Exhibits In Extreme Close Up: Black Sand

by Paul Dancstep • April 7, 2017

Exhibits In Extreme Close Up is a series in which we examine pieces of our exhibits using a scanning electron microscope.

Black Sand is one of the most popular exhibits at the Exploratorium. It consists of two large horn-shaped magnets and a volume of ferromagnetic powder which clings to the magnets, coating them in a spiky plumage.

This black sand, called magnetite, has a lovely glittery quality to it and is extremely soft to the touch. On its own, magnetite just looks like an unremarkable pile of sand. But placed near a magnet it springs to life, forming fuzzy resilient fingers that align themselves with the magnetic force field.

All of our black sand comes from Ocean Beach in San Francisco. If you pick up a fist full of beach sand you'll see many different grain colors. The tiniest of these grains appear to be a dark dust. This is the magnetite that we must separate out for the exhibit.

Over the years Exploratorium staff members have come up with many clever methods for sorting black sand from beach sand. Recently Gaily Ezer, one of our Field Trip Explainers, built this ingenious contraption:

Black Sand Sorter

Beach sand goes in the top and slides through a narrow channel, flowing past a spinning cylinder covered in magnets. These magnets attract the grains of magnetite and fling them into a separate compartment at the back of the device. The result is some of the purest, most velvety black sand we've ever had in the exhibit.

At 120x magnification, these grains of magnetite look like round tumbled stones.

At 1000x magnification, we can examine the features of an individual piece of magnetite. Long ago, this tiny speck of iron oxide was part of the Sierra Nevada mountains. It broke off and was gradually carried to the ocean by rain and river water and then washed up on the beach for us to collect.