Concretions in central Wales are so common that it is almost embarrassing to include them in my rock collection, however I picked this one up because it illustrates beautifully the distinctive surface of a cone-in -cone concretion. It is only part of a concretion, and the curvature at the edge suggests that it was an oblong shape with the length being about twice the width.. This one was collected on the Waen, Trefeglwys, Powys.
Very often referred to as "volcanic glass" Obsidian is neither a rock or a mineral. It is classed as a mineraloid because it is not crystalline. Odsidian is formed in rhyolitic eruptions, and cools so quickly that crystals do not have time to form. Its composition is generally 70% of SiO2 The three specimens are from Mexico, The larger one is colloquially known as Snowflake obsidian. The snowflake appearance is due to inclusions of spherulites in the form of clusters of radiating crystals of cristobalite. The brown specimen is coloured by trace amounts of iron.
These specimens were collected at a road widening excavation near Enville, Which is a small village between Bridgenorth and Stourbridge. The geology at the point of collection is the Enville member, sandstone, in the Salop formation. The smaller nodule appears to be very hard silica with a flaky appearance. The slab shaped nodule is composed of small irregular grains of sand. The larger spherical nodule or concretion is composed of very fine grains of sand, and on examination under magnification has numerous minute blue coloured inclusions, which I think are silica based, and are shown in the inset picture. There is also a distinct reddish ring at it's centre, which could have been the "seed" for the growth.
This specimen of breccia was collected about 15 years ago from the Hafren forest car park area, near Llanidloes, Mid Wales. The illustration is low tech, having been produced on my photocopier, before digital cameras. I keep a photographic record of all my rocks and minerals, both printed, and on my computer. The breccia is Silurian in age, and is poorly sorted, with clasts of different sizes. The clasts are mostly angular and of different rock types, making it a polymitic specimen. The specimen is clast supported. i.e. the clasts are in close contact with each other, with the matrix filling the voids. All these features suggest that the clasts did not travel far, perhaps debris at the base of a cliff , or at the base of a steep slope.
This was collected a few years ago, when specimens were plentiful at the foot of the exposure. The specimen is the product of contact metamorphism, which creates high temperature and low pressure conditions. The dark red and black colour of the rock is due to it's constituent minerals, which are biotite, amphiboles, and some pyrite, but with no quartz or feldspar ( plagioclase).