Process for preparing a composition comprising synthetic mineral particles and composition

Abstract

A process for preparing a composition including synthetic mineral particles, in which a hydrogel which is a precursor of the synthetic mineral particles is prepared via a coprecipitation reaction between at least one compound including silicon, and at least one compound including at least one metal element, characterized in that the coprecipitation reaction takes place in the presence of at least one carboxylate salt of formula R.sub.2COOM in which: M denotes a metal chosen from the group made up of Na and K, and R.sub.2 is chosen from H and alkyl groups including fewer than 5 carbon atoms. A composition including synthetic mineral particles which is obtained by such a process is also described.

Claims

1. A process for preparing a composition comprising synthetic mineral particles, in which a hydrogel precursor of said synthetic mineral particles is prepared by a coprecipitation reaction between: at least one compound comprising sodium metasilicate, and at least one compound comprising at least one metal element, wherein said coprecipitation reaction takes place in the presence of at least one carboxylate salt of the formula R.sub.2COOM, wherein: M denotes Na or K, and R.sub.2 is H or an alkyl group having 1 to 4 carbon atoms, and wherein said carboxylate salt of the formula R.sub.2COOM is added before the coprecipitation reaction takes place.

2. The process according to claim 1, wherein said at least one compound comprising at least one metal element is a dicarboxylate salt of the formula M(R.sub.1COO).sub.2, wherein: R.sub.1 is H or an alkyl group having 1 to 4 carbon atoms, and M denotes at least one divalent metal having the formula Mg.sub.y(1)CO.sub.y(2)Zn.sub.y(3)Cu.sub.y(4)Mn.sub.y(5)Fe.sub.y(6)Ni.sub.y(7)Cr.sub.y(8); each y(i) representing a real number of the interval [0; 1], such that $\underset{i=1}{\overset{8}{.Math.}}y\left(\right)=1.$

3. The process according to claim 1, wherein after said coprecipitation reaction, said hydrogel precursor is then subjected to a hydrothermal treatment to obtain said synthetic mineral particles.

4. The process according to claim 1, wherein a coprecipitation medium comprising said hydrogel precursor is subjected directly to a hydrothermal treatment.

5. The process according to claim 2, wherein R.sub.1 and R.sub.2 are selected from the group consisting of CH.sub.3, CH.sub.3CH.sub.2, and CH.sub.3CH.sub.2CH.sub.2.

6. The process according to claim 1, wherein the groups R.sub.1 and R.sub.2 are identical.

7. The process according to claim 1, wherein said synthetic mineral particles are silicate mineral particles.

8. The process according to claim 1, wherein said synthetic mineral particles are phyllosilicate mineral particles.

9. The process according to claim 1, wherein said hydrogel precursor of said synthetic mineral particles is a hydrogel of the formula
(Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.11,nH.sub.2O, wherein: x is a real number of the interval [0; 1], and n is a number of molecule(s) of water associated with said hydrogel precursor.

10. The process according to claim 1, wherein said at least one carboxylate salt of the formula R.sub.2COOM is present in an amount so as to give, based on silicon, a molar ratio R.sub.2COOM/Si of from 0.1 to 9.

11. The process according to claim 3, wherein said hydrothermal treatment is carried out at a temperature of from 150 C. to 400 C.

12. The process according to claim 3, wherein said hydrothermal treatment is carried out at a pressure of from 5 bar to 200 bar.

13. The process according to claim 2, wherein after said coprecipitation reaction, said hydrogel precursor is then subjected to a hydrothermal treatment to obtain said synthetic mineral particles.

14. A process for preparing a composition comprising synthetic phyllosilicate mineral particles, the process comprising: (i) preparing a hydrogel precursor of said synthetic mineral particles by a coprecipitation reaction comprising: (a) providing a first solution comprising at least one sodium metasilicate and at least one carboxylate salt of the formula R.sub.2COOM, wherein M is Na or K, and R.sub.2 is H or an alkyl group having 1 to 4 carbon atoms; and (b) rapidly adding to the first solution a second solution comprising at least one dicarboxylate salt of the formula M(R.sub.1COO).sub.2, wherein R.sub.1 is H or an alkyl group having 1 to 4 carbon atoms, and M denotes at least one divalent metal having the formula Mg.sub.y(1)Co.sub.y(2)Zn.sub.y(3)Cu.sub.y(4)Mn.sub.y(5)Fe.sub.y(6)Ni.sub.y(7)Cr.sub.y(8), each y(i) representing a real number of the interval [0; 1], such that $\underset{i=1}{\overset{8}{.Math.}}y\left(\right)=1;$ and (ii) after said coprecipitation reaction, subjecting said hydrogel precursor to a hydrothermal treatment to obtain said synthetic phyllosilicate mineral particles.

15. The process according to claim 14, wherein the synthetic phyllosilicate mineral particles have the formula
(Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.10(OH).sub.2, wherein x is a real number of the interval [0; 1], and M denotes at least one divalent metal having the formula Mg.sub.y(1)Co.sub.y(2)Zn.sub.y(3)Cu.sub.y(4)Mn.sub.y(5)Fe.sub.y(6)Ni.sub.y(7)Cr.sub.y(8), each y(i) representing a real number of the interval [0; 1], such that $\underset{i=1}{\overset{8}{.Math.}}y\left(i\right)=1.$

16. The process according to claim 14, wherein the at least one sodium metasilicate is Na.sub.2SiO.sub.3, the at least one carboxylate salt of the formula R.sub.2COOM is CH.sub.3COONa, the at least one dicarboxylate salt of the formula M(R.sub.1COO).sub.2 is Mg(CH.sub.3COO).sub.2, and the synthetic phyllosilicate mineral particles have the formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2.

Description

(1) FIGS. 1 to 3 show RX diffractograms, on each of which there is shown the relative intensity of the signal (number of counts per second) as a function of the inter-reticular distance in Angstroms.

(2) FIGS. 1 and 2 show the results of analyses carried out by X-ray diffraction on: a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 3 hours (curve 1), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 6 hours (curve 2), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 18 hours (curve 3), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 24 hours (curve 4), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 10 days (curve 5).

(3) FIG. 2 shows an enlargement of part of the diffractograms shown in FIG. 1, that is to say only the part of the diffractograms corresponding to the inter-reticular distances between 2 and 4 , thus permitting better visualization of the lines corresponding to the plane (003).

(4) FIG. 3 shows the results of X-ray diffraction analyses on a synthetic talc composition of the formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment at 300 C. for 18 hours (curve 3), and on that same synthetic talc composition after anhydrous heat treatment for 5 hours at 550 C. (curve 10).

(5) The RX diffractograms shown in FIGS. 1 to 3 were recorded on a CPS 120 device marketed by INEL (Artenay, France). This is a diffractometer with a curved detector, allowing detection in real time over an angle domain of 120. The acceleration voltage used is 40 kV and the intensity 25 mA. The Bragg equation giving the structural equidistance is: d.sub.hkl=0.89449/sin (with the use of a cobalt anticathode).

(6) This X-ray diffraction analysis confirms that there is great structural similarity between the phyllosilicate mineral particles of the talcose compositions prepared according to the invention and the particles of natural talc.

(7) In particular, the diffraction lines which respectively correspond to the planes (003) and (060) have positions which coincide perfectly with those of the reference diffraction lines for natural talc.

(8) Furthermore, analysis of the RX diffractograms of the prepared talcose compositions also allows the coherent domain to be determined for each talcose composition, that is to say the number of elementary laminae that are stacked without a major defect in the c* direction (of the reciprocal space of the crystal lattice of the synthetic mineral particles). The coherent domain can be determined for the lines (001) and depends especially on the full width at half maximum of the corresponding line and on the corresponding diffraction angle. In the case of the synthetic talc, it is determined in particular starting from the line (003).

(9) A natural talc from the LYAONING province (China) has, for example, a coherent domain of 70 laminae.

(10) The talcose compositions prepared by a process according to the invention have coherent domains which are similar to the coherent domains of natural talcs and greater than the coherent domains of synthetic talcs of the prior art for comparable temperatures and durations of hydrothermal treatment.

(11) Analysis of the RX diffractograms of the prepared talcose compositions also allows the ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) to be determined and compared with that of a natural talc. For a natural talc from the LYAONING province (China), the ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.61.

(12) 2Near-Infrared Analyses

(13) In infrared, it is known that natural talc has, in near-infrared, a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond.

(14) FIGS. 4 and 5 show near-infrared spectra, on each of which there is shown the intensity of the signal as a function of the wavelength expressed in cm.sup.1. FIG. 5 shows the near-infrared spectra of: a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 3 hours (curve 11), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 6 hours (curve 12), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 18 hours (curve 13), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 24 hours (curve 14), a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 10 days (curve 15), and a natural talc composition from the LYAONING province (China) (curve 16).

(15) FIG. 4 shows a near-infrared spectrum of a synthetic talc composition Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 prepared by a process according to the invention with precipitation of a silico-metallic hydrogel (i.e. containing silicon and metal) in the presence of sodium acetate and hydrothermal treatment of said hydrogel at a temperature of 300 C. for 18 hours (curve 13) as well as a near-infrared spectrum of that same synthetic talc composition after anhydrous heat treatment for 5 hours at 550 C. (curve 20).

(16) These spectra were acquired using a NICOLET 6700-FTIR spectrometer over a domain of 9000 cm.sup.1 to 4000 cm.sup.1.

(17) 3Microscopic Observations and Assessment of the Particle Size of the Particles

(18) In view of the considerable fineness of the powders of which the talcose compositions according to the invention can be constituted, the size and particle size distribution of the phyllosilicate mineral particles composing them were assessed by observation under a field-emission scanning electron microscope and under a transmission electron microscope.

(19) It is found that the particle size of the elementary particles varies between 20 nm and 100 nm.

(20) It has further been observed that the synthetic talc particles prepared by a process according to the invention exhibit a pearly effect which may be of interest in many industrial fields.

(21) The examples which follow illustrate the preparation process according to the invention and the structural characteristics of the compositions comprising synthetic mineral particles, and in particular of the talcose compositions comprising phyllosilicate mineral particles, so obtained.

Example 1Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(22) A first aqueous solution of sodium metasilicate comprising 42.43 g of pentahydrated sodium metasilicate Na.sub.2SiO.sub.3,5H.sub.2O and 140 ml of demineralized water is prepared, and the solution is stirred at ambient temperature (21 C.) for 10 minutes. 171 g of trihydrated sodium acetate CH.sub.3COONa.3H.sub.2O are then added to the solution, stirring of the solution being maintained for 10 minutes and the solution being maintained at a temperature of from 30 C. to 50 C. with the aid of a water bath. The concentration of sodium acetate in this first solution is 4 mol/l.

(23) A second solution of magnesium acetate comprising 32.17 g of tetrahydrated magnesium acetate Mg(CH.sub.3COO).sub.2.4H.sub.2O and 100 ml of 1M concentrated acetic acid is then prepared.

(24) Finally, when the first solution of sodium metasilicate and sodium acetate has returned to ambient temperature, the second solution is added rapidly to the first solution, with stirring and in a single batch.

(25) There is obtained a suspension of hydrogel precursor of synthetic talc particles, which is in the form of a hydrogel of milky consistency. At the end of the precipitation of the hydrogel, the concentration of sodium acetate in the hydrogel suspension is 4 mol/l, as during the subsequent hydrothermal treatment, the hydrogel obtained then being subjected directly to a hydrothermal treatment.

(26) The hydrothermal treatment of the hydrogel is carried out at a temperature of 300 C. for 3 hours at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor).

(27) To that end, the hydrogel suspension so obtained is placed directly in a closed titanium reactor. The titanium reactor is then placed in a furnace at a temperature of 300 C. for 3 hours.

(28) After cooling to ambient temperature, the reactor is opened and the suspension obtained is centrifuged. After centrifugation there are recovered on the one hand a talcose composition and on the other hand a supernatant solution comprising especially sodium acetate, which can then be recovered and optionally recycled.

(29) The talcose composition that has been recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation.

(30) The talcose composition recovered after centrifugation is finally dried by lyophilization for 72 hours.

(31) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 1). The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.77 (I=100); a plane (002) situated at a distance of 4.78 (I=33); a plane (020) situated at a distance of 4.55 (I=26); a plane (003) situated at a distance of 3.17 (I=61); a plane (060) situated at a distance of 1.52 (I=11).

(32) Such a talcose composition has a coherent domain of 12 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 1.49.

(33) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIG. 5 (curve 11). It has a vibration band at 7185 cm.sup.1 representing the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

Example 2Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(34) A hydrogel precursor of synthetic mineral particles is prepared according to the protocol described in Example 1.

(35) The hydrogel obtained is subjected directly to a hydrothermal treatment at a temperature of 300 C. for 6 hours at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor). The concentration of sodium acetate during the hydrothermal treatment is 4 mol/l.

(36) The talcose composition that is recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation and is finally dried by lyophilization for 72 hours.

(37) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 2).

(38) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.45 (I=100); a plane (002) situated at a distance of 4.72 (I=34); a plane (020) situated at a distance of 4.57 (I=20); a plane (003) situated at a distance of 3.14 (I=98); a plane (060) situated at a distance of 1.53 (I=18).

(39) Such a talcose composition has a coherent domain of 19 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 1.06.

(40) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIG. 5 (curve 12). It has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

Example 3Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(41) A hydrogel precursor of synthetic mineral particles is prepared according to the protocol described in Example 1.

(42) The hydrogel obtained is subjected directly to a hydrothermal treatment at a temperature of 300 C. for 18 hours at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor). The concentration of sodium acetate during the hydrothermal treatment is 4 mol/l.

(43) The talcose composition that is recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation and is finally dried by lyophilization for 72 hours.

(44) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 3).

(45) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.64 (I=100); a plane (002) situated at a distance of 4.74 (I=30); a plane (020) situated at a distance of 4.59 (slight shoulder); a plane (003) situated at a distance of 3.15 (I=100); a plane (060) situated at a distance of 1.52 (I=5).

(46) The intensity I of the corresponding lines that is given is normalized relative to the most intense line of the diffractogram, the intensity of the most intense line being taken as 100. It is observed that such a composition prepared by a process according to the invention has, in X-ray diffraction, lines corresponding to the planes (001) and (003) of very high intensities relative to the other lines, indicating crystallinity very similar to that of a natural talc. Furthermore, the line corresponding to the plane (002) has a higher intensity than the line corresponding to the plane (020), the diffraction line characteristic of a plane (020) being in part coincident with the diffraction line characteristic of a plane (002) and being present only in the form of a slight shoulder.

(47) Such a talcose composition has a coherent domain of 25 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.96. The near-infrared spectrum of the synthetic talc composition obtained is shown in FIGS. 4 and 5 (curve 13). It has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

Example 4Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(48) The talc obtained in Example 3 is subjected to an anhydrous heat treatment or annealing for 5 hours at 550 C.

(49) The synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained after annealing was characterized by X-ray diffraction and near-infrared analysis.

(50) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 3 (curve 10).

(51) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.51 (I=98); a plane (002) situated at a distance of 4.70 (I=29); a plane (003) situated at a distance of 3.14 (I=100); a plane (060) situated at a distance of 1.52 (I=5).

(52) The intensity I of the corresponding lines that is given is normalized relative to the most intense line of the diffractogram, the intensity of the most intense line being taken as 100.

(53) Relative to the X-ray diffractogram of the synthetic talc before annealing, an increase in the intensity of the line (002) as compared with the intensity of the line (020) is observed, the line (020) then being coincident with the diffraction line characteristic of plane (002). There is also observed an increase in the intensities of lines (001), (002) and (003), that is to say, more generally, of the intensities of the lines (001). Such annealing therefore allows the crystallinity of the synthetic talc prepared to be increased further, the structural characteristics of that talc then being even more similar to those of a natural talc.

(54) Such a talcose composition has, after annealing, a coherent domain of 28 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.98.

(55) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIG. 4 (curve 20). The near-infrared spectrum of the synthetic talc composition obtained after annealing has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The intensity of this vibration band has increased as compared with the near-infrared spectrum of the synthetic talc before annealing, which indicates an increase in the crystallinity of the synthetic talc obtained. There is also observed a decrease in the intensity of the vibration band corresponding to the presence of water bonded to the talc at lamina edges (situated at 5273 cm.sup.1) as compared with the curve 13 of the synthetic talc before heat treatment, revealing a reduction in the number of molecules of water bonded to the talc at lamina edges during the annealing.

Example 5Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(56) A hydrogel precursor of synthetic mineral particles is prepared according to the protocol described in Example 1.

(57) The hydrogel obtained is subjected directly to a hydrothermal treatment at a temperature of 300 C. for 24 hours at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor). The concentration of sodium acetate during the hydrothermal treatment is 4 mol/l.

(58) The talcose composition that is recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation and is finally dried by lyophilization for 72 hours.

(59) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 4).

(60) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.54 (I=100); a plane (002) situated at a distance of 4.73 (I=31); a plane (003) situated at a distance of 3.15 (I=96); a plane (060) situated at a distance of 1.52 (I=7).

(61) It is observed that the line corresponding to the plane (002) has a higher intensity than the line corresponding to the plane (020), the diffraction line characteristic of a plane (020) being partially coincident with the diffraction line characteristic of a plane (002) and being present only in the form of a very slight shoulder.

(62) Such a talcose composition has a coherent domain of 24 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.96.

(63) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIGS. 4 and 5 (curve 14). It has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

Example 6Preparation of a Composition Comprising Synthetic Mineral Particles According to the Invention

(64) A hydrogel precursor of synthetic mineral particles is prepared according to the protocol described in Example 1.

(65) The hydrogel obtained is subjected directly to a hydrothermal treatment at a temperature of 300 C. for 10 days at a pressure of 80 bar (8 MPa) (saturation vapor pressure of the water in the reactor). The concentration of sodium acetate during the hydrothermal treatment is 4 mol/l.

(66) The talcose composition that is recovered is then subjected to two successive cycles of washing with demineralized water and centrifugation and is finally dried by lyophilization for 72 hours.

(67) The X-ray diffractogram of the synthetic talc composition of formula Si.sub.4Mg.sub.3O.sub.10(OH).sub.2 so obtained is shown in FIG. 1 (curve 5).

(68) The X-ray diffractogram of this talcose composition has diffraction lines corresponding to the diffraction lines of talc, and in particular the following characteristic diffraction lines: a plane (001) situated at a distance of 9.51 (I=92); a plane (002) situated at a distance of 4.73 (I=26); a plane (003) situated at a distance of 3.14 (I=100); a plane (060) situated at a distance of 1.52 (I=5).

(69) It is observed that the line corresponding to the plane (002) has a higher intensity than the line corresponding to the plane (020), the diffraction line characteristic of a plane (020) being coincident with the diffraction line characteristic of a plane (002) and difficult to distinguish therefrom.

(70) Such a talcose composition has a coherent domain of 34 laminae (using line (003)). The ratio between the intensity of the diffraction line characteristic of plane (001) and the intensity of the diffraction line characteristic of plane (003) is 0.81.

(71) The near-infrared spectrum of the synthetic talc composition obtained is shown in FIG. 5 (curve 15). It has a vibration band at 7185 cm.sup.1 representative of the vibration of the Mg.sub.3OH bond of the talc. The near-infrared spectrum also has a vibration band at 5251 cm.sup.1 which is characteristic of a synthetic talc according to the invention and corresponds to the presence of water bonded to the talc at lamina edges.

(72) It is thus noted that, the more the duration of the hydrothermal treatment increases (as with an anhydrous heat treatment), the more the synthetic talc composition obtained is similar to a natural talc, in particular with regard to the values of coherent domains and the ratio between the intensity of the RX diffraction line characteristic of plane (001) and the intensity of the RX diffraction line characteristic of plane (003) (for a same hydrogel precursor and a constant hydrothermal treatment temperature).

(73) It can thus be noted that the near-infrared spectra of the various synthetic talcs according to the invention are very similar to a near-infrared spectrum of a natural talc (curve 16, FIG. 5), with the exception of the vibration band between 5000 cm.sup.1 and 5500 cm.sup.1 which, for a natural talc, does not have such a high intensity as for synthetic talcs according to the invention.

(74) The invention can be the subject of many other applications and of different variants with respect to the embodiments and examples described above. In particular, said talcose composition can comprise phyllosilicate mineral particles in which different metals are situated at octahedral sites, such that in (Si.sub.xGe.sub.1-x).sub.4M.sub.3O.sub.11, nH.sub.2O, M has the formula (Co.sub.0.5Ni.sub.0.5).