The Effect of Grinding on Tremolite Asbestos and Anthophyllite Asbestos

The six commercial asbestos minerals (chrysotile, fibrous actinolite, crocidolite, amosite, fibrous tremolite, and fibrous anthophyllite) are classified by the IARC as carcinogenic to humans. There are currently several lines of research dealing with the inertisation of asbestos minerals among which the dry grinding process has received considerable interest. The effects of dry grinding on tremolite asbestos and anthophyllite asbestos in eccentric vibration mills have not yet been investigated. Along the research line of the mechanical treatment of asbestos, the aim of this study was to evaluate the effects of dry grinding in eccentric vibration mills on the structure, temperature stability, and fibre dimensions of tremolite asbestos from Val d’Ala, (Italy) and UICC standard anthophyllite asbestos from Paakkila mine (Finland) by varying the grinding time (30 s, 5 min, and 10 min). After grinding for 30 s to 10 min, tremolite asbestos and anthophyllite asbestos showed a decrease in dehydroxylation and breakdown temperatures due to the increase in lattice strain and the decrease in crystallinity. Moreover, after grinding up to 10 min, tremolite and anthophyllite fibres were all below the limits defining a countable fibre according to WHO.


Introduction
Tremolite Ca 2 Mg 5 Si 8 O 22 (OH) 2 asbestos and anthophyllite Mg 7 Si 8 O 22 (OH) 2 asbestos are classified as carcinogenic substances group 1 by the IARC [1].In fact, occupational and environmental exposure to tremolite asbestos and anthophyllite asbestos are associated with lung diseases [2][3][4][5][6][7][8].In the past, large outcrops of tremolite asbestos and anthophyllite asbestos within altered ultramafic bodies have been mined in many parts of the world [9][10][11][12][13].Specifically, tremolite asbestos was mined in South Korea [14], in the Teeter quarry in southeast Pennsylvania, and in the Centreville quarry in north Virginia [9,11] while anthophyllite asbestos was mainly mined in Finland and North Carolina [8,15].In Turkey, after separating it from chrysotile (asbestos) during the milling process, tremolite was used for stuccoing the exterior surfaces of buildings [16], and a similar process was used in Cyprus [17] and all over the Middle East [18].The mining and use of tremolite asbestos and anthophyllite asbestos were reduced compared to more commercially available types of asbestos such as chrysotile, crocidolite, and amosite [19][20][21].However, despite their relatively limited use, tremolite asbestos and anthophyllite asbestos have been used in many commercial products such as cements, wall plasters, roofing materials, laboratory materials, in battery boxes, and as fillers in rubber etc. [8,[21][22][23].According to the existing regulations, tremolite asbestos and anthophyllite asbestos are also banned worldwide [21].This danger is connected with the fibrous nature of asbestos minerals, especially as a respirable form i.e., fibres with a length >5 µm, a width <3 µm, and an aspect ratio >3:1 [24].However, tremolite asbestos and anthophyllite asbestos are considered to be contaminants of more widely used minerals such as chrysotile, talc, or vermiculite that are being exploited commercially (e.g., cement, cosmetics, asbestos-containing materials) [17,23,25].It is important to note that chrysotile is only banned in 28% of the countries worldwide, while the other countries permit "the safe use" of chrysotile [26].For example, tremolite asbestos was found in chrysotile outcrops in Quebec Province in Canada.The consumption of asbestos containing chrysotile has recently increased in India, which uses approximately 100,000 tons of asbestos per year, 80% of which is imported from other countries [27].In this regard, it is worth remembering that, in India, the mining, production, and use of asbestos is very poorly regulated despite the serious risks to human health [27,28].Furthermore, the presence of asbestos in talc has attracted much attention due to potential asbestos exposure when using products containing talc (e.g., cosmetics, filler in composites).For example, talc ore mined in upstate New York contains large amounts of both tremolite asbestos and anthophyllite asbestos, which led to high mortality rates among talc workers [12,[29][30][31].In this scenario, lawsuits against the multinational medical devices, pharmaceuticals, and consumer packaged goods manufacturing company that produces baby talcum powder have been instituted.According to the claimant, the use of the company's powder contributed to develop some forms of cancer [32].Moreover, several studies have been recently carried on tremolite and anthophyllite asbestos naturally occurring in different parts of the world [8,[33][34][35].This is due to the relatively widespread diffusion of asbestos-containing rocks that are excavated for numerous civil engineering projects such as housing settlements, railway lines, motorways etc. e.g., [36].Today, following the ban of its use, asbestos must be safely disposed of or made inert.In countries where asbestos minerals are banned and remediation policies are promoted, numerous attempts have been made to neutralize asbestos minerals and many studies have focused on effective asbestos inertisation methods (e.g., thermal treatment, dissolution by acid, biological treatment, thermochemical treatment) [37][38][39][40][41][42][43].Amorphisation and the modification of the fibrous morphology of chrysotile asbestos by mechano-chemical treatment has been also successfully applied by several authors [44][45][46][47].These authors showed that chrysotile asbestos completely disappeared after several hours of milling, during which part of the mechanical energy transferred to solid system is converted into heat and a part is used to destroy the crystalline framework.Bloise et al. [48] recently demonstrated that in addition to chrysotile, grinding amosite and crocidolite asbestos for up to 10 min led to almost total amorphisation and size reduction of the fibres.The reaction of tremolite when subjected to various kinds of mechanical and chemical corrosion with HCl, HNO 3 , aqua regia, NaOH is described only in one paper [49], which showed that tremolite fibres were significantly modified by grinding and chemical corrosion.However, no studies have yet been carried out on tremolite asbestos and anthophyllite asbestos subjected to grinding and subsequently characterized according to degree of crystallinity, thermal behaviour, and fibre size.In this context, the aim of this study is to evaluate for the first time the effects that dry grinding process for different time in eccentric vibration mills has influence on tremolite and anthophyllite asbestos in terms of their degree of crystallinity, temperature stability, and fibre size.Using X-ray powder diffraction (XRPD), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM) has given us a better understanding of the phase evolution during grinding and the changes in particle size of tremolite asbestos and anthophyllite asbestos.We believe that our study can provide new insight on how to safely dispose of asbestos through the grinding process.

Materials and Methods
For the study, the following mineral fibres have been used: (1)  This sample contains minor impurity of clinochlore and very minor impurities of antigorite and talc [50].
(2) UICC anthophyllite asbestos from Paakkila (Finland) with chemical formula: Ca 0.04 (Mg This sample contains minor impurities of clinochlore and biotite and very minor impurities of vermiculite and talc [50]. These two samples, have been extensively characterized as far as the thermal behaviour, impurity content, crystallinity, and other characteristic features [50][51][52].The samples in fibrous form (1 g) were placed in a grinding container (100 mL) that incorporate an anvil ring of 1285 g and puck of 1388 g to pulverize the samples by an eccentric vibratory motion (container and ring set are made out of tungsten carbide alloy containing 9% of cobalt).The temperature never exceeds 60 • C during the grinding process.Dry grinding of the samples was conducted using Bleuler Rotary Mill (Sepor, Los Angeles, CA, USA) for 30 s, 5 min, and 10 min at a speed of 900 revolutions per min (rpm).A qualitative phase analysis of the samples before and after thermogravimetric analyses was performed by XRPD using a Bruker D8 Advance X-ray diffractometer (Bruker, Billerica, MA, USA) operating in reflection mode at 40 kV and 40 mA.Scans were recorded in the range of 3-66 • 2θ, with a step size of 0.02 • 2θ and a counting time of 3 s.Mineral phases were identified using DIFFRAC plus EVA software by comparing the experimental peaks with reference patterns stored in the 2005 PDF2.In the present work, the weight change and the energetic transformations were evaluated on samples by simultaneous Differential Scanning Calorimetry and Thermogravimetric Analysis (DSC/TGA: Netzsch STA 449 C Jupiter, Netzsch-Gerätebau GmbH, Selb, Germany).During DSC/TGA analysis, the samples were heated at a rate of 10 • C•min −1 in the 25-1100 • C temperature range under an air flow of 30 mL•min −1 .Approximately 20 mg of sample was used for each run.Derivative thermogravimetry (DTG), derivative differential scanning calorimetry (DDSC), exo-and endo-thermic peaks were obtained using Netzsch proteus thermal analysis software (Netzsch-Gerätebau GmbH, Selb, Germany).Instrumental precision was checked by six repeated collections on a kaolinite reference sample revealing good reproducibility (instrumental theoretical T precision of ±1.2 • C) DSC detection limit <1 µW, theoretical mass sensitivity of 0.10 µg.Size, crystallinity, structural features, and chemical composition of single fibres were determined using a Jeol JEM 1400 Plus (120 kV) transmission electron microscope (TEM, Jeol, Tokyo, Japan) equipped with Jeol large-area silicon drift detector SDD-EDS (Jeol, Tokyo, Japan) for microanalyses.Unambiguous crystallinity of individual fibres was achieved by selected area electron diffraction (SAED).For TEM investigations, the sample was put in isopropyl alcohol and then sonicated.Three drops of the obtained suspension were deposited on a Formvar carbon-coated copper grid.In order to describe the size (length and diameter) of the fibres, several hundred TEM micrographs were recorded and 20 single fibres of tremolite and anthophyllite (chemically and morphologically verified) for each grinding time (30 s, 5 min, and 10 min) were measured.Twenty fibres of both types of mineral were used to make reliable comparisons between the ground tremolite and anthophyllite samples, since after 10 min of grinding the tremolite fibres, no more than 20 fibres were observed by TEM.

Tremolite
Figure 1a shows the X-ray diffraction patterns of tremolite asbestos ground for three different times (30 s, 5 min and 10 min).The X-ray diffraction pattern of the 30 s mechanically ground tremolite asbestos is characterized by sharp peaks (JCPDS card 44-1402) due to its high degree of crystallinity.After 5 min of grinding, all reflections are preserved even if broadened (Figure 1a).When the grinding time is increased up to 10 min, the intensity of the peaks is reduced to the point that they are no longer distinguishable from the background (Figure 1a) except for the (110), (310), (151), and (260) reflections that were still slightly visible.This trend is mostly due to amorphisation of the sample ground for 10 min than the sample ground for 5 min.Moreover, after 5 min grinding, the diagnostic reflections of clinochlore (JCPDS card 29-0853) present as an impurity in the sample [50] are no longer visible. of the sample ground for 10 min than the sample ground for 5 min.Moreover, after 5 min grinding, the diagnostic reflections of clinochlore (JCPDS card 29-0853) present as an impurity in the sample [50] are no longer visible.

Anthophyllite
After 30 s of grinding, anthophyllite asbestos is highly crystalline, with clinochlore and biotite present as impurities (Figure 1b).After 5 min of grinding, most of the anthophyllite asbestos reflections disappeared, while (200), ( 210), ( 440), (600), and (501) reflections (JCPDS card 42-0544) are less intense but preserved, suggesting that anthophyllite with lattice strain coexists with a fraction of residual crystalline fibres in the sample.After 10 min of grinding, all reflections became gradually indistinct in the pattern (Figure 1b).It is evident that no crystalline anthophyllite was detected by XRPD after 10 min grinding.Mechanical grinding also affected the structure of clinochlore (JCPDS card 24-0506) and biotite (JCPDS card 42-1437) whose characteristic reflections disappeared after 5 min of grinding (Figure 1b).

Tremolite
Figure 2 shows the thermal behaviour of tremolite asbestos ground for 30 s, 5 min, and 10 min in selected temperature range.After 30 s grinding, using the maximum mass loss rate (DTG) due to the loss of hydroxyls groups when tremolite is heated up to 1100 °C, the dehydroxylation temperature of tremolite is at about 1046 °C (Table 1) even if dehydroxylation started at 1000 °C and ends at approximately 1090 °C [53,54].After 5 min grinding, tremolite dehydroxylation occurs at a lower temperature, as shown by the DTG peak at approximately 954 °C (Figure 2), which shifts to 916 °C and becomes weaker in the sample ground for 10 min (Figure 2).DDSC confirmed the structural collapse of tremolite which occurs at lower temperatures, when the grinding time was increased from 30 s to 5 min (Table 1).Although the DDSC curve did not show detectable signals after 10 min grinding (Figure 2), when heated tremolite decomposes into pyroxene, silica and water [55] whereby the double-chain units of tremolite amphibole split into single-chain pyroxene.In fact, diopside CaMgSi2O6 (JCPDS card 38-0466) and cristobalite SiO2 (JCPDS card 03-0257) is identified by XRPD as the main crystalline phase formed after combined thermal treatments at 1100 °C and grinding for 30 s, 5 min and 10 min (Supplementary Figure S1).

Anthophyllite
After 30 s of grinding, anthophyllite asbestos is highly crystalline, with clinochlore and biotite present as impurities (Figure 1b).After 5 min of grinding, most of the anthophyllite asbestos reflections disappeared, while (200), (210), (440), (600), and (501) reflections (JCPDS card 42-0544) are less intense but preserved, suggesting that anthophyllite with lattice strain coexists with a fraction of residual crystalline fibres in the sample.After 10 min of grinding, all reflections became gradually indistinct in the pattern (Figure 1b).It is evident that no crystalline anthophyllite was detected by XRPD after 10 min grinding.Mechanical grinding also affected the structure of clinochlore (JCPDS card 24-0506) and biotite (JCPDS card 42-1437) whose characteristic reflections disappeared after 5 min of grinding (Figure 1b).

Tremolite
Figure 2 shows the thermal behaviour of tremolite asbestos ground for 30 s, 5 min, and 10 min in selected temperature range.After 30 s grinding, using the maximum mass loss rate (DTG) due to the loss of hydroxyls groups when tremolite is heated up to 1100 • C, the dehydroxylation temperature of tremolite is at about 1046 • C (Table 1) even if dehydroxylation started at 1000 • C and ends at approximately 1090 • C [53,54].After 5 min grinding, tremolite dehydroxylation occurs at a lower temperature, as shown by the DTG peak at approximately 954 • C (Figure 2), which shifts to 916 • C and becomes weaker in the sample ground for 10 min (Figure 2).DDSC confirmed the structural collapse of tremolite which occurs at lower temperatures, when the grinding time was increased from 30 s to 5 min (Table 1).Although the DDSC curve did not show detectable signals after 10 min grinding (Figure 2), when heated tremolite decomposes into pyroxene, silica and water [55] whereby the double-chain units of tremolite amphibole split into single-chain pyroxene.In fact, diopside CaMgSi 2 O 6 (JCPDS card 38-0466) and cristobalite SiO 2 (JCPDS card 03-0257) is identified by XRPD as the main crystalline phase formed after combined thermal treatments at 1100 • C and grinding for 30 s, 5 min and 10 min (Supplementary Figure S1).

Anthophyllite
The destruction of anthophyllite closely resembles the collapse of tremolite.In Figure 3, the broad DTG event, which starts above 800 °C and ends at approximately 900 °C, is centred at 868 °C (Table 2) and is related to the dehydroxylation of anthophyllite grinding for 30 s.This result is in agreement with the literature data [55].The dehydroxylation temperature of anthophyllite decreases at 823 °C and starts at about 800 °C in the samples ground for 5 and 10 min (Figure 3).The broad endothermic event on DDSC curve at 850 °C (Figure 3) is related to the structural breakdown of anthophyllite grinding for 30 s, which shifts downward to approximately 816 °C for the sample ground for 5 and 10 min (Figure 3, Table 2).The product of the structural collapse is a pyroxene phase, cristobalite, and water [56].This reaction is also observed even after grinding the sample.In fact, after heating at 1100 °C, the XRPD patterns of anthophyllite ground for 5 min and 10 min shows enstatite MgSiO3 (JCPDS card 19-0606) and cristobalite SiO2 (JCPDS card 03-0257) (Supplementary Figure S2).

Anthophyllite
The destruction of anthophyllite closely resembles the collapse of tremolite.In Figure 3, the broad DTG event, which starts above 800 • C and ends at approximately 900 • C, is centred at 868 • C (Table 2) and is related to the dehydroxylation of anthophyllite grinding for 30 s.This result is in agreement with the literature data [55].The dehydroxylation temperature of anthophyllite decreases at 823 • C and starts at about 800 • C in the samples ground for 5 and 10 min (Figure 3).The broad endothermic event on DDSC curve at 850 • C (Figure 3) is related to the structural breakdown of anthophyllite grinding for 30 s, which shifts downward to approximately 816 • C for the sample ground for 5 and 10 min (Figure 3, Table 2).The product of the structural collapse is a pyroxene phase, cristobalite, and water [56].This reaction is also observed even after grinding the sample.In fact, after heating at 1100 • C, the XRPD patterns of anthophyllite ground for 5 min and 10 min shows enstatite MgSiO 3 (JCPDS card 19-0606) and cristobalite SiO 2 (JCPDS card 03-0257) (Supplementary Figure S2).

TEM Characterization
Figure 4 was obtained through low magnification TEM observation to capture the whole mesh of the grid in each micrograph, as grinding time increased for tremolite (Figure 4a-c) and anthophyllite (Figure 4d-f).

TEM Characterization
Figure 4 was obtained through low magnification TEM observation to capture the whole mesh of the grid in each micrograph, as grinding time increased for tremolite (Figure 4a-c) and anthophyllite (Figure 4d-f).

Tremolite
After 30 s grinding, tremolite fibres exhibit a prismatic rod-shaped morphology (Figure 4a), and most fibres (70%) are >5 µm, width <3 µm (Figure 5a,b), and are therefore classified as countable fibres according to the WHO counting criteria [2].After 5 min grinding, few fibres and numerous amorphous particles (SAED patterns with full halo rings) are detected (Figure 4b).Some fibres show impact ruptures transverse to the fibre axis, leading to a local decrease in diameter (Figure 6a).
Regarding the size of tremolite, after 5 min grinding, 100% of the fibres measured are <2 µm, and 95% have diameters <1 µm (Figure 5a,b).As grinding progressed up to 10 min, very few crystalline fibres with sharp corners and strongly reduced dimensions are detected (Figure 6b).One hundred per cent of the observed fibres are shorter than 1 µm, with 95% of diameters below 250 nm (Figure 5a,b).

Tremolite
After 30 s grinding, tremolite fibres exhibit a prismatic rod-shaped morphology (Figure 4a), and most fibres (70%) are >5 μm, width <3 μm (Figure 5a,b), and are therefore classified as countable fibres according to the WHO counting criteria [2].After 5 min grinding, few fibres and numerous amorphous particles (SAED patterns with full halo rings) are detected (Figure 4b).Some fibres show impact ruptures transverse to the fibre axis, leading to a local decrease in diameter (Figure 6a).
Regarding the size of tremolite, after 5 min grinding, 100% of the fibres measured are <2 μm, and 95% have diameters <1 μm (Figure 5a,b).As grinding progressed up to 10 min, very few crystalline fibres with sharp corners and strongly reduced dimensions are detected (Figure 6b).One hundred per cent of the observed fibres are shorter than 1 μm, with 95% of diameters below 250 nm (Figure 5a,b).

Tremolite
After 30 s grinding, tremolite fibres exhibit a prismatic rod-shaped morphology (Figure 4a), and most fibres (70%) are >5 μm, width <3 μm (Figure 5a,b), and are therefore classified as countable fibres according to the WHO counting criteria [2].After 5 min grinding, few fibres and numerous amorphous particles (SAED patterns with full halo rings) are detected (Figure 4b).Some fibres show impact ruptures transverse to the fibre axis, leading to a local decrease in diameter (Figure 6a).
Regarding the size of tremolite, after 5 min grinding, 100% of the fibres measured are <2 μm, and 95% have diameters <1 μm (Figure 5a,b).As grinding progressed up to 10 min, very few crystalline fibres with sharp corners and strongly reduced dimensions are detected (Figure 6b).One hundred per cent of the observed fibres are shorter than 1 μm, with 95% of diameters below 250 nm (Figure 5a,b).

Anthophyllite
After 30 s grinding, anthophyllite shows a needle-like crystal habit (Figure 4d) 60% of which met the size criteria defined for WHO countable fibres (Figure 5c,d) [2].After 5 min grinding, the particle agglomerates increase with the presence of few anthophyllite fibres (Figure 4e,f), some of which are preserved while others are clearly delaminated (Figure 7a).Regarding the size, 80% of the fibres are <5 μm long with 100% of their diameters < 1 μm (Figure 5c,d).As grinding increased up to 10 min, numerous amorphous agglomerates are observed (Figure 4f).Moreover, infrequent fibres with strongly rounded edges coexist with partially sharp-edged fibres (Figure 7b).One hundred per cent of the fibres are <2 μm long, with 75% of their diameters <250 nm (Figure 5c,d).

Anthophyllite
After 30 s grinding, anthophyllite shows a needle-like crystal habit (Figure 4d) 60% of which met the size criteria defined for WHO countable fibres (Figure 5c,d) [2].After 5 min grinding, the particle agglomerates increase with the presence of few anthophyllite fibres (Figure 4e,f), some of which are preserved while others are clearly delaminated (Figure 7a).Regarding the size, 80% of the fibres are <5 µm long with 100% of their diameters <1 µm (Figure 5c,d).As grinding increased up to 10 min, numerous amorphous agglomerates are observed (Figure 4f).Moreover, infrequent fibres with strongly rounded edges coexist with partially sharp-edged fibres (Figure 7b).One hundred per cent of the fibres are <2 µm long, with 75% of their diameters <250 nm (Figure 5c,d).

Anthophyllite
After 30 s grinding, anthophyllite shows a needle-like crystal habit (Figure 4d) 60% of which met the size criteria defined for WHO countable fibres (Figure 5c,d) [2].After 5 min grinding, the particle agglomerates increase with the presence of few anthophyllite fibres (Figure 4e,f), some of which are preserved while others are clearly delaminated (Figure 7a).Regarding the size, 80% of the fibres are <5 μm long with 100% of their diameters < 1 μm (Figure 5c,d).As grinding increased up to 10 min, numerous amorphous agglomerates are observed (Figure 4f).Moreover, infrequent fibres with strongly rounded edges coexist with partially sharp-edged fibres (Figure 7b).One hundred per cent of the fibres are <2 μm long, with 75% of their diameters <250 nm (Figure 5c,d).

Discussion
The reduction of the diffraction peak intensity and the decrease of the breakdown temperatures in the DTG/DDSC curves indicate that the structure of both fibrous tremolite and anthophyllite are rapidly damaged by mechanical treatment even after 5 min grinding.Our observations on XRPD data are consistent with a decrease in crystallinity and the formation of lattice defects as grinding increased from 30 s to 10 min for both tremolite and anthophyllite samples.Notwithstanding, XRPD shows that after 5 min grinding, tremolite is still crystalline.The reduction in particle size, the formation of lattice defects, or stressed structures induced by the grinding processes caused the original maximum mass loss rate (DTG) events related to dehydroxylation to decrease, as observed for other types of asbestos such as chrysotile, amosite, and crocidolite [48].Dehydroxylation is related to the loss of OH groups through a prototropic mechanism based on the concept of proton migration during grinding and their consequent combination with hydroxyl groups to form water molecules [48,57].It is important to note that the effect of grinding activation is the point heating of the mineral, which causes its dehydroxylation at the contact points between the grinding medium and the mineral structure [57].
The point contact heating at specific sites leads to the achievement of the high temperatures required for dehydroxylation as follows: Prototropy produces a water molecule as follows: This reaction occurs when fibres are mechanically activated.TGA of tremolite and anthophyllite grinding for 30 s, 5 min, and 10 min showed differences in temperature breakdown.Tremolite ranged from 1046 to 916 • C, while anthophyllite ranged from 871 to 823 • C, depending on the grinding time.However, greater reductions in dehydroxylation temperature were observed in tremolite ground for 5 min than in anthophyllite.In particular, the dehydroxylation temperature of the tremolite sample ground for 10 min is 130 • C lower than that of the sample ground for 30 s (Table 1).Anthophyllite dehydroxylation is anticipated of 46 • C for the sample ground for 10 min with respect to the sample ground for 30 s (Table 2).Moreover, 5 min of grinding before thermal treatment are sufficient to achieve the maximum decrease of temperature breakdown for both tremolite and anthophyllite asbestos, as this temperature does not change even after 10 min grinding (Table 3).A similar trend is observed for chrysotile, amosite, and crocidolite [48].We found that the decomposition temperatures of both anthophyllite and tremolite asbestos after grinding for 30 s, 5 min, and 10 min are higher than those reported for other asbestos such as amosite and crocidolite that underwent the same treatment [48] (Figure 8; Table 3).This probably depends on the correlation between the Mg content, which is higher in anthophyllite and tremolite than in amosite and crocidolite, and the decomposition peak temperature as previously demonstrated in the literature [38,50,58].The results of this study also prove that fibrous tremolite and anthophyllite decomposition is simultaneous with dehydroxylation as grinding increased from 30 s to 10 min.Therefore, crocidolite is the only amphibole asbestos with consecutive rather than concurrent dehydroxylation and structural breakdown processes [50,55,59] even after grinding [48].
Morphometric analyses were performed on 150 meshes observed by TEM in order to quantify the size (length and diameter) of all visible fibres, the results of which are reported in Figure 5. Regarding the length, the histograms show that after 30 s grinding 70% of tremolite and 60% of anthophyllite are generally >5 μm (Figure 5).One hundred per cent of tremolite fibres are <5 μm after 5 min grinding and even less after 10 min (Figure 5a,b).Ten minutes of grinding is required to reduce the length of 100% of the anthophyllite fibres under 5 μm.As regards diameters, grinding prompted the longitudinal splitting of both tremolite and anthophyllite fibres parallel to (110) cleavage surfaces into thinner fibrils.After 30 s grinding, all fibres are characterized by a small diameter (<3 μm) with 73% of tremolite and 50% of anthophyllite diameters <1 μm (Figure 5c,d).After grinding up to 10 min, the diameter decreased considerably with the main mode corresponding to a fibre diameter < 250 nm.In accordance with WHO fibre regulations [2], all of the tremolite fibres measured in the samples ground for 5 min were generally <5 μm long.On the other hand, 20% of anthophyllite fibres ground for 5 min are still >5 μm long while after 10 min grinding, all of the anthophyllite fibres became shorter.

Conclusions
For the first time, we investigated the effects that dry grinding in eccentric vibration mills produce on tremolite asbestos (Val d'Ala, Piedmont, Italy) and anthophyllite asbestos (UICC, Paakkila mine, Eastern Finland Region, Finland) in terms of crystallinity, temperature stability, and the reduction of the fibre size.Grinding tremolite and anthophyllite asbestos leads to an increase the amorphization degree and a decrease in thermal decomposition temperatures.However, the existence of rare tremolite and anthophyllite fibres preserved within the samples after 10 min grinding was detected by TEM.Tremolite proved to be the most thermally-stable amphibole asbestos even after grinding treatment.In fact, tremolite asbestos was decomposed at approximately 916 °C when it was ground for up to 10 min and after 5 min grinding all XRPD peaks were preserved.The breakdown of anthophyllite asbestos occurs at approximately 822 °C after grinding up to 10 min.On the other hand, 5 min grinding before thermal treatment are sufficient to achieve a significant reduction of the breakdown temperature for both tremolite and anthophyllite, as this temperature remains almost the same even after 10 min of grinding.This study also confirms that This probably depends on the correlation between the Mg content, which is higher in anthophyllite and tremolite than in amosite and crocidolite, and the decomposition peak temperature as previously demonstrated in the literature [38,50,58].The results of this study also prove that fibrous tremolite and anthophyllite decomposition is simultaneous with dehydroxylation as grinding increased from 30 s to 10 min.Therefore, crocidolite is the only amphibole asbestos with consecutive rather than concurrent dehydroxylation and structural breakdown processes [50,55,59] even after grinding [48].
Morphometric analyses were performed on 150 meshes observed by TEM in order to quantify the size (length and diameter) of all visible fibres, the results of which are reported in Figure 5. Regarding the length, the histograms show that after 30 s grinding 70% of tremolite and 60% of anthophyllite are generally >5 µm (Figure 5).One hundred per cent of tremolite fibres are <5 µm after 5 min grinding and even less after 10 min (Figure 5a,b).Ten minutes of grinding is required to reduce the length of 100% of the anthophyllite fibres under 5 µm.As regards diameters, grinding prompted the longitudinal splitting of both tremolite and anthophyllite fibres parallel to (110) cleavage surfaces into thinner fibrils.After 30 s grinding, all fibres are characterized by a small diameter (<3 µm) with 73% of tremolite and 50% of anthophyllite diameters <1 µm (Figure 5c,d).After grinding up to 10 min, the diameter decreased considerably with the main mode corresponding to a fibre diameter < 250 nm.In accordance with WHO fibre regulations [2], all of the tremolite fibres measured in the samples ground for 5 min were generally <5 µm long.On the other hand, 20% of anthophyllite fibres ground for 5 min are still >5 µm long while after 10 min grinding, all of the anthophyllite fibres became shorter.

Conclusions
For the first time, we investigated the effects that dry grinding in eccentric vibration mills produce on tremolite asbestos (Val d'Ala, Piedmont, Italy) and anthophyllite asbestos (UICC, Paakkila mine, Eastern Finland Region, Finland) in terms of crystallinity, temperature stability, and the reduction of the fibre size.Grinding tremolite and anthophyllite asbestos leads to an increase the amorphization degree and a decrease in thermal decomposition temperatures.However, the existence of rare tremolite and anthophyllite fibres preserved within the samples after 10 min grinding was detected by TEM.Tremolite proved to be the most thermally-stable amphibole asbestos even after grinding treatment.In fact, tremolite asbestos was decomposed at approximately 916 • C when it was ground for up to 10 min and after 5 min grinding all XRPD peaks were preserved.The breakdown of anthophyllite asbestos occurs at approximately 822 • C after grinding up to 10 min.On the other hand, 5 min grinding before thermal treatment are sufficient to achieve a significant reduction of the breakdown temperature for both tremolite and anthophyllite, as this temperature remains almost the same even after 10 min of grinding.This study also confirms that tremolite and anthophyllite decomposition occurred concurrently with dehydroxylation as grinding increased.As far as the size is concerned, only 5 min grinding are required to achieve tremolite fibres with dimensions below the limits defining a countable fibre according to WHO [2], whereas 10 min of grinding are required to grind all of the anthophyllite fibres to a size < 5 µm.We believe that this study can provide novel and valuable insights into safe asbestos disposal through the grinding process and will prompt future developments of solutions for the safe disposal of asbestos containing materials.

Figure 5 .
Figure 5. Size distribution of measured fibres (length and diameter) and trend line of the tremolite (a,b) and anthophyllite (c,d).

Figure 5 .
Figure 5. Size distribution of measured fibres (length and diameter) and trend line of the tremolite (a,b) and anthophyllite (c,d).Figure 5. Size distribution of measured fibres (length and diameter) and trend line of the tremolite (a,b) and anthophyllite (c,d).

Figure 5 .
Figure 5. Size distribution of measured fibres (length and diameter) and trend line of the tremolite (a,b) and anthophyllite (c,d).Figure 5. Size distribution of measured fibres (length and diameter) and trend line of the tremolite (a,b) and anthophyllite (c,d).

Figure 6 .
Figure 6.(a) TEM images of tremolite after 5 min of grinding.The black square indicates the zoom area: impact ruptures.(b) TEM images of tremolite after 10 min of grinding, corresponding point analysis, and selected area electron diffraction (SAED) pattern carried out on the point indicated by full white square.

Figure 7 .Figure 6 .
Figure 7. (a) TEM images of anthophyllite after 5 min of grinding.Arrows in (a) indicate the delaminated anthophyllite.(b) TEM images of anthophyllite after 10 min of grinding, corresponding

Minerals 2018, 8 , 13 Figure 6 .
Figure 6.(a) TEM images of tremolite after 5 min of grinding.The black square indicates the zoom area: impact ruptures.(b) TEM images of tremolite after 10 min of grinding, corresponding point analysis, and selected area electron diffraction (SAED) pattern carried out on the point indicated by full white square.

Figure 7 .
Figure 7. (a) TEM images of anthophyllite after 5 min of grinding.Arrows in (a) indicate the delaminated anthophyllite.(b) TEM images of anthophyllite after 10 min of grinding, corresponding

Figure 7 .
Figure 7. (a) TEM images of anthophyllite after 5 min of grinding.Arrows in (a) indicate the delaminated anthophyllite.(b) TEM images of anthophyllite after 10 min of grinding, corresponding point analysis and SAED pattern carried out on the point indicated by full white square.Arrows in (b) indicate the sharp edges of anthophyllite.

Figure 8 .
Figure 8. Changes in the temperature of amphibole asbestos breakdown in relation to the grinding time (30 s, 5 min, and 10 min).

Figure 8 .
Figure 8. Changes in the temperature of amphibole asbestos breakdown in relation to the grinding time (30 s, 5 min, and 10 min).