Abstract

Chlorites are petrogenetically important minerals, exercise controls on petroleum reservoir qualities, are common in alteration zones during hydrothermal ore mineralization, and may form during carbon sequestration in sedimentary formations. Chlorite thermochemistry and structure have been investigated, in the present study, to facilitate an improved understanding of chlorite equilibria.

Three natural IIb chlorites were studied by powder diffraction and calorimetric methods (low-temperature relaxation calorimetry using a Physical Properties Measurement System [PPMS] and differential scanning calorimetry [DSC]). The samples include a low-Fe clinochlore [Mg-Chl] and two Fe-rich chlorites from Windsor [Fe-Chl(W)] and Michigan [Fe-Chl(M)]. The structure of each chlorite was refined in the ideal C2/m symmetry using Rietveld techniques. Lattice parameters for the Windsor chlorite are a = 5.3786(6) Å, b = 9.3176(9) Å, c = 14.2187(6) Å, β = 96.98(10)°. The Michigan chlorite returned a = 5.3897(3) Å, b = 9.3300(3) Å, c = 14.2376(2) Å, β = 97.043(5)° whereas the low-Fe clinochlore yielded a = 5.3301(3) Å, b = 9.2231(8) Å, c = 14.2912(4) Å, β = 97.03(10)°.

Heat capacities (Cp) for the three natural chlorites were measured using both PPMS (2–303 K) and DSC (282–564 K). Employing a combination of Debye-Einstein-Schottky functions, the lattice dynamics component of the Cp at lower temperature was evaluated allowing a separation of the magnetic spin ordering component of Cp from the lattice vibrational part. For Mg-Chl, Fe-Chl(W), and Fe-Chl(M), the polynomials defining the temperature dependencies of the heat capacities between 280 and 570 K are:  
Cp=1185.44(±68.93)9753.21(±2186.85)T0.51.9094(±1.0288)·107T2+3.3013(±1.5363)·109T3
 
Cp=1006.06(±48.46)4134.83(±1515.16)T0.540.0949(±6.9413)·106T2+5.9386(±1.0287)·109T3
and  
Cp=1268.60(±67.16)11983.09(±2107.07)T0.57.6037(±9.6417)·106T2+1.5398(±1.4187)·109T3,

respectively.

Standard state molar thermodynamic functions, CP, ST, (HTH0)/T, and Φ were evaluated for the samples. S298.15 for Fe-Chl(W), Mg-Chl, and Fe-Chl(M) were found to be 499.14 ± 3.40, 437.81 ± 3.00 and 515.06 ± 3.60 J mol−1K−1, respectively, whereas S° for Fe-Chl(W) and Mg-Chl were determined to be 578.24 ± 3.76 and 503.21 ± 3.60 J mol−1K−1, respectively; these values emphasize that configurational entropy effects are consequential for chlorite thermochemistry.

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