Introduction

Like the rest of the East African Rift System, the Main Ethiopian Rift (MER) has experienced an extremely complex tectonic history. The axial zone, or rift axis, of the MER is the youngest portion. It overlaps with the Wonji Fault Belt, which primarily originated during the Quaternary epoch (Mohr 1970; Boccaletti et al. 1998; Abebe et al. 2007; Corti 2009). Active NNE-SSW-trending extension fractures and normal faults with a right-stepping en-echelon layout are features of the Wonji fault belt. Two primary Pleistocene magmatic episodes are known along these faults: (1) basaltic flows that erupted between 2 and 1 Ma, followed by ignimbrites and a few silicic centers, and (2) axial silicic volcanics and basalts that erupted in the last ~ 650 Ka.

Huge industrial mineral reserves, including potash, salt, gypsum, limestone, diatomite, clay, and pumice, as well as some metallic mineral commodities, such as Au, Fe, and Mg, are present in this rift (Assefa 1985; Fentaw and Mengistu 1998; Tadesse et al. 2003; Ghebre 2019). Among the industrial mineral resources in MER, kaolin is the most common and important. This resource is also used in the production of cosmetics, fertilizers, insecticides, pigment, cement, ceramics, glass, floor tiles, bricks, and pottery (Murray 2006). Many geologists have undertaken studies and found many kaolin suites, including the Ammacio kaolin (Fentaw and Mengistu 1998; Bedassa et al. 2019; Getaneh and Gezhagne 2020; Getaw et al. 2020). Fentaw and Mengistu (1998), Getaneh and Gezahegn (2020), and Getaw et al. (2020), for example, investigated the genesis, industrial use, mineralogy, and geochemistry of the kaolin suites from Bombowha, Kombolcha, Belessa, Alemtena, Bensa, and Debre Tabor. The presence of kaolin in the Ammacio area has been documented by Berhanu et al. (2021), but a thorough investigation into the mineralogy, geochemistry, and physical characteristics of this kaolin has not yet been carried out to assess its appropriateness for usage in several application areas. The current investigation concentrated on the Ammacio kaolin occurrence’s geology, mineralogy, physicochemical characteristics, and its industrial applications.

Regional geologic and tectonic setting

According to Corti (2009), continental rifting in the MER has evolved in two different phases. The initial continental rifting in MER started mainly from the early Miocene to Pleistocene, and it is characterized by displacement along large boundary faults, formation of rift depression with local development of deep asymmetric basins (up to 5 km), and diffuse magmatic activity. During this stage, magmatism encompassed the whole rift, with volcanic activity affecting the rift basins, the major boundary faults, and limited portions of the rift shoulders. The second phase of rift evolution in MER started late Pleistocene to recent, and it is characterized by rift ward narrowing of the volcano-tectonic activity. In this stage, the main boundary faults were deactivated, and extensional deformation was accommodated by dense swarms of faults so-called Wonji fault belts in the thinned rift depression. This progressive thinning of the continental lithosphere under constant, prolonged oblique rifting conditions controlled the magmatic conditions of the rift floor (Fig. 1).

Fig. 1
figure 1

Simplified geological map of central Main Ethiopia Rift (MER). Modified after (Abebe et al., 2010). (1) Pre-Tertiary sediments and crystalline basement; (2) Oligocene (32–29 Ma) and lower Miocene (12–8 Ma) plateau volcanics; (3) Miocene–Pliocene rift-shoulder trachytic–rhyolitic volcanic and pyroclastic layers; (4) Plio-Pleistocene rift floor; (5) Quaternary central volcanic and basaltic lava flows, associated scoria cones and phreato-magmatic deposits; (6) Quaternary lacustrine sediments and interbedded pyroclastic; (7) faults; (8) major rift border faults; (9) major transversal tectonic lineaments in the basement; and (10) Wonji Fault Belt segments. The bold red square area shows the study area (Ammacio). In the inset: YTVL, Yerer-Tullu-Wellel volcano tectonic lineament, MER, Main Ethiopian Rift

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Regardless of geological and geochronological investigations, different authors (e.g., Corti et al. 2001; Abebe et al. 2007; Beutel et al. 2010) distinguished two major phases of volcanic episodes in the MER during Pleistocene (Fig. 1). The first episode of volcanic eruption in MER is characterized by several layers of pyroclastic rocks associated with trachytic and rhyolitic lava domes and flows together with some important central volcanoes (5–3 Ma) and covers the MER shoulders and floor. The second phase of volcanic eruption in MER is distinguished by uncompact pumiceous fall and flow deposits, rhyolitic–trachytic lava flows forming central volcanic edifices, fissural basaltic lava flows with associated scoria and phreatomagmatic cones, and interbedded lacustrine deposits with age reaches up to 650 Ka. In general, the MER is mainly characterized by a bimodal basalt-rhyolite association volcanism with rare intermediate magma composition (Peccerillo et al. 2003). It is covered by Miocene to Quaternary volcanic rocks predominantly, pyroclastic fall and flow, trachyte, basalts, lacustrine, and Quaternary sediments (Fig. 1; Abebe et al. 2007; Rooney 2010).

Materials and methods

Field geology

Field geology was conducted for the purpose of geological mapping, characterization of lithological units, collection of structural data, and samples for laboratory analysis. Chip samples of potential host rocks have been collected for mineralogical and geochemical analysis. Fresh kaolin grab samples were taken from locations where kaolinization is predominant. Samples were taken with a geological hammer from the wall of vertically exposed kaolin. All samples, each weighing just 1 kg, were labeled and packed in plastic bags for subsequent laboratory testing. A geological map of the Ammacio occurrence is also being prepared at a scale of 1:25,000.

Petrography

Thin sections are prepared for the selected six samples. Typical four tuff and two basaltic rock samples were collected from the Ammacio area for thin section preparation based on lithological variability and the importance of identifying them during fieldwork. This helps to identify the minerals found in the rock and to understand the texture of the rock and the parent rock responsible for the Ammacio kaolin. The thin-section preparation is carried out in the Central Geoscience Laboratory of the Geological Survey of Ethiopia. A petrographic description of these rocks was carried out in the petrological laboratory of the Arbaminch University geology department.

X-ray diffraction (XRD) analysis

This analysis is performed to know the minerals present in the Ammacio kaolin occurrence and to know what percentage of the mineral was found. This qualitative and quantitative information about the minerals present in the kaolin deposits is determined by XRD. XRD analysis was performed at Adama Science and Technology University. Seven kaolin samples are selected for XRD analysis to determine mineral composition. The diffractometer was fitted with a copper tube and operated at 40 kV and 30 mA ranging from 3° to 80° of 2Ɵ for about 1 h per sample. Match 3 and Origin 2018 software were used for the qualitative and quantitative determination of the mineralogy that is present in the kaolin samples. The sample preparation goes through three main steps. First, the kaolin sample is air-dried, and then, the sample is ground with mortar. Finally, the sample is pulverized (1 g of the pulverized sample is used for this analysis). In preparation, care was always taken to wash all materials (mortar and sieve) to eliminate contamination.

Physical tests

To determine the potential industrial application of the Ammacio kaolin, a number of key tests (i.e., color, grain size, specific gravity, pH, Atterberg limit, and bulk density) were performed. A dry-sieving method was used to perform the analysis. Seven kaolin samples were selected for grain size analysis, weighing 1 kg for each sample to be used. The selected sample was separately conical-cut and quartered to obtain homogeneous and representative fractions. The quartered samples were dried in an oven at a temperature of 105 ℃ for 24 h. The weight of each sample was measured and placed in a set of stacked sieves ranging in opening diameter from 2.36 mm to 75 microns and in a pan. The sieves were electrically shaken for 10 min. After sieving, the sample retained on each sieve and pan was measured on an electronic balance and its mass was recorded. The analysis was performed in the Soil Mechanics Laboratory of the Civil Engineering Department of Arbaminch University.

Bulk density

The average bulk density value of the Ammacio kaolin samples is used as an input parameter to estimate the tonnage of kaolin deposits found in the area. The test will be conducted on seven samples at the Geological Survey of Ethiopia.

pH analysis

At Arbaminch University’s soil mechanics civil engineering laboratory facility, the pH level of the Ammacio kaolin was evaluated. Each 10-g sample of kaolin was taken, mixed with 25 ml of distilled water, and left in the solutions for 30 min to assess the pH of the samples. Place the pH sensor electrode into the solution, wait 5 min, and then take a pH meter reading.

Color assessment

Both the preliminary and fired color assessments of the Ammacio kaolin were conducted at the Geological Survey of Ethiopia by comparing the kaolin samples visually with the Munsell soil color chart.

Attenuated total reflection Fourier transform infrared (ATR-FTIR)

This analysis is conducted at the Addis Ababa Science and Technology University. This analysis is complementary to that of XRD, mainly for qualitative and quantitative study of mineral components found in the kaolin samples. FTIR spectrum of the kaolin sample was measured on spectrum FTIR (Perkin Elmer) in the frequency range 4000 cm−1–400 cm−1 (at resolution 4 cm−1, number of scans: 4 per second) using KBr pellets.

Scanning electron microscope analysis

The morphology of kaolin is important to elucidate the industrial properties of the resource. For this purpose, four representative kaolin samples were selected. One-gram samples that penetrated 0.60 microns and were retained on a 0.30-micron sieve were used for analysis. The grains are washed and air-dried to remove the moisture content of the kaolin samples. The Ammacio kaolin occurrence’s morphology was determined using an SEM (JCM-6000Plus) following the sample’s grinding into a powder (less than 2 µm) at the Geological Survey of Ethiopia. Three scans were performed on the sample with a monitor magnification power of 150 × , operating at 10 V, and covering a field of view of 795 µm to obtain the morphology of the Ammacio kaolin. This analysis was performed at the Adama Science and Technology University Biological Laboratory.

Geochemical analysis

Representative samples of kaolin and its host rock were collected based on color, composition, and grain size variations. Accordingly, 11 representative fresh samples were selected (four host rocks and seven kaolin) in the first phase of the work. As a preliminary step in sample preparation, remove the weathered part from the surface of the sample and crush it into the desired sizes. In the second step, the crushed fresh sample was crushed in a jaw crusher, and finally, the crushed sample was crushed into micron-sized particles in an automatic agate ball milling machine. After the preparation, the samples were pulverized to 75 microns to produce a suitable particle size. The powdered samples were sent to the University of Arbaminch Geophysical and Geochemical Laboratory Center for major and trace element analysis to determine the quantification of each sample by X-ray fluorescence (TN9000) on standard grade. The XRF TN9000 machine model was used to hold the samples for elemental analysis. To produce the X-rays that will energize the sample for a specified amount of time (specifically, 120 s), the machine was run at its maximum voltage and current of 60 kV and 1 mA, respectively, using a rhodium anode X-ray tube and a scintillation detector with a current of 40 mA and a voltage of 40 mV.

Results and discussion

Results from field investigations, mineralogical, and physico-geochemical analyses are presented here. The analyses are helping to characterize and determine the potential industrial applications of the Ammacio kaolin. Geologically, the Ammacio area lies in the southwest of the central Main Ethiopian Rift. The various lithological units that occur in the northern, southern, and central parts of the study area are basalt, unwelded tuff, and Quaternary sediments. They were described on both exploratory and hand sample scales and finally plotted on the geological map of the study area at a scale of 1:25,000 (Fig. 2). In addition, the geological cross-section is also prepared to show the vertical stratigraphic continuity of the unit (Fig. 3). The detailed geological character of each unit is discussed below.

Fig. 2
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Lithological map of Ammacio, southwest of central MER

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Fig. 3
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Photomicrograph of Ammacio plagioclase-phyric basalt in PPL (A, C) and XPL (B, D) at 40 × magnifications power (where plg—plagioclase)

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Geologic setting and petrography

Porphyritic basalt

Porphyritic basalt is the oldest rock unit in the study area and is mainly laid at the bottom of the study area’s stratigraphy (Fig. 2). This rock unit is exposed by road cuts and rivers in the western part of the target area. Locally, this rock unit is undergoing exfoliation (spheroidal) weathering, and it is dark gray in color with large visible and shiny crystals of plagioclase and olivine minerals. From the stratigraphy investigations of the targeted area, this rock unit is directly overlaid by tuff rock units and shows sharp contact. The petrographic study result shows that the porphyritic basalt is composed of 75% groundmass, 18% plagioclase, 4% olivine, 2% pyroxene, and 1% opaque minerals (Fig. 3).

Unwelded tuff

This rock unit was exposed in the central and eastern parts of the study area (Fig. 2). Physically, the tuff unit varies from light gray to brownish yellow and shows moderate to high welding features from top to bottom. Contrary to popular belief, this rock unit was exposed to road cuts, hillsides, stream cuts, riverbeds, and banks. The Ammacio tuff unit was predominantly exposed around the Harme River, which is the major tributary of the Gibe River, and overlaid directly by porphyritic basalt and unconsolidated sediment. Moreover, this rock unit was locally affected by numerous numbers of joints. Furthermore, the rock mainly forms cliff topography. Despite the topographic features, the geological structure of the study area played a greater role in the percolation of natural water through the tuff unit and facilitated its weathering to the kaolin resources. The petrographic results reveal that the tuff unit is mainly composed of alkali-feldspar (13%), volcanic glass (83%), quartz (4%), and opaque minerals (1%) (Fig. 4).

Fig. 4
figure 4

Photomicrograph of Ammacio tuff in PPL (A, C) and XPL (B, D) at 40 × magnification power (San = Sandine, Qtz = quartz, Plg = Plagioclase, Alt = alteration)

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Quaternary sediments

These lithological units are younger and directly overlie the tuff, porphyritic basalt, and kaolin deposits of the study area (Fig. 2). These sediments are light gray and reddish brown in color. Moreover, the sediments are made up of silt, clay, and sand-sized fragments, kaolinite, organic matter, and rock pieces. These lithological units are visible throughout the entire area under investigation. These sediments are mostly found along the Harme River. The sediment ranges in thickness from 0.75 to 15 m, with the top of the kaolin deposit recording the thinnest thickness.

Morphology of kaolin deposit

The morphology of the Ammacio kaolin was studied using scanning electron microscopy techniques. The shape and size of kaolin control its industrial applications. In the paint, pigment, and abrasive industries, the shape of the kaolinite is taken into account. Grain shape and size usually control the viscosity of the kaolin (Murray 2007). The SEM analysis of the Ammacio kaolin shows stacked, platy, and pseudohexagonal morphology (Fig. 5).

Fig. 5
figure 5

Morphology (A, C) and a diagrammatic sketch of some kaolinite particles (B, D), (Hemi = hemispherical)

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Physical characteristics of the Ammacio kaolin occurrence

Grain size distribution

Grain size is one of the most critical physical properties of kaolin, which determines its fitness for various industries. It influences the viscosity of the clay (Bain 1971; Murray 2006). In the present study, this analysis was conducted on seven representative kaolin samples. According to the result, silt plus clay in the kaolin spans from 67.56 to 83.79%, this is below the standard required for filler (greater than 90%). The grain size characteristics of the studied kaolin occurrence correspond to the demands of different industries. At different magnitudes, the grain size distribution of the studied kaolin falls on average at 76.91% silt plus clay and 26.09% sand size (Table 1). Consequently, Ammacio kaolin fails in the filler industry; yet dry gridding and crashing are necessary to maximize the kaolin’s size for the necessary industries. The kaolin has a decreasing grain size from bottom to top due to the degree of weathering in the Ammacio area.

Table 1 Grain size proportion of the Ammacio kaolin occurrence
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Bulk density

Bulk density is a direct measurement that reflects the density of the constituent minerals or materials that include open spaces. The bulk density of the Ammacio kaolin is an important physical property used to evaluate the geologic origin and industrial application. Bulk density indirectly may yield significant data on the environment in which the kaolin originated (Murray 2006). Kaolin obtained from hypogene has a bulk density of greater than 2 gm/cm3, whereas kaolin formed through weathering has a bulk density of less than 2 gm/cm3 (Murray 2006). The average bulk density of the Ammacio kaolin is 1.36 g/cm3, which is in a range of supergene processes. In addition, the bulk density of the Ammacio kaolin commonly plays a crucial role in its economic value (Table 2).

Table 2 Different physical tests conducted for the Ammacio kaolin
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Other physical properties of the Ammacio kaolin

Most samples from Ammacio kaolin occurrence are pale yellow (2.5Y 8/3) in color (according to the Munsell soil map); some kaolin samples show light yellowish brown (2.5Y 6/3) and pink (7.5Y 7/4) colors (Table 2). Besides color, particle size distribution is the most important physical parameter driving the commercial viability of kaolin deposits (Prasad et al. 1991; Bloodworth et al. 1993). The average specific gravity value of the Ammacio kaolin is 2.39, which is within the range of the theoretical value of kaolin (2.16–2.68) (Murray 2006). In addition, the pH of the Ammacio kaolin ranges from 6.58 to 7.43, with an average of 6.99, which is between the ideal pH of kaolin (4–9) (Murray 2007). On the other hand, the plasticity of kaolin has a similarly greater effect on the industrial applications of kaolin (Bain 1971; Murray 2007). Accordingly, the Ammacio kaolin has a 20.25% average plasticity and a 1.87 shrinkage limit; as a result, the Ammacio kaolin is important to produce fired ceramic products (Table 2).

Mineralogical characterization

The mineralogical composition of the Ammacio kaolin occurrence is given in Tables 3 and 4. The quantitative XRD and ATR-FTIR analysis results indicate that kaolinite is the dominant mineral with a considerable amount of quartz (Tables 3 and 4, Figs. 7 and 8). The average kaolinite and quartz content for the Ammacio kaolin is 72.65% and 24.78%, respectively. Ammacio-kaolin XRD patterns are characterized by sharp diffraction peaks (Fig. 6). High-structural defect kaolinite is expected to have broad peaks (Murray 2007). The XRD peaks are also related to the degree of weathering in such a way that the peaks become sharper with increasing intensity of weathering (Duzgoren-Aydin et al. 2002). The presence of kaolinite in the Ammacio kaolin is supported by two clearly observable signals around 3693 and 3620 cm−1 in the FTIR spectral band (Table 4 and Fig. 7). In addition, its crystallinity is also shown by having an apparent peak in the wavenumber region of 800 and 750 cm−1. However, the mineralogy of the Ammacio kaolin was analyzed using XRD, ATR-FTIR, and a light microscope, and the results are presented below.

Table 3 Mineralogical compositions of the Ammacio kaolin
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Table 4 FTIR spectra summarized for the Ammacio kaolin (Saikia et al. 2003; Vaculíková et al. 2011)
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Fig. 6
figure 6

XRD pattern of the Ammacio kaolin samples (K = kaolinite; Q = quartz, F = feldspar)

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Fig. 7
figure 7

A–D FTIR spectra of the Ammacio kaolin samples

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XRD pattern

The XRD pattern of the Ammacio kaolin samples shows variations in intensity and crystallinity (Fig. 6). The differences in the X-ray peaks (sharpness and resolution) are related to the degree of crystallinity of kaolin, which has a higher crystallinity and shows sharp X-ray diffraction peaks (Ritz et al. 2011). In the Ammacio kaolin, kaolinite is observed at 12°, 21.18°, 25°, 26.5°, 30°, 32°, 33.8°, 35.4°, 38.58°, 51.7°, 58.7°, and 62.55°, whereas quartz is seen at 27°, 42°, 50°, and 58.6°, corresponding to 2Ɵ (Fig. 6). The sharpness and intensity of an X-ray diffraction peak can usually be regarded as an indication of the degree of crystallinity and the perfection of atomic arrangement within the structural framework of a mineral. Thus, the nature of quartz reflection should be acceptable as a standard upon which the perfection of the atomic arrangement of the kaolinite and quartz minerals can be compared. When the intensity of a mineral increases, the crystallinity of the mineral also increases.

ATR-FTIR analysis

ATR-FTIR analysis is used for mineralogical identification and quality characterization of clay minerals (Madejová 2003; Saikia et al. 2003; Saikia and Parthasarathy 2010). According to Saikia et al. (2003), the ATR-FTIR analysis technique is complementary and commonly used with XRD analysis for clay mineral studies. According to the authors, OH and Si–O functional group absorption bands between 400 and 4000 cm−1 on the ATR-FTIR spectra region are ordinarily used for the identification of different clay mineral families. Kaolinite is identified by having double peaks between 3697 cm−1 and 3620 cm−1, whereas its crystallinity is differentiated by its double peaks at 3697 cm−1 and 3620 cm−1 and two obvious peaks between 800 and 750 cm−1 (Fig. 7, Table 4). The two clear, observable peaks in the high infrared region, mainly around, indicate the presence of kaolinite. In similar fashion, the kaolinite crystallinity is supported by having two obvious peaks in the lower inferred region, mainly around 798 cm−1 and 750 cm−1 (Fig. 7, Table 4).

Geochemistry

Major and trace element geochemistry

The SiO2 concentration of the fresh rock ranges from 69.4 to 74.12% (Table 5) and falls within the felsic rock composition (Dill et al. 1997) (Fig. 8). Therefore, a sample is chemically classified as a rhyolitic tuff. Most of the major elements are concentrated in the parent rocks, except for Al and Ti, which show a strong positive correlation between them (Fig. 9E). This is due to their immobility during volcanic rock alteration (Bedassa et al. 2019). All other major elements, including iron, have been mobilized and are negatively correlated with Al (Fig. 9 A–D and F). Since the mobility of Fe during weathering is a function of Eh and pH conditions, it is usually hypothesized that acidic and reducing conditions facilitate the removal of Fe from the weathering profile (Middelburg et al. 1988). It is noteworthy that the linear regression line shows both positively and negatively correlated elements with the tuff and the kaolin deposit. HFS elements such as Th, U, Zr, and Nb are enriched in the kaolin samples because they are immobile during surface weathering processes (Fig. 10B, D, and E). However, LILEs such as K, Rb, Ba, and Sr are usually mobile and depleted from the kaolin (Fig. 10A, C, and F). Enrichments of HFSEs and depletions of LILEs support strong weathering of feldspar in the study area.

Table 5 Major oxides and trace elements of the Ammacio tuff and kaolin samples
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Fig. 8
figure 8

TiO2 vs. Al2O3 discrimination diagram of the Ammacio kaolin for its host rock prediction (after Dill et al. 1997)

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Fig. 9
figure 9

A–D Variation diagram of Al2O3 vs. major oxides from the Ammacio kaolin samples and its host rock (arrows indicate the general trend of weathering)

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Fig. 10
figure 10

A–D Variation diagram of LOI vs. selected trace elements from the Ammacio kaolin occurrence (the long arrowhead line indicates the direction of intensity of weathering)

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Rock classification

According to Dill et al. (1997) classification scheme of volcanic rocks (Fig. 8), the parent rock of the Ammacio kaolin falls under the rhyolitic field. In addition to the mineralogical study, the high percentage of K2O and Na2O relative to CaO supports that the parent rock unit of the Ammacio kaolin is initially enriched in alkali-feldspar minerals rather than plagioclase. Overall results support that the parent rock of the Ammacio kaolin is more felsic in composition. High concentrations of selected incompatible and compatible trace elements such as Rb, Ba, Sr, Th, U, K, Na, and Zr. Table 5 indicate a more felsic source rock of the Ammacio kaolin.

Resource estimation and evaluation

The resource estimate for the Ammacio kaolin occurrence was completed using a conventional three-parameter approach (Table 6). The parameters are the area (A) from the geological map and the area (Fig. 2), the thickness (h) from the section measurement (Fig. 3), and the bulk density (Table 2) from the laboratory result. The total tonnage of the Ammacio kaolin occurrence is 74.6 metric tons. This total resource is the only one being addressed in the study area. It does not fully represent the kaolin occurrence within the Gibe Woreda.

Table 6 Kaolin blocks of the study area
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Thus, resource (kaolin) = (A)*(h)* ρ (g/cm3)

$$\begin{array}{c}\text{Block}-\text{A }= A ({\text{m}}^{2}) *\text{h }(\text{m})* \rho (\text{g}/{\text{cm}}^{3})\\ =\text{ 1,882,720 }{\text{m}}^{2}*20\text{m}*1.41\text{ g}/{\text{cm}}^{3}\\ =\text{53,092,704 tone}\end{array}$$
$$\begin{array}{c}\text{Block}-\text{B }= A ({\text{m}}^{2}) *\text{h }(\text{m})* \rho (\text{g}/{\text{cm}}^{3})\\ =\text{ 541,586}{\text{m}}^{2}*11\text{m}*1.34\text{g}/{\text{cm}}^{3}\\ =\text{ 7,744,679.8 tones}\end{array}$$
$$\begin{array}{c}\text{Block }-\text{ C }= A ({\text{m}}^{2}) *h (\text{m})* \rho (\text{g}/{\text{cm}}^{3})\\ =\text{ 615,125 }{\text{m}}^{2}*10\text{m}*1.34\text{ g}/{\text{cm}}^{3}\\ =\text{ 8,242,675 tones}\end{array}$$
$$\begin{array}{c}\text{Block}-\text{D }= A ({\text{m}}^{2}) *h (\text{m})* \rho (\text{g}/{\text{cm}}^{3})\\ = 454449 {\text{m}}^{2}*9\text{m}*1.34\text{ g}/{\text{cm}}^{3}\\ =\text{ 5,480,654.9 tones}\end{array}$$
$$\begin{array}{c}\text{Total kaolin resource of the area }= (\text{Block}-\text{A}) + (\text{Block}-\text{B}) + (\text{Block}-\text{C}) + (\text{Block}-\text{D})\\ =\text{ 74,560,713.7 tones}/74.6\text{Mt}\end{array}$$

Based on the level of geological knowledge and confidence, the resource is grouped under infrared resources, because the thickness of the kaolin is not so well known.

Source area weathering and host rock characterization

The rock change during weathering indicates the degradation of alkali and alkaline earth elements. As the intensity of weathering increases, the content of mobile elements such as CaO, Na2O, and K2O decreases, while the content of Al2O3, TiO2, and LOI increases (Fig. 9). The weathering status of the headwaters can be measured by the molecular fraction, or weight percent, of major oxides. Various indices, such as the chemical index of weathering (CIW) (Harnois 1988) and the chemical index of alteration (CIA) (Nesbitt and Young 1996), are used to assess the degree of weathering and changes. The weathering rate of Ammacio tuff shows moderate to intense weathering. This shows that with increasing chemical weathering intensity, the content of highly mobile elements decreases relatively while the content of immobile elements such as Al, Ti, and LOI increases. If the CIA value is 100, it means that feldspar has completely changed to kaolinite, gibbsite, and chlorite. Headwater weathering has a moderate or average intensity when the CIA is between 60 and 75 (Nesbitt and Young 1989). The CIA values of the examined kaolin range from 77.4 to 94.4%, with an average of 84.4%, showing that the area has experienced an intense degree of weathering (Table 5, Fig. 11).

Fig. 11
figure 11

Chemical index of alteration (CIW) vs. chemical index of weathering (CIA) plot for the Ammacio kaolin clay samples (after Bukalo et., 2017)

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This is also evident from the A-CN-K (Al2O3-(CaO + Na2O)-K2O) and A-CNK-F (Al2O3-(CaO + Na2O + K2O)-Fe2O3) ternary diagrams (Nesbitt and Young 1989), in which the samples are predominantly plotted near the aluminum apex (Fig. 12), indicating moderate to intense chemical weathering. The CIW values of the examined kaolin are high, ranging from 86.5 to 98.6%, with an average of 92.4% indicating a high degree of weathering of the parent rock. As the degree of weathering increases, the CIA values increase towards 100, at which point all Ca, Na, and K are removed from the weathering residue and finally enriched by immobile elements such as Al and Ti. Furthermore, the trend line (red arrow in Fig. 12 shows the direction of intensity of weathering) is parallel to the A-CN and CNK boundary and extends to the A peak, indicating potassium leaching and aluminum enrichment. This indicates that further weathering has led to the decomposition of K-bearing minerals. Additionally, the decreasing trend of grain size distribution and the increasing trend of Al2O3 from the host rock to the kaolin suit support intensive weathering processes. The enrichment of HFSEs (like Nb, Zr, Th, and U) and depletions of mobile trace elements (like Rb, Sr, and Ba) also reflect the intense weathering process of the study area (Fig. 10).

Fig. 12
figure 12

A A-CN-K ternary diagram and B A-CNK-F ternary diagram. In both cases, the arrow indicates the general direction of the weathering trend (after Nesbit and Young, 1996)

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Grade and quality assessment of the Ammacio kaolin

The total amount of kaolinite plays an important role in the kaolin grade (Ekosse 2010). Kaolin deposits with greater than 75% kaolinite are considered high-grade (Awad et al. 2018). The kaolinite content in the Ammacio kaolin ranges from 52.70 to 87.80%, with an average of 72.65% (Table 3). However, only four kaolin samples (KA09, KA06, KA05, and KA04) have a kaolinite greater than 75%, while the remaining kaolin samples have a kaolinite less than 75% (Table 3). As a result, the kaolin deposit in the study area was classified as low grade. In addition, Al2O3/SiO2 has also been used to determine kaolin grade (Getaneh and Gezahegn 2020). Accordingly, Al2O3/SiO2 greater than 0.5 belongs to high-grade kaolin and vice versa. Contrary to this, Al2O3/SiO2 for the Ammacio kaolin ranges from 0.23 to 0.56, with an average of 0.39 (Table 5). Al2O3/SiO2 is increasing with an increasing proportion of kaolinite minerals (Table 5). The lowest 0.23 Al2O3/SiO2 was recorded in the sample (KA01), which contained 52.76% kaolinite, whereas Al2O3/SiO2 0.56 was recorded in the sample (KA09), which contained 87.8% kaolinite. The result shows that only one sample (KA09) has a value above 0.5 (0.56) for Al2O3/SiO2, while all samples are below 0.5 (Table 5). Therefore, the result also confirms that the Ammacio kaolin falls in the low-grade range (Fig. 13).

Fig. 13
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Al2O3/SiO2 vs. kaolinite binary diagram that evaluates the grade of the Ammacio kaolin

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The industrial application of kaolin depends on its quality, whereas the quality of the kaolin is influenced by the intensity of weathering, its mineralogy, and its physicochemical properties (Murray 2006). The weathering phenomena combined with the physicochemical and mineralogical properties of the Ammacio kaolin suggest that it is useful for various industrial applications. Kaolin with a high crystallinity nature is ordinarily suitable for industries like paint and paper by yielding a lower viscosity (Prasad et al. 1991; Murray 2006, 2007; Bukalo et al. 2017). FTIR and XRD results show that the Ammacio kaolin is well-crystalline (Figs. 7 and 8). This is supported by the sharp peaks of the XRD pattern and the presence of two peaks nearly at around 800 and 750 cm−1 and 3693 cm−1 and 3621 cm−1 in the ATR-FTIR spectrum region. Moreover, the smooth and definite edge of the SEM results also support this (Tables 4, Figs. 5, 7, and 8).

The physical properties of kaolin, plasticity, and linear shrinkage limits are the major controlling factors that contribute to the manufacturing of fired industrial products. Kaolin has optimum plasticity and shrinkage limits favorable for the manufacturing of craved-fired products (Prasad et al. 1991). The plasticity index of the Ammacio kaolin ranges from 19.81 to 22.18%, with an average of 20.25% (Table 2), which is within the standard range of the kaolinite plasticity index (20–42%) (White 1942). Furthermore, the shrinkage limit of the Ammacio kaolin ranges from 1.09 to 2.58%, with an average of 1.87% (Table 2), which is in an acceptable range. All results show that it is suitable for the production of a variety of required fired products.

The quality of the kaolin is also assured by its preliminary appearance and fired color. These colors are commonly governed by the presence and amount of ferruginous and titaniferous mineral impurities. Kaolin with low concentrations of mineral impurities is unremarkably white in color and commonly used for the manufacturing of paint, paper, and white-fired products (Prasad et al. 1991; Bloodworth et al. 1993; Siddiqui et al. 2005; Murray 2007). However, the Ammacio kaolin consists of feldspar and quartz impurities and a high concentration of TiO2 and Fe2O3. As a result, it shows a pale yellow, light yellowish brown, and pink preliminary color and a creamy-white, reddish-yellow, and light reddish-brown color after being fired at 900℃ (Table 2). Typically, these impurities can lower the quality of the kaolin suites. Therefore, beneficiation or impurity removal techniques should be employed (like fort floatation or magnetic separation methods), to use the raw kaolin sample directly for the manufacturing of paint, paper, and white-fired products. In similar fashion, the color of the Ammacio kaolin occurrence can be enhanced by physical (crushing and calcination) and chemical (the traditional leaching process using various chemical agents such as oxalic acid, which dissolves iron and lowers Fe levels). The overall quality of the Ammacio kaolin occurrence is enhanced through ore dressing methods.

Industrial applications of the Ammacio kaolin

Different kaolin-consuming industries require different physicochemical properties of kaolin (Prasad et al. 1991). To assess the suitability of kaolin for different industries, an assessment of the geological, mineralogical, and physicochemical properties of the Ammacio kaolin has been conducted. The mineralogy and physicochemical properties of kaolin regulate its potential industrial uses (Prasad et al. 1991; Nyakairu et al. 2001; Murray 2007; Bukalo et al. 2017). The mineralogy and physico-chemical characteristics of kaolin must meet varied standards depending on the industries that use it to make various industrial products (Prasad et al. 1991). Some industries need strict requirements whereas others do not (Bloodworth et al. 1993). But to evaluate its potential field of industrial uses, the Ammacio kaolin is assessed in terms of mineralogy and physicochemical properties. Because kaolin-consuming companies require kaolin that meets the specification criteria of the customers’ aims to utilize the kaolin for filler, ceramics, pharmaceuticals, cosmetics, paper coating, pigments, agricultural sectors, etc. For instance, kaolin geochemistry is quite specific to be used for the manufacturing of fiberglass, refractories, and cosmetic products.

Ceramic industry

The type of ceramic goods made from kaolin is determined by its physicochemical qualities (Bloodworth et al. 1993; Pruett 2016). To put it another way, the varied physico-chemical characteristics of kaolin lead to diverse kinds of ceramic products. However, the quality of kaolin that may be utilized to make ceramic goods is affected by plasticity, linear shrinkage, and coloring impurities. The fabrication of ceramic items often uses plastic or kaolin with low levels of coloring impurities and a low shrinkage limit (Bain 1971; Bloodworth et al. 1993; Murray 2007). Therefore, the Ammacio kaolin’s shrinkage limit is between 1.09 and 2.58% (Table 2), which is within the acceptable range (0.5–4.8%) for kaolin’s shrinkage limit to be utilized for ceramic manufacturing purposes as per IS 2840 (1965) specification.

Pruett (2016) claimed that the chemistry of kaolin is crucial for its usage in the production of fiberglass and refractories. SiO2 (58.22 wt.%), Al2O3 (20.21 wt.%), Fe2O3 (2.93 wt.%), Na2O (1.57 wt.%), K2O (2.09 wt.%), TiO2 (1.55 wt.%), and LOI (12.88 wt.%) are all present in the Ammacio kaolin. As a result, the Ammacio kaolin chemistry has to be improved to be used for producing various ceramic goods. As a result of the Ammacio kaolin’s higher SiO2, Fe2O3, Na2O, K2O, and TiO2 content, as well as its comparable LOI value and low Al2O3 content (Table 7), it meets the requirements for kaolin ceramic manufacturing. The Ammacio kaolin’s pink, pale yellow, and yellowish-brown hues are caused by high levels of Fe2O3 and TiO2. Unfortunately, these elements made the Ammacio kaolin less white and prevented its direct use in the ceramics manufacturing industry. The ratio of SiO2/Al2O3 (greater than 1.8) in the Ammacio kaolin, on the other hand, supports the existence of silicate mineral impurities in addition to kaolinite. The primary phases in the Ammacio kaolin samples, according to mineralogical findings, are kaolinite and quartz, with minor phases of plagioclase, halloysite, and k-feldspar (Tables 3 and 8). According to Murray (2007), halloysite, which is present in kaolin in concentrations of 5–10%, improves the fired brightness and translucency appearance of manufactured ceramic items. Moreover, halloysite, on the other hand, is used to produce fine tableware; nevertheless, other mineral impurities such as plagioclase, quartz, and k-feldspar decrease the quality of the Ammacio kaolin, rendering it unsuitable for use in the direct fabrication of various industrial products.

Table 7 Assessment of Ammacio kaolin for ceramic type specification (Bloodworth et al. 1993)
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Table 8 Suitability of Ammacio kaolin for filler based on IS: 505 (1978) specifications
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The pale-yellow and pink color of the Ammacio kaolin is caused by the presence of iron-bearing minerals. Using froth flotation processing, the pale-yellow kaolin is given a white tint (Yoon et al. 1992), and the pink color of the Ammacio kaolin may be directly used in the Grade-I ceramic manufacturing industry. Alternately, superconducting or gradient magnetic separation methods can be used to eliminate the paramagnetic coloring bodies in the kaolin suit (Wastion 1994). On the other hand, the Ammacio kaolin meets the specifications demanded by all ceramic-grade production industries in terms of silt, clay, and sand percentage.

Filler

According to IS: 505 (1978) requirements, the Ammacio kaolin has lower LOI and Al2O3 values and higher SiO2, Fe2O3, and K2O contents than what is needed by the filler industries, which include those in paper, rubber, paint, and plastic (Table 8). The Ammacio kaolin occurrence has a higher MnO concentration than what the filler industry demands. In the paper, paint, rubber, and plastics industries, the Ammacio kaolin occurrence cannot thus be used directly as filler. For use in filler industries, the Ammacio kaolin must first undergo beneficiation.

Moreover, kaolin with a pH of 4.5 to 7.5 is needed for filler in the paint, paper, plastic, and rubber industries (IS: 505 1978). The Ammacio kaolin’s pH values range from 6.58 to 7.43, with an average of 6.99 (Table 2), which is within the standard requirements required by the filler industry. The pH and appropriate proportions of silt and clay particle size of Ammacio kaolin (Table 1) make it acceptable for use as a filler in a variety of industries, including those that produce paper, paint, plastics, and rubber, according to IS 505 1978 requirements. In comparison, the Ammacio kaolin has kaolinite concentrations ranging from 52.7 to 87.8%, with an average of 72.65%, and other mineral values ranging from 1.7 to 35.9%, with an average of 27.34% (Table 3). As a result, the proportion of kaolinite in the Ammacio kaolin is lower than the specification required by the filler industries; nevertheless, the content of other minerals is higher than the requirements of IS: 505 (1978) for the filler industries. Because of this, the Ammacio kaolin needs to be optimized to be used in the necessary filler industries.

Paper coating

Various investigators (e.g., Prasad et al. 1991; Bloodworth et al. 1993; Murray 2007) claimed that the presence of a higher silica content imparts an abrasive effect to kaolin clay. The Ammacio kaolin samples’ ATR-FTIR, XRD, and SiO2/Al2O3 results indicate the presence of an appreciable quartz component, which poses an abrasive concern for the kaolin deposit. Furthermore, the Ammacio kaolin exceeds the specifications needed by the paper coating industry with its high content of TiO2, Fe2O3, and K2O and low Al2O3 and LOI values. These circumstances prevent the Ammacio kaolin from being applied directly to the paper coating industry. To raise the grade of the Ammacio kaolin and enable its usage as a coating clay in the paper coating industry, beneficiation is required. The Ammacio kaolin is ideal for paper manufacture because it serves as a coating clay since its pH value is within the range defined by Siddiqui et al. (2005).

In the Ammacio kaolin, kaolinite accounts for an average of 72.65% of the mineral composition, whereas other minerals make up 27.34% of the kaolin (Table 3). The kaolinite proportions in the Ammacio kaolin are below the minimal requirements for paper-coating industries, according to Prasad et al. (1991) and Siddiqui et al. (2005). Therefore, before employing the Ammacio kaolin in the paper-coating production industry, it needs to be beneficiated. The platy form (Fig. 5) and adequate proportions of silt and clay kaolinite particles (Table 1) make the kaolin in the study area more suitable for paper-coating manufacture (Murray 2006, 2007) (Table 9).

Table 9 Ammacio kaolin for paper coating application (Prasad et al. 1991; Siddiqui et al. 2005)
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Pharmaceutical industries

The largest consumer of kaolin, the pharmaceutical industry, is known for its rigorous standards for kaolin (Prasad et al. 1991). The finest kaolin is employed in poultices; as a dusting material during surgical operations, as a tablet or capsule diluent, as a suspending agent, and as an absorptive for gastrointestinal disorders (Table 10). The Ammacio kaolin’s clay and silt content, which ranges from 67.56 to 83.79% with an average of 76.91% (Table 1), makes it appropriate for the pharmaceutical industry. However, to use kaolin as a pigment in the pharmaceutical industry, titanium dioxide (TiO2) and barium (Ba) concentrations must be below the detection limit (Christian 2018). The study area kaolin samples commonly contain 1.55 wt.% TiO2, which inhibits the Ammacio kaolin from being used as a source of raw materials for the pharmaceutical industry. The Ammacio kaolin is inappropriate for pharmaceutical manufacturing due to the high concentrations of TiO2 and barium (247.14 ppm) in the samples. On the other hand, the Ammacio kaolin needs to be beneficiated using the right methods before it can be used in the pharmaceutical industry.

Table 10 Assessment of Ammacio kaolin major element geochemistry for the pharmaceutical industry (Lopez-Galindo et al. 2007)
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Cosmetics industry

The cosmetics production sector uses kaolin, which has a low concentration of non-toxic ingredients (Madikizela et al. 2017). The quantity of hazardous substances, including arsenic, cadmium, and mercury, in kaolin from the Ammacio confirms the specifications needed by the cosmetic manufacturing industries (Table 11). As a physical UV filter, however, the Ammacio kaolin’s high TiO2 content (an average of 1.55 wt.%) is crucial since it reflects ultraviolet light and deflects sunlight. Similarly, the Ammacio kaolin samples have Pb (20.05 ppm) contents that are significantly higher than what is necessary for the production of cosmetics. The Ammacio kaolin samples had excessive levels of lead, which prevented direct use of the material for producing cosmetics.

Table 11 Assessment of Ammacio kaolin for the cosmetics industry (Madikizela et al. 2017)
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According to IS: 1463 (1983), the Pb content and LOI for kaolin used in cosmetics may not exceed 5 ppm and 15 wt.%, respectively. However, the Ammacio kaolin’s very high Pb content (avg. 20.05 ppm) and low LOI value (avg. 12.88 wt.%) prevent it from being used in the cosmetic production industry. Furthermore, the pH of the Ammacio kaolin is 6.99, which is higher than the pH needed to produce face cream and scrubs. As a result, the Ammacio kaolin is not appropriate for face cream and scrub manufacturing directly. In contrast, the Ammacio kaolin has a pH level that is equivalent to what is needed for the manufacturing of face masks and foundation; therefore, it may be utilized to make these products.

Fiberglass industry

The SiO2, Al2O3, TiO2, and Fe2O3 concentrations in the kaolin for the fiberglass industry must be within the specified range as per Wilsons’ (1966) standard (Table 12). Comparatively, the Ammacio kaolin has less Al2O3 and more SiO2, TiO2, and Fe2O3 than what is needed by the fiberglass manufacturing sector. The Ammacio kaolin must thus be beneficiated before it can be used to manufacture fiberglass products.

Table 12 Specification of Ammacio kaolin for fiberglass industries (Watkins 1986)
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Conclusion

The following findings are made as a result of geological, mineralogical, and physico-chemical analyses of the Ammacio kaolin occurrence:

The XRD, SEM, and ATR-FTIR results show that

  1. 1.

    The Ammacio kaolin is well-crystalline and primarily composed of the minerals kaolinite and quartz.

  2. 2.

    Mineralogical and geochemical results revealed that the Ammacio kaolin is low grade.

  3. 3.

    The physical tests, major, and trace element distributions in combination with high values of (CIW = 92.4%) and (CIA = 84.4%) demonstrate the moderate to intensive kaolinization of feldspar underwent in the study area.

  4. 4.

    The main impurities in the Ammacio kaolin occurrence include high levels of TiO2 and Fe2O3, as well as the presence of minerals like feldspar and quartz.

  5. 5.

    The Ammacio kaolin needs a suitable beneficiation method to improve its quality so that it can be used in various industries.

  6. 6.

    The grain size distributions, Atterberg limit, linear shrinkage, bulk density, and pH promote the prospective uses of kaolin in a variety of industries, including paper, plastic, paint, ceramics, rubber, pharmaceuticals, pottery, bricks, agriculture, and Portland cement.

  7. 7.

    A total of 74.6 million tons is estimated within the given mineral resource category using the standard technique for resource estimation.