COLUMN: Dredged sand is a black and white issue (w/POLL RESULTS)

As a professional geologist, here’s the reality of the black sand.

I’ve had the opportunity to see black beach sand deposits all over the world. Some Pacific Rim beaches in volcanically-derived terrains are composed purely of the stuff, the result of physical weathering processes on the basaltic host rock that produce both fallout and granularization of the associated heavy minerals.

Such heavy minerals are termed “mafic” in geological circles, which is an acronym for “manganese and ferric in combination.” The term alludes to the fact that the main metallic elements in such minerals are manganese and iron.

It is the preponderance of these metals which produces the dark colors characterisitic of mafic extrusive lavas, the solidified volcanic rocks, and the resulting and ultimate detrital fraction ... sand.  These mafic minerals are most commonly represented by limonite, magnetite, ilmenite, rutile, psilomelane, augite, eclogite and a host of others. Of these, the ones that most likely occur in Florida beach sand are magnetite, rutile, ilmenite, and psilomelane.

Consistent with their presence in mafic lavas, mafic minerals are also present in the volcanic and sedimentrary rocks that comprise the Appalachian Mountains, whose erosion over geologic time gave rise to the remarkably and very pure quartzose sand that inhabits Florida Panhandle beaches.

Mafic minerals being composed of heavy metals are heavier than quartz and would more readily concentrate in sand dredged from the bottom of channels or the ocean.

As a result, such sand contains a much higher percentage of these minerals and will naturally appear darker when dredged, and will contrast with the more pristine white sand we all know and love when placed topside. 
 
Joseph Hunt is a professional geologist living in San Diego, CA.

To read another geologist's take on the matter, click here.

To see the final poll results on the matter, click here.