*3.1. Single Crystals*

The combination of a{100} and o{111} forms making cubo-octahedron crystals is the most common combination form of pyrite crystals (Figures 5 and 6a,b). The dominant forms show a trend of transformation between a{100} and o{111} forms (Figure 5). Such crystals are sometimes elongated cubes because one pair of faces developed to a greater extent (e.g., Figure 5c). The size of single crystals is in a range from ~200 nm (Figure 5h) to >40 μm (Figure 5d). Some crystals have smooth surfaces (e.g., Figure 5a), some are covered by clay minerals (e.g., Figure 5d), and others show etched structures on the surface, especially on faces of o{111} (e.g., Figure 5c). Sometimes incomplete transformations between two forms result in face going missing during crystal development (Figure 6a,b).

**Figure 5.** SEM images of single pyrite with combinations of a{100} and o{111} faces and their corresponding crystal shapes (color drawings on the right), indicating preferential orientation growth in the (100) and (111) directions from (**a**–**h**). (**a**) Pyrite crystals have smooth surfaces. (**b**,**d**,**g**) Pyrite crystals covered by clay minerals. (**c**,**e**) Etching pits on the pyrite crystal surface, especially on faces of o{111}.

**Figure 6.** SEM images of single pyrite with combinations of a{100}, o{111}, e{210}, and f{310} faces and their corresponding crystal shapes (color drawings on the right) which show incomplete transformations between two forms result in face going missing during crystal development (**a**–**f**).

Combination forms of a{100}, o{111}, e{210}, and f{310} were also observed (Figure 6c–f). The octahedral form is dominant in these crystal habits and shows combinations with the e{210} form (Figure 6c) or with both a{100} and e{210} forms (Figure 6e). The pyritohedral form is dominant in only a few crystals (Figure 6d). A combination of a{100}, o{111}, and f{310} forms in one crystal of pyrite was observed, though the edges were ambiguous owing to its ultrafine size (300 nm, Figure 6f).
