tuff - a very pure lithified white volcanic ash - Unit of 5 Student Specimens
This tuff was mined for years as the scouring agent in Old Dutch Cleanser. It is a pure ash with no included clasts, is not very dense, and will rub off on students' fingers. It looks just like a block of solidified scouring powder. It is similar to diatomaceous earth in appearance, though more dense than diatomite.
Many tuffs contain angular clasts (from the Greek klastos = broken) - fragments of other rocks welded in, but this one doesn't. As an ash is being blasted out of the vent, fragments of the surrounding rocks are being torn loose and blasted out with it. Angular clasts make a tuff easy to separate from a rhyolite, a lava with the same composition as tuff or granite. The rhyolite is entirely crystalline, though the crystals are often difficult to see without a 10x lens.
Students should see a variety of tuffs. Tuffs are extremely variable, though most are relatively light in color and light in weight. They should be able to tell you where the included angular clasts came from, and should use the presence of clasts to separate a tuff from a rhyolite. It might be a surprise to discover that Ajax and other scouring powders are mined, not manufactured.
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