*3.2. Pyrite Crystal Aggregates*

Except those crystals that appeared clearly as single crystals in the sediments, most of them are in the form aggregates. Five types of pyrite crystal aggregates were observed in the spring sediments:


shown in Figure 8b. Octahedral nanocrystals prefer to attach to o{111} faces while cubic ones prefer a{100} faces (Figure 8e–g). Some of them grow in a certain direction (Figure 8g).

V Irregular pyrite nanocrystals aggregate as spherulites (Figure 8e,f,h). Some of the small single aggregates (~500 nm) attach to the surfaces of large pyrite crystals (white arrows in Figure 8e,f). Some large aggregates (1–5 μm) attaching to other mineral surfaces are covered by thin films containing organic carbon and sulfur, as measured by EDS (Figure 8h).

**Figure 7.** SEM images of pyrite aggregates. (**b**,**d**,**f**) are amplifications of the highlighted areas in images (**a**,**c**,**e**), respectively.

**Figure 8.** SEM images of pyrite aggregates. (**b**) showed octahedral pyrite crystals highlighted in (**a**). (**d**) An aggregate of octahedral pyrite crystals on the surface of one bigger crystal in (**c**). (**f**,**h**) Irregular pyrite nanocrystals aggregate as spherulites. (**g**) The linear arrangement of pyrite nanocrystals on the surface of a big pyrite crystal (**e**).

*3.3. Intergrowth Texture*

Crystal Intergrowths Appear in Four Types:

I Intergrowth of single crystals. The cubical pyrite intergrowth texture was very common in the hot spring sediments. Cubical pyrite crystals with a size range of 5 to 10 μm show intergrowth with each other, which are sometimes coated by clay minerals (Figure 9a). The octahedral crystals ranging from 300 nm to 1 μm were observed to have intergrowth with each other (Figure 9b) and were covered by thin biofilms, as indicated by EDS measurements.


**Figure 9.** SEM images of different types of intergrowths of pyrite crystals (**a**–**f**). The inset at the bottom-left of (**b**) was the amplification of the highlighted area. (**c**) Pyrite crystals intergrowth. (**e**) Pyrite twin crystals. (**f**) Pyrite crystal parallel growth.

The single forms commonly observed that make pyrite polyhedrons include: cube a{100}, pyritohedron e{210}, and octahedron o{111} [74]. There are other single forms, such as pyritohedron f{310}, {210}, {211}, {321}, and a small quantity of {221}. Combination forms of pyrite crystals were found to be common in these hot spring sediments. As reported in previous studies, cubical pyrite crystals are the most common, while octahedrons are rare among all single forms [75], yet the octahedron form is not rare in the hot spring sediments studied. Normally, the habit of a crystal is confined by the crystallographic structure and defects, and its crystallization environments (mainly temperature and the degree of supersaturation, e.g., [16]). In terrestrial hot springs, the fluctuation of temperature in microenvironments is uncommon, and therefore it has negligible impacts on crystal habits [14].
