Potential Soil Remineralizers from Silicate Rock Powders (SRP) as Alternative Sources of Nutrients for Agricultural Production (Amazon Region)

Abstract

The demand for mineral fertilizers has increased over the years. In the states of Amazonas and Roraima, acquiring conventional fertilizers used in agriculture is challenging due to the distance from large production centers. In these regions, alternative fertilizers are needed to maintain food security. However, research in agrominerals of silicate rock powders (SRP) is still incipient. The objective of this research was to characterize three important Units of Agrogeological Interest in the Manaus/Boa Vista axis in the Amazon region of Brazil: (i) EBAM Quarry: quartz monzonites from the Água Branca suite, partially potassified, (ii) Granada Mining Quarry: basalts from the Apoteri formation, and (iii) Samauma Quarry: riolites from the Iricoumé Group. Samplings were carried out followed by chemical analyses for determining macro and micronutrients, in addition to potentially toxic elements; petrographic analyses were performed for mineralogical characterization as well as granulometric analyses of the powders collected in the quarries. The results showed that the EBAM quarry rock powder meets the standards established by Brazilian legislation. It also has low levels of potentially toxic elements and only 15% quartz, indicating good safety in the use of this SRP, in addition to the large supply of the material already crushed, for which there is still no market. At the Granada Mineração quarry, SRP also has the necessary characteristics to classify as a soil remineralizer, including K₂O content above 1%. At Pedreira Samauma, although SRP does not qualify as an agromineral, it has more than 5% K₂O and 77% aphanitic matrix, which could result in a more accessible release of abundant K and Si to the soil–plant system. Using laser granulometry analysis, it was possible to make some considerations about the effects of the crushing process in such different lithotypes and, finally, to characterize and classify the prospects of greatest interest for “rochagem” in the Amazon.

1. Introduction

To improve food security and the balance of ecosystems, agroecological strategies, and conservationist practices are needed to restore degraded environments and maintain soil quality [1]. Food production aimed at eradicating hunger in the world is one of the goals established among the 17 Sustainable Development Goals (SDGs), which are part of the so-called “Agenda 2030” of the United Nations [2]. Brazil’s prominent role in SDG 2 (end hunger, achieve food security and improved nutrition, and promote sustainable agriculture) stems mainly from its cultivated area, climate, and the future availability of land for agriculture.

It is known that nutrients extracted annually by crops are responsible for plant development and productivity gains. However, in tropical soils, most of the time, the availability of these elements needs to be increased or improved [3]. It is estimated that there is a significant gap globally, in which only half of the potassium (K) removed from soils is replaced by fertilizers, with the production of fertilizers dominated by five countries in the northern hemisphere (i.e., Russia, Canada, Germany, Belarus, and China) that produce more than 80% of the K distributed globally [4]. This fact makes the Brazilian market dependent on importing mineral fertilizers, causing insecurity in the supply of these products. In 2020, the national production of fertilizers was 6.3 million tons, while 32.8 million tons of the same products were imported to meet national needs [5]. Without a supply of mineral fertilizers, agricultural productivity may be compromised and even unfeasible, affecting global food supply. In addition, “modern” fertilizers, or those with improved efficiency, manufactured for use in temperate soils, are soluble and quickly leached in tropical soils, which generally have low reactivity [6].

An alternative way to improve soil quality and maintain adequate crop development is using silicate rock dust (SRP), also known as agrominerals. Amending soils with rock powders is an ancient technique used until today in modern agriculture, such as limestone and phosphate rocks [7]. The weathering of silicate rocks is one of the fundamental geochemical processes for forming the planet’s environment and the primordial source of all mineral nutrients present in the soil [3].

In recent years, research with agrominerals has gained new impetus. In the context of tropical soils, SRP has led to impressive results, mainly in Brazil, where the “rochagem” movement led to the institutionalization of the research of SRP. The country already hosted three congresses focused on the SRP theme in the last two decades (2009, 2013, and 2016), where one of the key themes discussed was the establishment of the parameters that could be used to evaluate the potential of SRPs in fertilizing soils. In those events combined, more than a hundred scientific works were presented by national and international researchers. Those studies demonstrated many positive results from the use of SRPs as remineralizers as costs of acquiring SRP remineralizers are significantly less (up to 80%); a single application can be effective for up to five years; the fertility levels have been increasing in SRP remineralized soils over the last five years; productivity in SRP-fertilized fields equals or surpass the conventional fertilization ones; they have better radicular development, higher soil moisture levels, higher green mass in plants, and greater tillering; are characterized by no contamination of eutrophication of water due the slow nutrient release rate; and they meet the standards for organic production, a sector that grows an average of 35% annually [6]. As a result of those efforts, soil remineralizers were introduced as a category of agricultural input in Brazil by Law 12.890 of 10 December 2013, and the specifications of the materials to be used were consolidated in the normative instruction IN 05/2016 of the Ministry of Agriculture, Livestock and Supply–MAPA [8].

Remineralizers have great potential to supply agriculture in a complementary way and synergy with the fertilizers already used. The 2050 National Fertilizer Plan projects a highly relevant scenario for soil remineralizers in agriculture, with the feasibility of important research and development projects by 2025 in this area; large-scale use of remineralizers is aimed to be implemented by 2030, finally reaching, in 2050, the consolidation of the remineralizer production chain, becoming a reference in the subject and significantly reducing the country’s dependence on fertilizer imports. Currently, with 25 registered products, the production is estimated at 1 million tons/year, and projections for 2050 are to reach the mark of 10,000 registered products, with a production of 18 million tons/year [9].

Research that has been conducted in Brazil has sought to explore the aptitude of SRP in grain production in large monoculture systems, which presupposes that the SRP levels used must be compatible with the technical incubation capacity of hundreds, sometimes thousands of hectares, which results in the application of relatively small amounts of SRP per hectare. Doses commonly used in these cases are around 4–6 Mg ha⁻¹ [3]. However, it is important to highlight that this means for each kg of soil, considering only the 20 cm on the surface, around 2 g of SRP will be applied, relatively modest values that even so have shown agronomic efficiency and increased productivity in several cases.

For example, the protected cultivation of vegetables with high added value is common in the green belt near Manaus. These crops are between 0.1–0.5 ha in area. Thus, considering the proximity of the PRS prospected in this work and the agricultural production model, they could be tested in much more expressive quantities and still be logistically and economically viable.

The compounds then generated by adding enormous amounts of SRP to the soil will undoubtedly show some dynamism in their textural behavior, as the SRP added to the surface and reactive horizon of the soil begins to transform into clay minerals rich in silica, with a high specific surface and clay-sized particles [10]. Such transformations promote the release of chemical elements that form these primary minerals (e.g., Ca, Mg, K, Fe, Si), which, in their ionic form, become nutrients in the soil solution, increasing base saturation (SB), cation exchange capacity (CEC) and decreasing soil acidity. Experiments with basalt powder demonstrate significant improvement in these parameters using ground basalt at doses between 1 and 100 Mg ha⁻¹ [11,12,13,14,15]. The positive effects of several PRS, promoting changes in soil reaction, nutrient supply, pH, and SB of soils, in addition to changing physical and biological aspects, have already been proven [10,11,12,13,14,15,16].

The remineralizers have slow and gradual solubility due to the weathering of the minerals present in the rock powders. On the other hand, in an experiment conducted in the laboratory, the release of K and Na from rock powders was 5 to 10 times higher in citric acid than in acetic acid or water [17], demonstrating that organic acids, present in abundance in the surface horizon of the soil, rich in organic matter, play an essential role in chemical weathering and nutrient release by mineral fragments in SRP. Some studies have already shown the effects of the colonization of specific groups of bacteria in different mineral phases, consolidating the term “mineralosphere” as the differences between types and diversity of bacteria are demonstrated on the surface of fragments of some minerals and the soil in general, promoting weathering and contributing to the release of nutrients for plants [18,19].

Brazil has a great diversity of geological terrains, representing a high potential for using SRP as soil remineralizers [20]. As a result, the prospecting of rocks suitable for use in agriculture must consider regionally available resources, reducing agricultural production costs, and promoting the mitigation of environmental liabilities. In this context, the goal of this study was the chemical and mineralogical characterization of three rocks available in the context of BR-174. This road connects Manaus (AM) and Boa Vista (RR), two essential capitals in the northern region of Brazil. Approximately 2.9 million people live in the municipalities crossed by this road, corresponding to 59% of the population of the states of Roraima and Amazonas [21]. Thus, the main focus was the characterization of already crushed materials from quarries intended for civil construction present in the area, whose crushing fines can be researched for the development of soil remineralizers.

2. Materials and Methods

2.1. Rocks and Quarrys

This article reports a project started at the Manaus superintendence of Geological Survey of Brazil (SGB/CPRM), in the State of Amazonas. In this project, work was carried out to prospect Agromineral Interest Units (AIUs) in the context of BR-174, the highway that connects the Capitals Manaus (AM) and Boa Vista (RR). As a result of these efforts, 60 samples were collected and analyzed at 36 different stations, and later the data obtained were released through the Mineral Resource Report No. 28 of the SBG [22]. Among all the rocks studied, three prospects drew attention due to their chemical, mineralogical, and logistical attributes. The three developments studied are located on or near the BR-174 in the Amazon region (Figure 1).

The EBAM Quarry (PB-01) is located west of the BR-174, about 150 km from Manaus, in the municipality of Presidente Figueiredo, AM (Figure 1). The gravel produced is used in construction and road paving sectors. It produces from cobble (150 to 100 mm) to gravel powder (less than 4 mm); its main product is gravel 0 (5 to 13 mm). It mainly serves the Manaus market, producing about 27,000 m³ of material per month in the following proportions in general: 35% gravel 0, 20% gravel 1 (13 to 20 mm), 40% gravel powder, 5% tailings (fine crushing that is discarded in granulometry < 30 mm. The quarry sells its entire production of gravel 0 and 1, but the gravel powder and the discard, due to their inferior geotechnical characteristics attributed to the excess of iron, have little application and demand in construction but comprise 45% of the result of the crushing process, so that the material accumulates in the yards forming piles of “waste”. In April 2019, stock quantities were estimated at approximately 100,000 m³ of tailings and 125,000 m³ of gravel powder.

The Granada Mineração Quarry (PB-10), formerly Boa Vista Mineração, is the second largest Quarry visited by the project, second only to the EBAM Quarry. Its production consists of gravel powder, gravel 0, and gravel 1, and it is mostly used in asphalt plants in Roraima, but also serves the construction market in the region. The Quarry has the operational capacity to process approximately 11,000 m³ of rock per month, and its crushing operation results in 50% gravel 1, 30% gravel 0, and 20% gravel dust. In March 2019, the Quarry stock was approximately 25,000 m³. However, all fractions have a market and virtually no tailings in the Quarry.

Samauma Quary (PB-05) is located east of BR-174, about 190 km from Manaus. The quarry produces from gravel powder to gravel 2 but generally operates processing 5000 m³ of rock per month, and its crushing process results in 50% of gravel 1, 30% of gravel 0, and 20% of gravel powder. All fractions produced serve the local market, and there are no disposals of tailings available.

2.2. Sampling

A series of samples and information were collected at the Quarries: (i) rock samples at the mining front, always taking into account the proportion of each lithotype at the mining front, estimating the contribution to the dust resulting from the mixture of different outcropping rocks; (ii) samples of gravel powder, with around 500 g for carrying out lithochemical analyses, properly homogenized and divided; and, (iii) socioeconomic questionnaire carried out with supervisors and/or technical managers at the units, to gather information relevant to the study, such as monthly production, comminution process, equipment used, volume sold in different fractions, material applications, consumer market, the volume of waste in the plant, among other information that was relevant in each particular case.

2.3. Lithochemical Analysis

In the rocks and SRP, the constituent chemical elements (macro and micronutrients and/or heavy metals) were evaluated to verify the agromineral potential of the researched materials, considering the Brazilian standard for soil remineralizers [8].

The analyses carried out were (1) melting with lithium tetraborate and reading by X-ray Fluorescence (FRX) to obtain the major oxides and loss on ignition, with the main objective of measuring macronutrients (calcium, magnesium, potassium, and phosphorus), in addition to other elements of interest for research (silicon, aluminum, chromium, sodium, manganese, and iron). For the analysis, 0.5 g of sample was used, which undergoes melting in an automatic machine with lithium tetraborate, and then the reading of the melted pellet is carried out using a Magix RX spectrophotometer by Malvern Panalitycal, Malvern, UK; (2) chemical analysis for ten trace elements, focusing on base metals, through the opening with four acids and reading by inductively coupled plasma atomic emission spectrometry (ICP-OES—5300, Perkin Elmer, Waltham, MA, USA) and inductively coupled plasma mass spectrometry (ICP-MS—Nexion, Perkin Elmer, Waltham, MA, USA), to define the levels of micronutrients cobalt, copper, molybdenum, nickel, and zinc, in addition to heavy metals, cadmium, and lead. For this analysis, 0.25 g of sample was used, in which the decomposition is carried out opening by digestion with HCl, HNO₃, HClO_4, and HF; (3) analysis of nine volatile trace elements with geochemical affinity with gold through digestion by aqua regia and quantification by ICP-OES/ICP-MS to define the arsenic and mercury levels (potentially toxic elements—PTEs); in this analysis the decomposition of 0.25 g of the sample is performed through the opening with aqua regia (HCl and HNO₃).

2.4. Petrographic Analysis

The petrographic analysis aimed at describing the lithologies and characterizing the mineral phases, focusing on identifying minerals with crystalline opening conditions and their alteration and dissolution features. Petrography is the primary tool for statistically determining the content of crystals or quartz grains (free silica) that compose rocks. One of the main factors for evaluating the aptitudes of an SRP for remineralization is to define the free silica content, which is limited to 25% of the total composition of the rock for its use as an agromineral [8]. The determination of free silica contents was carried out by modal counting of the minerals in the rock-thin sections, adopting a regular mesh of 2 mm, which allowed a count of about 250 points per section.

In volcanic rocks, where it is difficult to define the mineral assemblage from petrography used, the approach of X-ray Diffraction (XRD) was used due to the small size of the crystals. The analysis was carried out using an X-ray diffractometer, model X’PERT PRO MPD (PW 3040/60), from Malvern Panalytical, Malvern, UK, with a Goniometer PW3050/60 (Theta/Theta) and a ceramic X-ray tube with Cu anode (Kα1 1.5406 Å), model PW3373/00, long fine focus, 2200 W, 60 kv. The detector used is of the RTMS type, Pixcel/1D. Data acquisition was performed using the X’Pert Data Collector software, version 2.1a, and data processing was performed using the X’Pert HighScore software, version 3.0d, also from Panalytical. The following analysis conditions were used: Voltage (kV): 40; Current (mA): 40; Scan range (° 2θ): 5–70 (MA) and 5–50 (RJ); Step size (° 2θ): 0.02; Scan mode: Continuous; Counting time(s): 50; Divergence slit: Slit Fixed 1/2°; Mask Fixed 10 mm; Anti-scatter slit Name: 5.7 mm. Minerals were identified by comparing the diffractogram obtained with patterns (sheets) from the International Center for Diffraction Data—Powder Diffraction File—ICDD-PDF database, (Newtown Square, PA, USA).

Scanning electron microscopy (SEM) analyses were performed using a Zeiss model LS15 microscope (Carl Zeiss Meditec AG, Jena, Germany). The material was analyzed and imaged using a tungsten filament in high vacuum mode (<3.0 × 10⁻⁵ mPa). Thin polished sections were covered with a 20 μm thick Cr film using a high vacuum metallizer. Backscattered Electron (BSE) images of minerals and textures of quartz monzonite and basalt were obtained with an acceleration voltage of 20 kV, the incoming current between 100 and 120 pA, with a working distance of 8.5 mm and magnifications between 60 and 2500 times. Analyzes of the chemical composition of the minerals were by X-ray Energy Dispersive Spectrometry (EDS) on an Oxford Instruments X-Act SSD 10 mm² detector (Oxford Instruments, Abingdon, UK). Analytical results were acquired at a working distance of 8.5 mm, with a voltage of 20 kV and an incoming current of 700 pA to 4.2 nA, to maintain an output count rate of about 2000 cps, both in spot analyses and element profiles. The results were standardized by energy spectra of standards from the Oxford Instruments AZTec program (Oxford Instruments, Abingdon, UK). In the treatment, results with a standard deviation above 10% of the element’s concentration were discarded from the calculation of the mineral composition, which may be present in the minerals at low concentrations as impurities in the crystalline reticulum or indicate an incipient secondary alteration of the analyzed minerals. They may also represent spurious elements of the host mineral detected due to the strong intensity and high thickness of the incident electron beam. The mineral classification was done by consulting the percentages of elements and mineral formulas on the Mineralogy Database website.

2.5. Granulometric Analysis

The samples were submitted to granulometric analysis using a Mastersizer 2000 laser granulometer with Hydro 2000MU disperser (Malvern, UK). The samples were previously sieved to separate the fraction greater than 1 mm. As a dispersant medium for the analysis, tap water was used.

3. Results and Discussion

3.1. Água Branca Suite (EBAM Quarry/PB-01)

The Água Branca suite is characterized in the literature as an expanded granitic series composed predominantly of monzogranites with some variation; however, up to dioritic terms. The rocks of this unit are tardicolisional granites of high-K calc-alkaline nature, of Orosirian age and, associated with the Iricoumé volcanism, make up a large part of the basement in the region of occurrence [23]. The general appearance of the rock is a medium to coarse isotropic inequigranular granite, classified as quartz monzonite (PB-01A), representing 60% of the mining front (Figure 2A).

In shear zones and fractures, the rock becomes reddish. This effect was attributed to the percolation of hydrothermal fluids that promoted potassification that covers 35% of the mining front and is represented by the sample classified as potassified monzonite quartz (PB 01B) (Figure 2C). In the faults and fractures associated with the process, veins of calcite and oxides were placed.

Estimated at 5% in the mining front are dikes and xenoliths of mafic fine phaneritic material of dark greenish color tone, represented by sample PB-01C, classified as diabase (Figure 2E). The result of the crushing process is a mixture of these three lithotypes, minding their proportions at the mining front.

In the thin section (Figure 2B), the quartz monzonite (PB-01A) is relatively altered; it is possible to observe a great dissemination of venules with alteration of sericite and epidote in the specimen. About 80% of the K feldspar found shows strong concentric zonation, with exsolution of albite (pertite) and a considerable level of sericitization sometimes controlled by compositional zonation or developing along the lines of pertite exsolution. Microcline crystals with tartan twinning and little or no weathering are generally tiny and rare. The plagioclase phase was mainly determined by the presence of polysynthetic twinning and, therefore, may be underestimated about K feldspar. The mafics are represented mainly by nuclei of orthopyroxenes and clinopyroxenes mantled by amphibole with the presence of some associated biotite and chlorite. Finally, using a magnification of 400×, the presence of tiny prismatic apatite crystals and anhedral epidote clusters within the feldspar crystal is noticeable. It is essential to point out that the quartz contents identified in the sample representing most of the mining (PB 01A) are around 15% (Figure 2B).

Using Energy Dispersive Electroscopy (EDS), it was possible to determine that the plagioclase crystals in the PB-01A slide are predominantly labradorite and andesine with very close Ca and Na contents. K-feldspars, in turn, are perthitic, formed by intercalations of orthoclase and albite (potassium and sodium). Some crystals show zonation with plagioclase in the core, mantled by k-feldspar, which denotes the typical crystallization sequence of the rock (Figure 3). Pyroxenes are augites with little or no aluminum in their composition, being richer in Fe and Ca than in Mg. The main identified P-bearing mineral has a composition compatible with minerals from the apatite group in tiny 0.01 mm crystals included in the feldspars. The opaque minerals are mostly iron oxide with some titanium and manganese content. Venules cutting the plagioclase crystals are very small and difficult to characterize. However, EDS identified chemical compositions compatible with sphalerite (ZnS), epidote (Ca and Fe), and sericite (K), minerals commonly associated with these features.

The potassified quartz monzonite sample (PB-01B) bears similarities with the quartz monzonite (PB-01A). However, in the potassified quartz monzonite, feldspars with polysynthetic twinning (Pl) are much rarer. PB-01B also has fewer venules and less sericitization compared to the first. This is probably due to the hydrothermal potassification event. No trace orthopyroxene or clinopyroxene nuclei were identified here; the predominant mafics are amphiboles (Figure 2D). The quartz contents observed in slides are lower than those of monzonite quartz (PB-01A).

The diabase sample (PB-01C) shows an advanced degree of alteration. It is composed of highly sericitized plagioclase slats and phenocrystals, some small pyroxene nuclei, also very degenerated, and immersed in an aphanitic matrix, which seems to be formed by secondary minerals, probably sericite, actinolite, epidote, and opaques, in addition to some quartz and biotite (Figure 2F).

3.2. Apoteri Formation (Granada Mineração Quarry/PB-10)

The Apoteri Formation is composed of continental tholeiitic basalt flows and dykes associated with the context of the Central Atlantic Magmatic Province—CAMP in the Tacutu Rift, a NE-SW aulacogene that developed in the late Triassic and early Jurassic [24].

In the quarry, the mining front is composed solely of massive, fine, dark gray phaneritic basalts, significantly fractured, isotropic, except for occasional white venules of millimeter thickness, formed chiefly by calcite. Centimetric columnar disjunctions observed at this location would characterize a central zone of effusion [25] (Figure 4A,B).

In a thin section, the rock is significantly altered, probably due to some hydrothermal event, which affected the optical properties of the minerals, making their identification difficult. It is possible to observe the very altered plagioclase slats. XRD analysis indicates the presence of minerals from the smectite group. There are also small crystals of medium to high birefringence of pyroxene, involved in a mass of alteration minerals, in addition to opaque minerals (Figure 4C). The rock is slightly porphyritic, with plagioclase phenocrysts up to 1 mm long. A notable and common feature of these basalts is diktitaxitic cavities filled with isotropic clay minerals with aphanitic texture and green color (Figure 4D,F).

These cavities make up about 10% of the volume of the rock and originate from the concentration of volatile elements during the crystallization of the anhydrous phases, and are finally filled by microcrystals of clay minerals from the smectite group [25,26,27,28,29]. Points investigated by EDS within these diktitaxitic cavities in rocks of the Apoteri Formation showed a composition compatible with minerals from the chlorite group (corrensite), rich in iron and magnesium, and that the Ca existing in the rock was identified mainly in pyroxenes, probably augites, also rich in iron and magnesium. Some of the opaques in the rock were also investigated and are mainly composed of iron but have some titanium content (Figure 4E) [22].

The diffractogram of sample PB-10B (Figure 5) shows peaks that indicate the presence of minerals from the smectite, chlorite, and vermiculite groups. This information corroborates the hypothesis that family this family’s diktitaxitic cavities are filled with minerals. The altered plagioclase, in turn, did not present any calcium content, being composed only of sodium, aluminum, silicon, and oxygen [22], which indicates that some albitization process occurred in this phase, its principal constituent being the mineral albite (Na), probably partially altered to smectite. These minerals are of great value for amending soils, as they are poorly weathered clays and have a larger specific surface, which contributes positively to increasing the soil’s cation exchange capacity (CEC).

3.3. Iricoumé Group (Samauma Quarry/PB-05)

The Iricoumé Group consists of a series of acid to intermediate effusive rocks of the Orosirian age that outcrop in the Guiana Shield and are in a remarkable state of preservation. In the literature, they range from rhyolites, rhyodacites, and felsic trachytes to andesitic basalts, in addition to tuffs and ignimbrites [30,31].

The rock in the mining front is a porphyritic volcanic of aphanitic matrix and grayish-pink color with greenish-colored millimeter veins; the rounded quartz phenocrysts of about 2 mm and the prismatic feldspar phenocrysts of up to 4 mm present a whitish color (Figure 6).

In thin sections, it is possible to observe in detail the aphanitic and felsic matrix of the rock. The advanced degree of weathering of Kfs and Pl phenocrystals is also observed, in addition to estimating, by modal counting, that only 3% of the rock is composed of quartz phenocrystals since the stone is composed of almost 80% of aphanitic matrix. Venules filled with aphanitic material with a strong green color were identified, which indicates the presence of microcrystalline epidote.

Performing XRD analyses complementary to petrography helped in a better characterization of the mineralogy of the sample (Figure 7). As in the samples from the Apoteri Formation, sericite was not identified as an alteration mineral, but as small fractions of kaolinite and smectite, in addition to quartz, potassium feldspar, and plagioclase.

3.4. Lithochemical Considerations and Agromineral Potential

The lithochemical analyses conducted and expressed in

Table 1. Chemical characterization of the levels of macro and micronutrients in addition to potentially toxic elements in the samples.

Amostra	CaO	K2O	MgO	SB	P2O5	SiO2	Fe2O3	As	Cd	Hg	Pb	Mn	Ni	Co
	%							mg kg−1
PB 01A	4.28	3.29	2.69	10.26	0.41	60.3	8.57	<1	0.65	<0.01	27.5	1239.4	24.8	20
PB 01B	1.91	4.96	0.68	7.55	0.18	66.2	5	2	0.22	<0.01	32.9	852.1	2.7	5.6
PB 01C	8.56	1.63	9.16	19.35	0.36	47.4	11.6	<1	0.14	<0.01	7.9	1781.7	190.8	49.3
PB 01E	3.89	3.5	2.1	9.49	0.38	61	7.5	1	0.14	0.01	20.1	1084.5	14	15.4
PB-10A	7.91	0.85	5.26	14.02	0.17	52.9	8.47	<1	0.08	<0.01	3.5	1471.83	44.3	44.7
PB-10B	7.98	0.71	5.23	13.92	0.18	53.3	8.61	<1	0.08	<0.01	4	1394.37	46.3	45
PB-10C	8.49	1.14	5.3	14.93	0.17	53.5	8.47	<1	0.11	<0.01	4.9	1394.37	49	45.4
PB-05A	0.78	5.35	0.22	6.35	0.03	73.5	1.88	<1	0.12	0.02	23.7	542.25	2.2	1.4
PB-05C	0.8	5.2	<0.1	6	0.03	74.3	1.9	2	0.23	0.03	22.7	619.72	0.9	1.3
Legislation (1)		≥1	-- (2)	≥9	≥1	≥0.05	≥0.1	<15	<10	<0.1	<200	≥1000	≥50	≥50

⁽¹⁾ The normative instruction for soil remineralizers established by the Ministry of Agriculture, Livestock and Food Supply (MAPA) in Brazil [8]. ⁽²⁾ Not established. In bold are the values that comply with the legislation.

demonstrate the amounts of macronutrients, micronutrients, and potentially toxic elements in rock samples and PRS collected in the quarries. The SRP is represented by sample E, in the case of EBAM quarry (PB-01), and by sample C at Granada (PB-10) and Samauma (PB-05) quarries. The comparison between the evaluated materials and the standardized by the IN 05/2016 of MAPA [8] is represented in Figure 8.

The results show that the SRP samples collected at the EBAM and Granada quarries (PB-01E and PB-10C) fully meet the chemical criteria of the standard for soil remineralizers [8], although some rock samples from these two quarries, individually, do not meet all parameter of the standard. For example, potassified quartz monzonites (PB-01B), which do not meet the 9% sum of bases (SB), or basalts (PB-10 A and B), where K₂O contents are less than 1%. At the Samauma quarry (PB-05), the SB values are far below the norm. However, this material has the most expressive K₂O levels among those studied. All materials have declarable levels of silica and iron. Again, the materials from EBAM and Granada quarries stand out, which also have declarable levels of manganese. It is essential to point out that the cobalt content in the SRP at Granada quarry (PB-10C) is close to the standardized declarable content.

Regarding the SRP from the Samauma quarry (PB-05C), it is considered that aphanitic rocks are more susceptible to the release of nutrients to the soil–plant system, considering that the rapid cooling of the volcanic matrix does not allow the organization of cohesive and well-developed crystalline reticulum, which results in the fine texture of the mineral phases observed in this lithotype. This would facilitate the weathering processes of the soil on the constituents of this material. This fact, associated with the low levels of free silica, represented by the quartz crystals (Figure 9) and the high levels of K₂O, give relevance to the prospect, even though it does not meet the standard for soil remineralizers [8].

The samples in the EBAM Quarry showed quartz (free silica) contents below 15.5% (Figure 9), far below the maximum 25% established by the norm for soil remineralizers [8], in addition to detecting the presence of mineral apatite that carries P (an essential macronutrient for plants) and alteration minerals (epidote and sericite) that show the susceptibility of rocks to weathering.

In the rocks of the Granada quarry (Figure 9), no quartz or K-feldspar (minerals with low weathering potential) were found in the samples, which are formed mainly through plagioclase and pyroxene, which is very positive for its application as a remineralizer. In addition, 10% of diktitaxitic cavities were characterized by their modal composition. This also positively impacts rockstone, as these cavities are filled with 2:1 clays with a high cation exchange capacity (CEC), which contributes to improving nutrient retention in soils.

In an agronomic study of SRP incubation conducted with rocks from the Apoteri Formation (Granada Mineração) in a dystrophic yellow Oxisoil developed on the Boa Vista Formation, with doses varying between 0 and 96 Mg ha⁻¹, ground in granulometry smaller than 0.053 mm, it was observed that there was an increase in pH values, going from 4.8 to 5.5, at the dose of 50 Mg ha⁻¹, in addition to a decrease in potential acidity and important increases in Ca, Mg, Zn and Cu contents in the soil [12]. The authors concluded that basaltic rock dust can be used as an alternative source of nutrients aiming at the correction and fertilization of cultivated species.

The granulometric analyses of the SRP (PB-01E; PB-10C; PB-05C) consistently demonstrated that less than 2% of these SRPs are in the clay fraction. The silt fraction is represented by percentages ranging from 0.41%–11%; fine sand 4%–20%, and the coarse sand fraction represents the remaining 80%–95%. It is essential to point out that the methodology used in the laboratory to determine the granulometry of the SRP, laser ray diffraction, is very different from the method traditionally used to texture soils (pipetting). However, the differences between laser granulometry and pipetting in the sand fraction are negligible [32]. These results show that such powders, produced in this type of enterprise, can be classified as low reactivity, functioning as a reserve for the slow release of nutrients due to their low specific surface [33]. The comminution of these fragments into smaller and more reactive fractions would depend on milling techniques, which would substantially enhance their reactivity. However, costs related to milling must be considered.

In all projects covered in this study, crushing is accomplished in two stages; jaw crushers are used in the initial stage, and cone crushers are used in the secondary stage so that it is possible to establish some relationships regarding the effect of crushing on the different rocks studied (Figure 10).

It was noted that the smallest particles identified by the granulometric analyses were generated by crushing the rocks, which generally had a larger granulometry (PB-01). When crushed, Basalt, a finer granulometry rock, in turn (PB-10), generated a more condensed granulometric curve and larger fragments. While the powder from monzonite quartz (PB-01) already has fragments from 0.6 µm in diameter, in addition to 1.5% clay and 11% silt by texture, the basalt powder has its smallest fragments measuring 26 µm. Therefore, it does not have clay-sized fragments, in addition to presenting less than 0.5% of fragments in the silt fraction. The porphyritic rhyolite sample (PB-05) has an intermediate behavior.

In phaneritic rock with larger crystals, the comminution process promotes the shattering of the crystals, resulting in the generation of silt and clay-sized fragments. The quartz monzonite from the EBAM quarry is formed, primarily, by crystals of 1 mm (1000 µm) in diameter or larger; 64% of these are feldspars with up to 6 mm in diameter. The rhyolite, in turn, has about 16% of feldspar phenocrysts with a size of 0.5 mm or larger immersed in a cohesive microcrystalline matrix. In basalt, large phenocrysts are rare; most of its granulometry has sizes between 0.1 and 0.3 mm (100–300 µm) in diameter. It is possible to observe a sharp increase followed by a plateau outlined by the PB-10C granulometric curve at these values (100–300 µm). This behavior and the absence of fragments in the silt and clay fractions in the basalt powder indicate that the comminution process is promoting the separation between the different rock crystals and not their shattering in the case of basalt.

The shattering of crystals and the decrease in granulometry at silt and clay levels are promoted more forcefully by milling processes, different from crushing, already known in the universe of agrominerals, such as mechanoactivation techniques [34].

4. Conclusions

The silicate rock powders (SRP) presented in the study have chemical, physical, and mineralogical characteristics that qualify them to be researched as soil remineralizers. Furthermore, all the materials have meager amounts of potentially toxic elements. The existence of established extraction, crushing, and distribution logistics is also evidenced, in addition to the fact that the prospects are located in an easily accessible region with a high demand for agricultural inputs. Finally, it is concluded that only the rock powders from the EBAM Quarries (PB-01) and Granada Quarry (PB-10) had adequate nutrient and mineral contents to be classified as soil remineralizers by IN05/2016. Despite not meeting the standard, SRP from the Samauma quarry (PB-10) has relevant K₂O (PB-05) content and can be exploited as simple fertilizers. Overall, it is important to highlight that one of the essential conditions for registering an agromineral as a soil remineralizer is the evaluation of the agronomic efficiency of the product, which is obtained through tests carried out with plants in a greenhouse or the field.