It's interesting to see how the terms "rhyolite with fabric" and "Jovian fabric" have been used in the marketing of the "monolith quarry" at Rhosyfelin. Over and again the terms are used (mostly by archaeologists) with a pretence that they are scientific or diagnostic terms that are crucial in the arguments about the precise provenancing of Stonehenge rhyolite fragments to Rhosyfelin and to nowhere else. These extracts are typical:
More than 1200 chippings from the 2008 Stonehenge Riverside Project (Parker Pearson 2012) and the SPACES project (Darvill & Wainwright 2009), and from excavations at Stonehenge in 1980 (Pitts 1982), have been characterised as ‘rhyolite with fabric’ (Ixer & Bevins 2010; Bevins et al. 2011). Most of these have been found in the centre of Stonehenge, but they also occur in its environs, almost as extensively as the spotted dolerite chippings. Six ‘rhyolite with fabric’ chippings were recovered from Aubrey Hole 7 in 2008. Of the 27 from the Stonehenge Avenue, one was found in a layer beneath the Avenue’s banks, and was thus deposited before 2480–2280 cal BC (see Darvill et al. 2012), indicating that ‘rhyolite with fabric’ was present at Stonehenge before the Early Bronze Age. The fabric of this particular type of rhyolite is macroscopically typically planar, with a prominent foliation developed on the millimetre scale. In thin section, the foliation is seen to be slightly lensoidal, and contains flattened, ovoid lithic clasts (2–5cm) of microtonalite. Locally, the fabric is extremely well developed and described as ‘Jovian’ because it resembles the swirling weather patterns on Jupiter (Ixer & Bevins 2011). The main rock is commonly traversed by thin quartz veins that are tightly folded, with their folds being axial planar to the foliation, suggesting that the rock fabric is most probably not a primary texture but a later (tectonic) flattening fabric. A strong petrographic match for these rhyolite fragments has been found with outcrops in the Pont Saeson area just north of Mynydd Preseli, specifically the outcrop of Craig Rhos-y-felin, belonging to the Fishguard Volcanic Group, of Ordovician age (Ixer & Bevins 2010; Bevins et al. 2011). This match is closest for samples from the north end of the outcrop’s near-vertical western edge ............
From Parker Pearson et al, 2022:
It is likely that just one bluestone pillar was extracted from the outcrop at Craig Rhos-y-felin (Fig. 4). This striking outcrop is situated in the bottom of a steep-sided valley along which flows a stream that rises close to Carn Goedog and forms a tributary of the River Nevern (Parker Pearson et al., 2015; 2019). Initial geological sampling at 19 locations around this outcrop through the stratigraphy of the rhyolitic body revealed just one location, close to the northern tip of the outcrop, where the ‘Jovian’ micro-structure of spherical to lensoidal features (named after the famous ‘spots’ in Jupiter’s atmosphere) within the rhyolite matches the fabric identified within some of the flakes from Stonehenge and its environs (Ixer and Bevins, 2011).
The published thin sections from the Stonehenge samples and those collected from Rhosyfelin and the Pont Saeson area have produced no perfect matches. Ixer and Bevins have assured us in various publications that "rhyolite with fabric" (or "Jovian fabric") as seen in the Pont Saeson area does not occur anywhere else in North Pembrokeshire, in any of the other abundant rhyolite outcrops. They tell us that, but they have never actually demonstrated it through the presentation of evidence in an accessible journal. In several papers they discuss the geochemistry and petrography of some of the rhyolite outcrops of North Pembrokeshire, and find that they do not match any of the fragments collected from the Stonehenge debitage and referred to as Rhyolite Group C.
Williams-Thorpe and Thorpe (1991) demonstrated many years ago that on geochemical grounds the rhyolite debris at Stonehenge could be identified as having come from multiple North Pembrokeshire sources. They favoured Carn Alw as the source of four monoliths and some Stonehenge fragments, but this was later disputed by Bevins and Ixer.
If one reads the key research articles by Bevins and Ixer (2010 and 2011), one sees that neither the "rhyolite with fabric" nor "Jovian" labels are of very great importance. The labels are probably used in order to appeal to non-geologists. In fairness to them, they explain that detailed petrology and geochemistry are far more important in differentiating Group C rhyolites from the groups which used to be called A, B, D, E and F. But they don't make it easy for the rest of us. Now Group A samples are called andesites and samples from groups B, D and G are referred to as dacites. Confused? You are not alone. We must not forget that no known monolith at Stonehenge provides a match for any of the samples taken from the Pont Saeson area. The only reason for the emphasis on Rhosyfelin is that MPP, Ixer, Bevins et al think they they have found a quarry there. Sources still have not been found for all the other rhyolite fragments and rhyolite monoliths at Stonehenge. They may have come from North Pembrokeshire, and maybe not.
Also found on the web -- highly deformed mylonite from the western Alps:
Ixer and Bevins may be right in their claim that certain fragments at Stonehenge have come from the Pont Saeson area -- and that would be fine, as far as I am concerned. But the find of this interesting boulder down on The Parrog reminds us that there are other rhyolites in North Pembrokeshire that are foliated and light blue in colour, with a wide range of internal characteristics, and I would like to see some evidence that these outcrops have NOT contributed any of the rhyolitic debitage that has ended up at Stonehenge.