Good to Know Series – 07 – Natural Graphite – Part 3: Microstructural Evolution from Anthracite to Graphite

Today, we’re exploring the microstructural evolution of carbonaceous materials from anthracite to graphite using optical microscopy, Raman spectroscopy, and X-ray diffraction (XRD). The figure below illustrates how these materials transform with increasing structural order.

Summary of structural evolution from anthracite to graphite, as observed by optical microscopy, Raman spectroscopy, and X-ray diffraction. V - Vitrinite; I - Inertinite; F - fibers; DP - dense particles; Qtz - quartz; Ap - apatite.

Optical Microscopy

Let’s begin with the view through the microscope. For graphite derived from organic matter (i.e. metamorphic graphite—see the previous episode), we can observe several petrographic changes as the material evolves from anthracite to semi-graphite to graphite:

1. Increase in maximum reflectance (RMax%) and a decrease in minimum reflectance (RMin%).

2. Increase in bireflectance (optical anisotropy), which is the difference between RMax% and RMin%.

3. At the meta-anthracite stage, vitrinite reflectance exceeds that of inertinite; in the semi-graphite stage, inertinite becomes difficult to distinguish from vitrinite, forming what we refer to as dense particles.

4. Graphitic structures such as fibers and microcrystals become visible at the semi-graphite stage.

5. At the graphite stage, basal and prismatic sections are clearly recognizable. The basal surface shows the maximum reflectance and appears optically isotropic, while the prismatic surface shows the minimum reflectance and exhibits strong anisotropy.

Raman Spectroscopy

Raman spectroscopy is a powerful technique for characterising carbon-based materials. The spectra shown here were acquired from individual particles on polished surfaces (as opposed to bulk powders), since crystallinity and orientation vary at the particle level—information that also aligns with optical microscopy observations. Let’s summarise how the Raman features evolve:

(Nomenclature follows Henry et al., 2019)

1. At the anthracite and meta-anthracite stages, disorder bands (D1 to D6) are present. These diminish with increasing structural order.

2. In graphite, the D1 and D2 bands may still appear—not necessarily due to bulk disorder but rather due to structural discontinuities like edges, defects, or finite crystallite size. Additionally, laser polarisation and orientation effects can activate these bands. On prismatic surfaces, where edge planes are exposed, symmetry is broken and selection rules permit D1 scattering—even in otherwise well-ordered graphite. The D2 band (~1620 cm⁻¹) often appears as a shoulder on the G band and is associated with stacking disorder, interlayer stress, or surface effects.

3. The G band, characteristic of graphitic sp² carbon, is present in all samples and becomes sharper and more intense as structural order increases. On basal graphite surfaces, it may appear as the only band.

4. In semi-graphitic samples, the D1 band may appear more intense than the G band. This does not necessarily indicate disorder; rather, it reflects finite crystallite size, edge exposure, and residual stacking imperfections. A 532 nm laser enhances resonance with small sp² domains, amplifying the D1 intensity relative to the G band—typical of materials transitioning from disordered to ordered carbon.

5. The second-order Raman spectrum begins developing in anthracite, where three broad bands appear. At the semi-graphite stage, the ~2700 cm⁻¹ S2 band becomes sharp, symmetric, and dominant.

6. In graphite, S2 splits into two components—G’₁ and G’₂—a feature often taken as evidence of 3D order. However, this splitting only occurs on basal surfaces. On prismatic surfaces, where phonon pathways are disrupted by symmetry breaking, the S2 band remains a sharp single peak.

X-ray Diffraction

X-ray diffraction (XRD) is the most definitive method to assess crystallinity in carbon materials. In graphite, the interlayer spacing d₀₀₂ is 0.3354 nm; in more disordered, turbostratic materials, it is typically ≥0.344 nm. Here's how the XRD reflections evolve:

1. The (002) reflection, representing interlayer stacking along the c-axis, is dominant in all samples. In anthracite and meta-anthracite, it appears as a broad band, while in semi-graphite and graphite, it sharpens significantly.

2. The (004) reflection is diffuse in meta-anthracite, weak but visible in semi-graphite, and sharp in graphite. The (006) peak appears only in semi-graphite and graphite.

3. In well-ordered graphite, (002), (004), and (006) are strong and sharp, indicating well-aligned graphene layers.

4. In anthracite and meta-anthracite, 2D (hk) bands such as (10) and (11) appear, especially in meta-anthracite. These suggest that graphite-like planes exist in parallel stacks, though without long-range mutual orientation.

5. In semi-graphite, (hkl) reflections where h and/or k ≠ 0 (e.g., 110) begin to appear. These correspond to in-plane periodicity (a–b plane) and mark the onset of 3D crystallinity. Reflections with l ≠ 0 (e.g., 112), which indicate planes inclined relative to the layers, also begin to emerge—signalling structural coherence both within and across planes.

6. In fully graphitised material, both (00l) and (hkl) reflections are well-developed. Occasional rhombohedral reflections may be present as well, typically caused by shear deformation or mechanical processing during sample preparation.

This blog only scratches the surface of natural graphitisation. If you’re keen to explore further, I recommend the following resources. These are just a few among many valuable studies on the topic.

For Optical Microscopy:

• Kwiecińska, B., Murchison, D.G., Scott, E. (1977). Optical properties of graphite. Journal of Microscopy, 109, 289–302.

• Rodrigues, S., Marques, M., Suárez-Ruiz, I., Cameán, I., Flores, D., Kwiecińska, B. (2013). Microstructural investigations of natural and synthetic graphites and semi-graphites. International Journal of Coal Geology, 111, 67–79.

For Raman Spectroscopy:

• Cançado, L.G., Reina, A., Kong, J., Dresselhaus, M.S., 2008. Geometrical approach for the study of G′ band in the Raman spectrum of monolayer graphene, bilayer graphene, and bulk graphite. Physical Review B 77 245408-1-9.8.

• Henry, D.G., Jarvis, I., Gillmore, G., Stephenson, M., 2019. Raman spectroscopy as a tool to determine the thermal maturity of organic matter: Application to sedimentary, metamorphic and structural geology. Earth-Science Reviews 198, 102936.

• Katagiri, G., Ishida, H., Ishitani, A., 1988. Raman spectra of graphite edge planes. Carbon 26, 565–571.

• Sato, K., Saito, R., Oyama, Y., Jiang, J., Cançado, L.G., Pimenta, M.A., Jorio, A., Samsonidze, G.G., Dresselhaus, G., Dresselhaus, M.S., 2006. D-band Raman intensity, of graphitic materials as a function of laser energy and crystallite size. Chemical Physics Letters 427, 117–121.

• Tuinstra, F., Koenig, J.L. (1970). Raman spectrum of graphite. The Journal of Chemical Physics, 53, 1126–1130.

• Vidano, R.P., Fischbach, D.B., Willis, L.J., Loehr, T.M., 1981. Observation of Raman band intensity shifting with excitation wavelength for carbons and graphites. Solid State Communications 39, 341–344.

For X-ray Diffraction:

• Biscoe, J., Warren, B. (1942). An X-ray study of carbon blacks. Journal of Applied Physics, 13, 364–371.

• Franklin, R.E. (1951). The structure of graphitic carbon. Acta Crystallographica, 4, 253–261.