Sand Analysis

The mighty rivers like the Colorado wear away the rocks as water passes on it's way to the sea.  Tiny bits of decomposed rock are carried along in the water.  When water reaches a lake, it slows down and some of the sand particles are deposited to make beaches.  We start with this image of sand from Lake Powell to illustrate some of the basic things you should look for when investigating sand.  First, the size of the grains is always important.  Put a metric ruler under your microscope to get an idea of their size.  The shape of the material is also important.  Is it smooth or rough?  Third, can you identify the material that makes up the sand?  Is it made from minerals like quartz, feldspar or mica or is it made from tiny bits of broken shells?  

Some samples are best observed on a white background and others on a black background.  Try both to see which is best.

Here we see the same sample, magnified more on a black background.  See how different it looks.  Sand is a byproduct of weathering.  The particles in sand are usually very small bits of something very large upstream, like a huge mountain.  But not all sand is deposited from rivers.  On some beaches, wave action on coral underwater deposits tiny bits of coral on the beach.  From a distance, the color of the sand is related to the composition of the individual particles.  Lots of quartz will produce a shiny white beach.  Lots of feldspar will make a more orange colored beach.  Common black minerals in sand are mica and hornblend.

One winter day I found this expanse of black sand on a beach north of San Diego.  The waves had been big for a few days and most of the lighter sand with quartz and feldspar had washed out to sea leaving this heavy material behind.  I took a sample back to my lab for analysis.  It was very pretty with many interesting minerals.  It was also quite heavy as compared to regular sand.  I wondered why it was so dense so I grabbed a magnet off my refrigerator door and ran it through the sample.

Many of the black particles stuck to the magnet.  Those metallic particles are called magnetite.  I looked at what was left.  There was still some black minerals.  My guess is that they are hornblende.  The green crystals could be olivine.

If you ever drive the road to Hana on the island of Maui (Hawaii) be sure to stop at Waianapanapa Beach (3 miles before Hana).  It is a gorgeous black sand beach.  Unlike the sand above, you can see that these particles are dull in color and likely made from something other than magnetite or hornblende.  The sand here is composed of basalt (lava) which is weathered by waves and wind at the oceans edge.  At Waianapanapa you will also see many photogenic volcanic arches and sea stacks at the oceans edge.  This is a good clue as to why the sand on the beach is black.

Some beaches aren't made of minerals at all but rather tiny bits of broken shells.  Notice the size scale here.  These particles are quite large.  When you walk on a beach like this you can feel the coarseness of the sand under your feet.  Because there is very little rain in Baja, there is very little transport of material from the land.  As one might guess, the sand must be made from some material other than decomposed rock.  After time, wave action will smooth out the sharp edges of the shells.  In this picture you can see some smooth and some sharp pieces

Another example of a beach made of shell fragments.  If you spend some time sifting through the sample you should find some less eroded material.  You could eventually start to identify and classify each particle and make a hypothesis as to what type of mollusk that particle originally came from.

Like the two samples above, Hawaiian beach sand is primarily made up of the shells of micromolluscs (tiny snails) and broken and worn pieces of larger shells and corals.  On beaches with intense wave action (like this North Shore sample), the shell pieces are rounded by the energy of the waves.  On more protected Hawaiian beaches the sand is coarser and the particles are more defined (compare to the Cabo sample above). 

In French Polynesia there are no big rivers that flow to the sea nor is there freezing of rock to break it up.  Some of the minerals (mostly volcanic) find there way to the beach but the beaches in Tahiti are predominantly very white and made up of bits of coral.  The islands are mostly protected by barrier reefs so there is little wave action.  The sand particles are generally not smooth like those of the North Shore above and from this sample you can see why the beaches in Tahiti are so very white and soft.

You don't have to live near the beach to collect sand.  Take a trip to your local lumber company and go to the area where bricks and concrete are sold.  Likely you will find the big bags of sand used for sandblasting or to mix with cement.  Some is very fine and white, almost pure quartz.  Look around the floor.  There will surely be a spill from one of the bags.  Take a sample.   Hmmmm, I don't think it would be shoplifting but never cut open a bag for a sample!  From our scale, notice how small these particles are.

Yes, it really is green sand.  It is strange to walk on a green beach and there are very few green sand beaches in the world.  One is on the island of Guam and another is found on the big island of Hawaii (2 hour hike from South Point).  The beach formed by the erosion and concentration of olivine crystals derived from a surrounding lava cone.  Olivine is a mineral commonly found in basalt lava.  The waves  removed the lighter grains of sand leaving the denser olivine crystals behind to form the beach. 

This is not sand because the particle size is too small.  It is called ash and is presented here as a reference.  Like dust, the particles are extremely small but on close inspection (40X) we see very tiny bits of quartz (Silica) and other minerals, all with very rough edges.  It was ejected from the Mt. St. Helens volcano on May 18, 1980.  Can you explain why the grains aren't smooth?