minerals Article Reflectance Spectral Characteristics of Minerals in the Mboukoumassi Sylvite Deposit, Kouilou Province, Congo Xian-Fu Zhao 1,2, Zong-Qi Wang 1,*, Jun-Ting Qiu 3,* and Yang Song 2 1 Ministry of Land and Resources Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China; [email protected] 2 School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China; [email protected] 3 National Key Laboratory of Science and Technology on Remote Sensing Information and Image Analysis, Beijing Research Institute of Uranium Geology, Beijing 100029, China * Correspondence: [email protected] (Z.-Q.W.); [email protected] (J.-T.Q.); Tel.: +86-10-82251207 (Z.-Q.W.); +86-10-64962683 (J.-T.Q.) Academic Editor: Dimitrina Dimitrova Received: 20 March 2016; Accepted: 23 May 2016; Published: 14 June 2016 Abstract: 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. Keywords: reflectance spectrum; mineral classification and separation; sylvite deposit; Mboukoumassi; Congo 1. 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¨ MgCl2¨ 6H2O), 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. Minerals 2016, 6, 55; doi:10.3390/min6020055 www.mdpi.com/journal/minerals Minerals 2016, 6, 55 2 of 9 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,sensing, whichwhich hashas beenbeen successfullysuccessfully usedused inin lithologicallithological classificationclassification [1[1–5]–5] and alteration mineral identificationidentification [6–8], [6–8], provides a potentialpotential alternativealternative for halitehalite estimationestimation andand carnallite separation in the solid state. Materials thatthat interact with the electromagnetic waves emitted from the sunsun cancan bebe recordedrecorded byby hyperspectralhyperspectral sensorssensors inin thethe formform ofof aa spectrum.spectrum. A spectrum can essentially fingerprintfingerprint aa material;material; itit cancan be be used used to to search search for for material material with with a similara similar spectrum spectrum and and to investigateto investigate the the composition composition of theof the material material [9]. [9]. In this study, thethe reflectancereflectance spectraspectra (350–2500(350–2500 nm) of several mineral samples (anhydrite, glauberite, halite, carnallite, and tachydrite) were recorded from the Mboukoumassi sylvite deposit, Kouilou Province,Province, DemocraticDemocratic Republic Republic of of the the Congo. Congo. This This was was done done using using an ASDan ASD Inc. Inc. (Boulder, (Boulder, CO, USA)CO, USA) TerraSpec TerraSpec® spectrometer® spectrometer in an attempt in an toattempt understand to under the spectralstand the features spectral of thesefeatures minerals of these and evaluateminerals theand capability evaluate ofthe hyperspectral capability of remote hyperspe sensingctral technologyremote sensing in identification technology andin identification classification ofand minerals classification in the of sylvite minerals deposit. in the sylvite deposit. 2. Geological Geological Backgrounds Backgrounds The Mboukoumassi dep depositosit in in the the southwest southwest of of Kouilou Kouilou Province Province in inthe the Democratic Democratic Republic Republic of ofthe the Congo Congo is an is animportant important potash potash and and halite halite depos depositit in inthe the Congo Congo Basin Basin (Figure (Figure 1).1 ).The The deposit deposit is ishosted hosted in inCretaceous Cretaceous strata strata and and covered covered by by Quat Quaternaryernary sediments. sediments. The The Cretaceous sedimentary strata consist of basalbasal continentalcontinental sedimentary rocks,rocks, a middle stratum of rock salt, and a capping of marine sedimentary rocks. The continental sediment sedimentaryary rocks were formed before or coeval with the formation ofof the the African-American African-American continent continent rift. The rift. rock The salt layerrock bearssalt lightlayer pink, bears salmon-colored light pink, andsalmon-colored dark salmon and colored dark salmon carnallite, colored bluish carnallite, white to steelbluish gray white or lightto steel brown gray halite,or light bischofite, brown halite, and tachydrite.bischofite, and This tachydrite. unit is composed This unit of fouris composed sedimentary of four cyclothems, sedimentary with cyclothems, bischofite and with tachydrite bischofite in theand toptachydrite layer, and in the bituminous top layer, shaleand bituminous (or asphaltenes-halite) shale (or asphaltenes-halite) in the bottom layer. in the The bottom rock layer. salt layer The thickensrock salt fromlayer the thickens edges to from the centerthe edges of the to basin, the cent wither thickness of the basin, in the with range thickness of 170–600 in m. the The range marine of sedimentary170–600 m. The rocks marine were sedimentary deposited at rocks the edge were of depos the thenited African at the edge continental of the then margin African and continental differ from mostmargin other and marine differ from deposits most because other marine it lacks depos sulfatesits because and carbonates. it lacks sulfates and carbonates. Figure 1. Geological map for Mboukoumassi deposit (Mod (Modifiedified after Martiniand Bowles (1994) [[10]).10]). Minerals 2016, 6, 55 3 of 9 During the Cretaceous Period, the Mboukoumassi deposit was located in a rift basin where tectonicDuring movements the Cretaceous were lacking, Period, resulting the Mboukoumassi in negligible deposit deformation. was located This inis aevident rift basin from where the tectonichorizontal movements and highly were continuous lacking, deposit, resulting and in its negligible uniform deformation.thickness. Only This minor is evident small faults from and the horizontalsome caverns and can highly be found continuous in the strata. deposit, and its uniform thickness. Only minor small faults and someThe caverns dominant can be ore found minerals in the in strata. the Mboukoumassi deposit are carnallite and potassium-halite. The potassium-haliteThe dominant ore ore minerals bodies inare the discontinuous Mboukoumassi and deposit are lenticular are carnallite in shape, and as potassium-halite. opposed to the Thestratiform potassium-halite and continuous ore bodies carnallite are discontinuousore bodies. The and gangue are lenticular minerals in include shape, ashalite, opposed anhydrite, to the stratiformbischofite, tachydrite, and continuous plus minor carnallite amounts ore bodies. of dolomite, The ganguecalcite, and minerals clay. The include ore-bearing halite, anhydrite,stratum is bischofite,mainly composed tachydrite, of carnallite plus minor and amounts halite ofinterb dolomite,eds, where calcite, bituminous and clay. Theshale ore-bearing and bischofite stratum are isdeveloped. mainly composed The total average of carnallite thickness and haliteof strata interbeds, that hosts where ore bodies bituminous is 259.61 shale m. and bischofite are developed. The total average thickness of strata that hosts ore bodies is 259.61 m. 3. Samples and Analytical Methods 3. Samples and Analytical Methods The mineral samples analyzed in this study were collected from drill cores BS-3 and ZK143. The photographsThe mineral