Subject: Beach Sand

I (teach) 3rd grade; part of the 3rd grade curriculum deals with geology. I read somewhere that the sands of the beaches of the earth are made always (mostly) of quartz. Is this true and if so why?

    Yes, it is true that most of the world's beaches are made of quartz-rich sand. This true for most of the world's continents because the sands are derived from the weathering and erosion of the land masses and their mountains. The land masses and mountains are composed of rocks that are in turn themselves composed of many common minerals, such as quartz, feldspar, pyroxenes amphiboles and mica, for example. During the weathering process ground waters and dissolved carbon dioxide react with the rocks to break them down. Quartz has the particular ability to survive because it very stable (unreactive to weathering processes) and because it very hard (relative hardness is measured from one to ten on the "Mohs Scale", with Talc = 1, Gypsum = 2 (as hard as a finger nail), Calcite = 3, Fluorite = 4, Apatite = 5 (as hard as a knife blade), Feldspar = 6, Quartz = 7, Topaz = 8, Corundum = 9, and Diamond = 10; some students like to remember this sequence as "The Green Cow Flew Away, Felt Queer To Come Down". Most igneous rocks (the kind of rocks that form major mountain ranges like the Sierras and the Rocky Mountains) have a lot of quartz and feldspar and relatively little of the others (especially diamond, which together with its hardness and cleavage properties makes it so valuable). During weathering, feldspar (an aluminosilicate) rather rapidly converts to clays, while quartz, being unreactive silicon dioxide, survives the weathering process and eventually makes its way to accumulate along the beaches. The clays that were formed by the alteration of the feldspars are winnowed out from the surf zone and washed away to the open ocean to accumulate on the continental shelf and beyond. The quartz remains behind, traveling down the coast line (because the waves approach the coast line obliquely) as a narrow ribbon of sand in what's called long shore or littoral drift. (In reality, the beaches of the continents are actually different mixtures of quartz, feldspars and rock fragments, but quartz usually is dominant).
    Other localities, such as here on the tropical islands of Hawaii, have volcanic rocks that have little quartz but lots of feldspar. Here the feldspar again weathers away to form clays that either accumulate to form our soils or are washed away by stream erosion to the deep sea. What are our beaches made of? The answer is most are made of carbonate sands (fragments of coral and mollusk shells and tiny micro-organisms called foraminifera) that are derived within the surf zone by erosion of our surrounding reefs. Some of our other beaches are "black sands" that form in beach pockets and are composed of glassy volcanic debris. We also have green beach sands composed largely of the green mineral olivine that is separated from the other volcanic fragments by erosion.
    Did you know that, unlike gold, sand and gravel are California's principal mineral resource?

Craig R. Glenn, Associate Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822

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