*3.1. Mineralogy*

The results presented here are for a particular area of interest that is presented and displayed in Figure 2a (backscattered electron micrograph) and Figure 2b with the false-colored mineral map from Mineralogic. The analysis was carried out of a 360,000-μm<sup>2</sup> region where a quantified EDX analysis was taken at every 200 nm, and therefore results in approximately 1.8 million quantified analyses.

Figure 2a displays the BSE data acquired from the region of interest and provides some initially information on the mineralogy based on the average atomic number of the mineral. However, the BSE gray-level intensity is not suitable to quantify mineral abundance and texture in these samples. As Figure 2a,b outlines, minerals such as illite and epidote appear to have almost identical gray-level signatures, as do hornblende, quartz, and anorthite.

However, Figure 2b displays the overlaid false-colored mineral data that was acquired at a 200-nm spatial resolution. The false-color minerals clearly display the mineralogy and the textural variation across this sample.

Combined with the modal mineralogy data (Table 1), these results clearly show how anorthite (c. 39%), illite (c. 20%), hornblende (c. 20%), and quartz (c. 13%) are the dominant mineral in this sample. It is also clear from Figure 2b and Table 1 that all the minerals show predominantly fine-grained sizes (200 nm); however, rare large clasts of quartz and anorthite are observed while hornblende, illite, and epidote mainly consist of a fine-grained matrix.

The 200-nm resolution of the scan was able to identify two distinct mineralogical and textural groups in this region (Figure 5). Textural group 1 is formed by the former amphibolite minerals and their reaction products. It can be found in the center of Figure 2, and is dominated by an epidote- and hornblende-dominated matrix that is populated with large (up to 80 microns in length) remnants of the orthogneiss, mainly consisting of single grains of quartz and anorthite. A second textural group can be found that encapsulates textural group 1 and is dominated by an illite and fine-grained anorthite matrix, which represents the remnants of the orthogneiss. In this textural group, hornblende is a minor phase in the matrix. Figure 5c,d shows that this mineralogical and textural group is dominated by anorthite, illite, and quartz with a small amount of epidote intergrown within this matrix.

#### *3.2. Grain Size Distributions*

Whilst the hornblende is relatively granular within this matrix, the epidote is often found as relatively coarse grains up to 20 microns in length (Figure 6). The anorthite grains display bimodal distribution as they are found in the matrix as grains of 1–5 microns in size, and also as much coarser remnant clasts that can be 50 microns in length or more.

The average grain size in textural group 2 is clearly much larger than that in textural group 1, and the matrix is dominated by illite and anorthite with only minor amounts of epidote and hornblende. Quartz grains are more common and of a larger grain size than those within textural group 1 (Table 2).


**Table 1.** Modal mineralogy data from the selected region of interest displayed in the graphical abstract. This data displays the mineral, its area % occurrence in the sample, the average grain size of that mineral with its standard deviation, and its average composition in wt.%.

**Figure 5.** Two details of Figure 2: Combined false-colored mineral map (**<sup>a</sup>**,**<sup>c</sup>**) and BSE images (**b**,**d**) for textural group 1 (**<sup>a</sup>**,**b**) and textural group 2 (**<sup>c</sup>**,**d**). The width of view for each field is 46 microns.


**Table 2.** Modal mineralogy and grain size distribution data for textural groups 1 (TG1) and 2 (TG2) and comparative modal mineralogy and grain size distribution

*Minerals* **2019**, *9*, 665

**Figure 6.** Grain size distribution for anorthite, hornblende, quartz, illite, and epidote in the analyzed fault gouge for textural groups 1 (TG1) and 2 (TG2). The distribution is shown as grain area in μm<sup>2</sup> and was measured as a 2D grain size distribution. Most grains are in the smallest bin that corresponds to grain diameters down to 200 nm.

The grain size distribution per mineral can be extracted from the Mineralogic output. Figure 6 presents the grain size distribution for one of the originally present grains, quartz, and for one of the reaction products, illite, which formed after the breakdown of feldspar minerals. Both minerals show a considerable decrease in grain size, with most grains in the bin down to 200 nm.
