Muscovite - Animal? Vegetable? Mineral?

When I was a kid, like a lot of others, I collected rocks, mostly ones with fossils of sea shells. Half a century later I am looking at rocks again, driven by an interest in the geography and geology of the places where I hike. In an earlier post I documented my quest for garnet schist. Then in September, while hiking around the Dorsey tunnel on an eroded section of the trail, I came upon a few small rocks with a strange feature. They had what looked and felt like sheets of transparent plastic in them. I guessed they must be some kind of mica, but I had never seen anything like it before. Then last week my brother Chris, knowing of my renewed interest in rocks, handed me a small book, Rocks and Minerals, published in 1957 by Golden Press. There on page 97 was a perfectly drawn illustration almost identical to one of the rocks I had picked up. It was Muscovite, a type of mica as I had suspected. Here is a photo of my rocks, with a dime provided for scale and a knife blade to demonstrate the plastic sheet-like quality of the mineral.