The coticule is a finely grained metasedimentary rock of yellow color that has undergone a metamorphism that caused its schistosity. It is mainly composed of quartz, spessartine (manganese garnet) and micas. It appears as interstratified layers in hematite-rich purple slates of the lower Ordovician (= about 477 Ma).  

My photo with an optical microscope is a first, at least in the image processing that highlights the three essential components (size 5 to 20 microns or 0.005 mm to 0.020 mm) embedded in a microcrystalline matrix of micas and quartz, appearing in black on the photo (polarized light).

Generally the photos are not contrasted. As a result, the photos are then taken under the electron microscope.

The color of the matrix is of course yellowish. But I was able to blacken it because it is microcrystalline.  

The presence of manganese garnets, that is to say spessartine, is inherent to the term "coticule". If the presence of spessartine decreases or is absent, whereas quartz and iron oxides, including hematite, become more important and the composition of the rock slides towards that of the pseudocoticula. This gradual transition favors a variable sedimentary origin in cycles. It is random. The variability of the various types of "good" coticules (Vintage!) also supports this thesis. These compositional variations change the color of the rock. Thus, the first degradation results from an increase of quartz at the expense of spessartine. The color then takes on greenish hues and loses its abrasive properties.  

The schistosity printed the texture of the coticule, thus creating alignments of the composites. The phenomenon is clearly visible on chlorites.

The small garnets (cubic system) are colorless in the photo and also align in strings. Garnet crystals are characterized by a strong relief (additional analytical criterion). This means that the edges of the crystal are well highlighted, in relief.

The garnet crystals are most often dodecahedra, trapezohedra, etc., all of globular appearance.

Chlorites are phyllosilicates, laminated texture.  

Chloritoids also have a strong relief. Accompanied by garnets, they are witnesses to the weak metamorphism that has undergone the coticule.

The matrix appears in black under these conditions of taking the photo.

This contrast highlights the texture of the rock and its components.  

Interpretation of the photo:  


Legend of notes: Sps = spessartine, the manganese garnet.

Chl = Chlorite.Ctd = chloritoid.


The coticule is one of the most successful whetstones when you want to get a great finish. We have obviously imagined ersatz with garnets of the same size incorporated into an artificial matrix, but they have never worked. But the unique qualities of the coticule is that it is not subject to jams, waste eliminating spontaneously. This is due to the texture of the stone. Garnets and other silicates such as chlorites are included in a micaceous matrix of muscovite. In addition, the general texture derived from the schistosity has created strings of crystals aligned with spessartine. The debris is eliminated in minute parallel areas of weaker cohesion (hardness ~ 3.5). So coticule rock, soft to the touch, wears easily while its active components (crystals of spessartine) have a hardness of 7 on the scale of Mohs

Roger Warin.