This photograph looks rather like a scree-slope on a Lakeland fell but in fact each of these slate-like fragments is less than a millimetre across, smaller than the commas on this page. It’s a piece of roofing felt taken at 60x magnification through my microscope. The felt is bitumen-coated with a ‘green mineral’ finish, but it looks browner in my photograph. The flaky shapes and the colour make me guess that this is a green variety of Muscovite mica called fuchsite. The flakes (laminæ) of Muscovite are thin and surprisingly flexible, so they’re ideal as a coating on rolls of roofing felt.
There’s another mineral present; the rounded, glassy mineral near the bottom left-hand corner is a worn grain of sand, silica.
Fuchsite is rich in chromium but like other micas, as a form of silicate, it has a chemical composition based on aluminium, silica and oxygen (AlSi3O10). Micas are part of the group of minerals known as Phyllosilicates or sheet silicates, which take their name from phyllon, the Greek for leaf.
Muscovite is 2-4 on the Mohs scale of hardness, depending whether you’re testing the softer ‘sides’ or the harder face of the flakes. This means that it’s somewhere between a finger-nail and a pocket-knife in hardness, so the sparrows might swallow it as a form of ‘gritting’. The flakes might be used in the bird’s gizzard to help grind down the seeds and grain that form its staple diet. Fuchsite is made of clayey minerals so it might also have medicinal properties that help with digestion, just as we’d take kaolin, a fine white clay that is another form of sheet sillicate.
When Paul and I put the new roofing felt I predicted that the sparrows would love it: “It’s like putting a new sheet of Tydsan in a budgie’s cage!”
Tydsan is the trade name of sheets of sandpaper, cut to size.
“Our budgies had to make do with newspaper in their cages!” Paul tells me.