Hard to miss in the wintertime both at Crane Beach and at Plum Island are the layers and swirls of pink and purple sand. On a recent visit to Revere Beach I noticed there were also rivulets of pink and purple sands.
The pink and purple are mineral deposits of rose quart and garnet and come to north of Boston beaches via the White Mountains. Water and wind worn rock is carried in river waters until it meets the ocean and becomes deposited on barrier beaches. We mostly see the garnet and quartz deposits in winter as storms erode the dunes, leaving the heavier minerals exposed. During the spring and summer, the lighter white quartz sand blows back over the dunes and covers the heavier sand.
JEOL is a supplier of electron microscopes, ion beam instruments, mass spectrometers and NMR spectrometers. On a visit to Plum Island looking for Snowy Owls, several JEOL employees found purple sand. They analyzed it using an optical microscope, a scanning electron microscopes (SEM) and an energy dispersive X-Ray spectrometer (EDS).
At first look under the optical microscope, the granules of sand appeared like scattered jewels of many colors; predominantly glassy pink angular grains, with smaller quantities of milky white rounded grains, clear angular grains, black grains (some magnetic and some not), and even the occasional green.
What could be the cause of the purple color? The answer was one that came as no surprise to the scientist, but was exciting for the beach walkers because they had an exact answer to a question that no doubt is one that many people have when they visit Plum Island – which was actually named for its beach plum bushes, not the plum-colored sand.
When large amounts of fine grained pink is intermixed with a smaller number of darker grains and dampened by rain or sea water the human eye will “see” the sand as a much darker pink to almost purple. The two most common pink minerals are rose quartz (while quartz is one of the two most common minerals on earth, the pink rose quartz variety is not so common ,especially in the New England geology, and is found only in a few isolated pegmatite deposits in NH & southern Maine which are where most gemstones originate) and the solid solution series of almandine and pyrope garnet which is also a very common mineral (and is quite common in the Seacoast area from the abundance of metamorphic rocks called mica schist and from contact metamorphism. This is also why many commercial sandpaper products have a pink color as the angular hard gains of almandine / pyrope garnet are the perfect abrasive. The most likely candidates for the white and clear are any of the feldspars and or quartz. The green is most likely epidote. Just based on the optical examination these are no more than educated logical guesses (but still guesses).
Vern Robertson, JEOL’s SEM Technical Sales Manager, originally examined the grains under a low power optical stereo microscope with the above conclusions. In addition to providing technical and scientific support to JEOL SEM customers for a multitude of applications, Vern holds a degree in Geology. After a cursory look optically, it was time to get down to some spectroscopic analysis to determine the actual mineral species present in the sand.
Individual grains of various colors were selected and mounted for examination with the JSM-6010LA+ InTouchScope SEM and for analysis using EDS. The SEM allows much higher magnification imaging with greater depth of field than a traditional OM and the low vacuum capability allows examination of the sample without the traditional conductive coating that needs to be applied for SEM imaging. However, it generates images in only black & white (electrons have no color!). One specialized detector in the SEM, the Backscatter Electron Detector, yields images with the gray level intensity directly proportional to the average atomic number (or density). This means that minerals containing only lighter elements like O, Si are darker in appearance to minerals that contain heavier elements like Fe or any of the metallic or rare earth elements.
Once located, each grain can be analyzed with the EDS. When an electron beam hits a sample it creates not only an image from the emitted electrons but creates X-rays, which when collected in a spectrum, indicate what elements are present and at what concentrations. This allows not only the elemental composition of the individual grains to be determined but the concentrations can be compared to known stoichiometry of the suspected mineral grains. The combination of color and magnetic properties from OM examination and the chemical makeup of the individual grains yield the answer.
The purple color (or more appropriately, pink color) comes from the abundance of almandine-pyrope garnet with a nominal solid solution composition of Fe3+2Al2Si3O12 to Mg3+2Al2Si3O12. As expected, the white grains are a mix of feldspars but mostly K-feldspar (potassium alumino-silicates) and quartz SiO2. The black nonmagnetic grains were a mix of a pyroxene called augite which showed its characteristic strong cleavage, (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 , and a mix of ilmenite FeTiO3 and hematite Fe2O3 which are the magnetic components. The green was confirmed to be epidote Ca2(Al,Fe)3(SiO4) 3(OH). With the exception of the high concentration of garnets the rest are common minerals one would expect to find in sands.