, and the location and physical properties of the samples are given in Table 1. The elemental compositions of these samples ( Table 2) were determined using atomic absorption at the Chinese Academy of Geological Sciences.    The spectra of mineral samples from the Mboukoumassi deposit were measured using an ASD Inc. TerraSpec ® spectrometer at the China National Key Laboratory of Science and Technology on Remote Sensing Information and Image Analysis (NKLST-RSIIA). The technical specifications of the spectrometer are summarized in Table 3.  The experimental procedure proceeds as follows: the spectrometer is set up according to the Labspec4 user manual and connected to a laptop. Data are recorded and manipulated using the RS3 software package (v6.4, ASD Inc.). An accessory light source is used so that measurements can be carried out without the need for sunlight. A white reference is introduced to optimize and calibrate the instrument; the reflectance of which is automatically set to 1 by the RS3 software.
Then, the detector is carefully placed on to the samples to avoid possible influence from fluorescence arising from the environment. Two or more measurements were carried out for each sample on different parts of the sample to check for unique spectral characteristic(s). Five spectra were recorded for each measurement to monitor any variation during the measurement. The spectrum of the white reference is measured again after the experiment to check for any significant changes of the standard spectrum. The spectra of the white reference before and after the experiment are shown in Figure 3.
The spectra of mineral samples are exported as ASCII files using RS3 and ViewSpecPro ® software (v.6.2, ASD Inc.), respectively. These data are then converted into a spreadsheet file with Excel ® 2007 and plotted with CorelDRAW ® X5 (Corel Corporation, Ottawa, ON, Canada). The results are listed in Supplementary Materials and displayed in Figure 4. The name list of the spectral files and related mineral samples is given in Table 4.

Results
The spectra of the white reference before and after the experiment are approximately equal to 1, suggesting that no significant variation occurred during the experiment (Figure 3). The five spectra in each sample measurement are nearly identical, indicating no variation of absorption features during the measurement. This is shown in Figure 4, which displays the spectra of eight mineral samples.
Although the spectra of different parts of each mineral sample differ, they are similar in shape and show unique absorption features (Figure 4) Table 5 and are compared with those presented in the United States Geological Survey (USGS) spectral library.

Results
The spectra of the white reference before and after the experiment are approximately equal to 1, suggesting that no significant variation occurred during the experiment (Figure 3). The five spectra in each sample measurement are nearly identical, indicating no variation of absorption features during the measurement. This is shown in Figure 4, which displays the spectra of eight mineral samples.
Although the spectra of different parts of each mineral sample differ, they are similar in shape and show unique absorption features (Figure 4) Table 5 and are compared with those presented in the United States Geological Survey (USGS) spectral library.

Discussion
As the dominant potassium-bearing mineral in the Mboukoumassi deposit, it is important that carnallite is distinguished from gangue minerals, such as anhydrite, glauberite, halite, and tachydrite. Since carnallite and halite have the same origin and are typically associated, it is difficult to distinguish between them and also to distinguish them from other gangue minerals. The traditional dissolution method for separating carnallite from halite is time-consuming, as is manual sorting. Whichis also uneconomic. Fortunately, mineral identification based on mineral spectra provides a potential approach for solid state mineral separation. The result of this study have shown absorption peaks at 999, 1077, 1206, 1237, 1524 and 1765 nm in carnallite, which are distinct from those of halite (1660 nm), suggesting that these minerals can be distinguished using reflectance spectra.
In the Mboukoumassi deposit, two types of carnallite can be distinguished based on color: a light pink variety and a salmon-to dark salmon-colored variety ( Figure 2 and Table 1). Bulk-rock geochemistry suggests that they are similar in element composition, but the red shade of the carnallite is probably due to ferric oxide inclusions [11]. The two types of carnallite have distinct spectra, but their absorptions are almost identical, except for two absorptions at 532 and 793 nm. The high degree of spectral similarity between the two carnallite varieties suggests that carnallite separation based on mineral spectral absorption is more reliable than that based on the general spectral shape.
In addition to the absorption features reported in this study, spectra of carnallites from other deposits were also collected. As shown in Figure 5a, the spectra of carnallites from Carlsbad, New Mexico [12] are also characterized by absorption peaks at 999, 1077, 1206, 1237, 1524, and 1765 nm and show high absorption reproducibility. To investigate the mechanism of these absorptions, elemental compositions of the carnallite samples from the Mboukoumassi deposit were reviewed. Samples BS-3-94 and BS-3-102 have high Na, K, Mg, and Cl contents with minor Ca, Br, and S. These elements are not critical to absorption features in the 350-2500 nm reflectance spectrum.
Since hydration of or addition of hydroxyl to the mineral may result in spectral absorptions [13] a heating experiment was carried out to investigate the relationship between carnallite spectral absorptions and hydration or hydroxyl addition. Sample BS-3-102 was powdered to 200 µm then heated to temperatures of~400 and~750˝C in a muffle furnace. It was found that the spectral measurement for heated carnallite was the same as for raw carnallite. The spectra of sample BS-3-102 before and after heating are shown in Figure 5b; the absorption peaks of raw carnallite at 999, 1077, 1206, 1237, 1524, and 1765 nm disappeared or became weak after dehydration, suggesting that the above absorptions are related to hydration. Since carnallite undergoes deliquescence easily, the absorptions at 999, 1077, 1206, 1237, 1524, and 1765 nm may be able to be widely used as diagnostic features of carnallite and for distinguishing from halite. measurement for heated carnallite was the same as for raw carnallite. The spectra of sample BS-3-102 before and after heating are shown in Figure 5b; the absorption peaks of raw carnallite at 999, 1077, 1206, 1237, 1524, and 1765 nm disappeared or became weak after dehydration, suggesting that the above absorptions are related to hydration. Since carnallite undergoes deliquescence easily, the absorptions at 999, 1077, 1206, 1237, 1524, and 1765 nm may be able to be widely used as diagnostic features of carnallite and for distinguishing from halite.  Hunt (1972) [12]; and (b) spectral comparison between raw and heated carnallite samples. Note that the reflectance values of the carnallite sample from Hunt (1972) [12] were divided by fourfor clarity.  Hunt (1972) [12]; and (b) spectral comparison between raw and heated carnallite samples. Note that the reflectance values of the carnallite sample from Hunt (1972) [12] were divided by fourfor clarity.