Origin and metamorphism of graphite from Formiga, Minas Gerais (Brazil)

Rezende, Luiza Carneiro de; Chaves, Alexandre de Oliveira; Ramos, Sérgio Luis Lima de Moraes

doi:10.1590/2317-4889202120200083

Luiza Carneiro de Rezende ^**Corresponding author.

Universidade Federal de Minas Gerais - Belo Horizonte (MG), Brazil. E-mails: luizacrez@gmail.com, alochaves@yahoo.com.br

https://orcid.org/0000-0002-7520-4935

Alexandre de Oliveira Chaves

Universidade Federal de Minas Gerais - Belo Horizonte (MG), Brazil. E-mails: luizacrez@gmail.com, alochaves@yahoo.com.br

https://orcid.org/0000-0002-1291-5553

Sérgio Luis Lima de Moraes Ramos

Centro de Tecnologia em Nanomateriais e Grafeno, Universidade Federal de Minas Gerais - Belo Horizonte (MG), Brazil. E-mail: slramos@ctnano.org

https://orcid.org/0000-0001-7056-0722

*Corresponding author.

L.R. and A.C. made the field work together. L.R. made the petrographic analyses, thermometric treatment of Raman and diffraction data, edited the Figures and Tables and wrote the manuscript. A.C. provided the X-ray diffraction data and improved and completed the manuscript. S.R. provided the Raman data.

Competing interests: The authors declare no competing interests.

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Figure 1.
Kerogen maturation in terms of H/C and O/C atomic ratios.

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Figure 2.
Geology of meridional sector of São Francisco Craton.

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Figure 3.
Graphite schist sampling site. (A) Front view of trench; (B) place where samples LR44A and LR44B were collected; (C) sample LR44C, collected from quartz vein with presence of todorokite and manganite; (D) quartz vein where sample LR44C was collected.

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Figure 4.
Protomicrographies of samples (A, B) LR44A and (C, D) LR44B on (A, C) plane polarized light and (B, D) crossed polarized light. Samples present granolepidoblastic texture. Graphite occurs in aggregates in sample LR44A, while in sample LR44B it occurs in layers parallel to foliation (marked by phengite). Mineral abbreviations, in accordance to Whitney and Evans (2010Whitney D.L., Evans B.W. 2010. Abbreviations for names of rock-forming minerals. American Mineralogist, 95(1):185-187. https://doi.org/10.2138/am.2010.3371
https://doi.org/https://doi.org/10.2138/... ), are: Gr: graphite; Qtz: quartz; Phg: phengite.

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Figure 5.
Diffractogram of sample LR44C showing the presence of the biogenic Mn-rich minerals.

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Figure 6.
Diffractograms of samples 44A and 44B showing graphite patterns. All peaks are from graphite.

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Figure 7.
(A) Interplanar spacing (d(002)) versus crystallite size (Lc(002)) (Tagiri and Oba 1986Tagiri M., Oba T. 1986. Hydrothermal syntheses of graphite from bituminous coal at 0.5-5 kbar water vapor pressure and 300-600°C. Journal of Mineralogy, Petrology and Economic Geology, 81(7):260-271. https://doi.org/10.2465/ganko1941.81.260
https://doi.org/https://doi.org/10.2465/... ). All samples are classified as graphite, but they aren’t fully ordered; (B) Graphitization degree versus X-ray diffraction data temperature (Wada et al. 1994Wada I.H., Tomita T., Matsuura K., Tuchi K., Ito M., Morikiyo T. 1994. Graphitization of carbonaceous matter during metamorphism with reference to carbonate and pelitic rocks of contact and regional metamorphism, Japan. Contributions to Mineralogy and Petrology, 118(3):217-228. https://doi.org/10.1007/BF00306643
https://doi.org/https://doi.org/10.1007/... ). Samples presented similar X-ray diffraction temperature and are randomly arranged in the trendline.

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Figure 8.
Photomicrographs of graphite powder (extracted from sample LR44A) taken in plane polarized light. (A) Selected fields for analyses represented with squares; (B) a random powder chosen for Raman analyses, in which each cross represents a spot of analyse.

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Figure 9.
Raman spectra of analyses made on sample 44A presenting graphite pattern. Some samples show some interference due to the contamination of phengite.

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Figure 10.
Raman spectra of analyses made on sample 44B presenting graphite pattern. Some samples show some interference due to the contamination of phengite.

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Figure 11.
(A) d(002) versus half width at half maximum of the G-band (G HWHM) (Beyssac et al. 2002Beyssac O., Goffé B., Chopin C., Rouzaud J.N. 2002. Raman spectra of carbonaceous material in metasediments: A new geothermometer. Journal of Metamorphic Geology, 20(9):859-871. https://doi.org/10.1046/j.1525-1314.2002.00408.x
https://doi.org/https://doi.org/10.1046/... , Kwiecińska and Petersen 2004Kwiecińska B., Petersen H. 2004. Graphite, semi-graphite, natural coke, and natural char classification-ICCP system. International Journal of Coal Geology, 57(2):99-116. https://doi.org/10.1016/j.coal.2003.09.003
https://doi.org/https://doi.org/10.1016/... , Rantitsch et al. 2016Rantitsch G., Lämmerer W., Fisslthaler E., Mitsche S., Kaltenböck H. 2016. On the discrimination of semi-graphite and graphite by Raman spectroscopy. International Journal of Coal Geology, 159:48-56. https://doi.org/10.1016/j.coal.2016.04.001
https://doi.org/https://doi.org/10.1016/... ). Sample LR44A is classified as upper amphibolite facies and LR44B as amphibolite facies; (B) d(002) versus area ratio (R2) (Kwiecińska and Petersen 2004Kwiecińska B., Petersen H. 2004. Graphite, semi-graphite, natural coke, and natural char classification-ICCP system. International Journal of Coal Geology, 57(2):99-116. https://doi.org/10.1016/j.coal.2003.09.003
https://doi.org/https://doi.org/10.1016/... , Rantitsch et al. 2016Rantitsch G., Lämmerer W., Fisslthaler E., Mitsche S., Kaltenböck H. 2016. On the discrimination of semi-graphite and graphite by Raman spectroscopy. International Journal of Coal Geology, 159:48-56. https://doi.org/10.1016/j.coal.2016.04.001
https://doi.org/https://doi.org/10.1016/... ). Both samples are classified as granulite facies.

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Table 1.
X-ray diffraction data from LR44A and LR44B, graphite schist samples, in which d(002) represents interplanar spacing, FWHM is the full width at half maximum of the G-band, Lc(002) is the crystal size along stacking direction, GD is the graphitization degree and T is the metamorphic temperature.

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Table 2.
Raman spectroscopy data with values obtained by the IFORS method (Lünsdorf and Lünsdorf 2016Lünsdorf N.K., Lünsdorf J.O. 2016. Evaluating Raman spectra of carbonaceous matter by automated, iterative curve-fitting. International Journal of Coal Geology, 160-161:51-62. https://doi.org/10.1016/j.coal.2016.04.008
https://doi.org/https://doi.org/10.1016/... ) from graphite schist samples LR44A and LR44B. G HWHM represents the width at half maximum of the G-band in cm-1, R2 is the area ratio and T is the metamorphic temperature in °C.

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Table 3.
Comparison between X-ray diffraction and Raman spectroscopy data from samples LR44A and LR44B of graphite schist. d(002) represents the interplanar spacing, GD is the graphitization degree, T is the metamorphic temperature in °C, G HWHM represents the width at half maximum of the G-band and R2 is the area ratio.

(1)

(2)

(3)

(4)

X-ray diffraction data
Sample	2θ	d(002) (Å)	sd ±	FWHM (2θ)	Lc(002) (Å)	sd ±	GD	T(°C)	sd ±
LR44A1	26.533	3.357	0.005	0.347	236	25	55	455	23
LR44A2	26.411	3.372	0.01	0.402	203	21	47	432	22
LR44A3	26.666	3.340	0.005	0.441	185	19	49	437	22
LR44A4	26.446	3.368	0.01	0.366	223	23	51	443	22
LR44B1	26.556	3.354	0.005	0.330	247	26	57	462	23
LR44B2	26.494	3.362	0.01	0.392	208	22	50	439	22
LR44B3	26.578	3.351	0.005	0.467	175	18	46	427	21
LR44B4	26.413	3.374	0.01	0.297	275	29	58	466	23

Raman spectroscopy data
Sample	G HWHM	sd ±	Center	sd ±	R2	sd ±	T °C	sd ±
LR44Aa1p1	7.94	0.42	1,581.58	0.00	0.000	0.000	521	64
LR44Aa1p2	7.87	0.00	1,581.58	0.00	0.067	0.000	496	63
LR44Aa1p3	7.78	0.00	1,581.58	0.00	0.070	0.001	410	61
LR44Aa1p4	8.33	0.00	1,581.58	0.00	0.120	0.000	492	63
LR44Aa1p5	8.91	0.00	1,581.58	0.00	0.066	0.001	465	62
LR44Aa2p3	8.06	0.48	1,581.58	0.00	0.000	0.000	488	63
LR44Aa3p1	7.90	0.00	1,581.58	0.00	0.047	0.001	499	63
LR44Aa3p2	8.08	0.00	1,584.06	0.00	0.140	0.161	497	63
LR44Aa3p3	7.67	0.00	1,581.58	0.00	0.000	0.000	494	63
LR44Aa4p1	7.71	0.00	1,581.58	0.00	0.000	0.000	476	63
LR44Aa4p3	7.78	0.00	1,579.09	0.00	0.127	0.000	479	63
LR44Aa5p1	7.79	0.01	1,581.58	0.00	0.119	0.000	501	63
LR44Aa5p2	7.85	0.00	1,581.58	0.00	0.057	0.000	499	63
LR44Aa5p3	8.00	0.00	1,574.12	0.00	0.118	0.001	487	63
LR44Ba1p1	8.89	0.00	1,581.58	0.00	0.000	0.000	502	63
LR44Ba1p2	8.90	0.00	1,583.28	0.00	0.216	0.000	492	63
LR44Ba1p3	8.78	0.00	1,580.79	0.00	0.050	0.000	498	63
LR44Ba1p4	9.58	0.01	1,582.45	1.44	0.044	0.001	491	63
LR44Ba2p1	8.12	0.00	1,580.79	0.00	0.056	0.000	502	63
LR44Ba2p2	8.17	0.01	1,575.81	0.00	0.060	0.000	493	63
LR44Ba2p3	8.46	0.00	1,583.28	0.00	0.035	0.000	505	64
LR44Ba2p4	8.94	0.02	1,578.30	0.00	0.081	0.000	481	63
LR44Ba3p3	8.01	0.00	1,580.79	0.00	0.000	0.000	500	63
LR44Ba4p1	9.09	0.01	1,583.28	0.00	0.144	0.002	496	63
LR44Ba4p2	9.82	0.00	1,583.28	0.00	0.063	0.000	492	63
LR44Ba4p3	8.51	0.00	1,588.78	0.00	0.088	0.000	499	63
LR44Ba5p1	8.36	0.00	1,588.78	0.00	0.039	0.000	491	63
LR44Ba5p2	9.32	0.00	1,588.78	0.00	0.052	0.000	491	63

Sample	XRD							Raman
Sample	n	d(002)	std	GD	std	T °C	std	n	G HWHM	std	R2	std	T °C	std
LR44A	4	3.359	0.014	50.5	3.1	442	10	14	7.976	0.066	0.067	0.012	486	63
LR44B	4	3.360	0.010	52.7	5.8	449	19	14	8.782	0.004	0.066	0.000	463	64

[1] *Corresponding author.

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Origin and metamorphism of graphite from Formiga, Minas Gerais (Brazil)

Abstract