Star Sand, a Foraminiferan


Foraminifera, or foraminiferans, are an order of the animal kingdom, of the class Sarcodina. There are more than 18,000 species of foraminifera—both fossil and recent—and most of them are marine. Foraminifera are single-cell protozoans, and are known for their calcium carbonate exoskeletons. The exoskeletons are typically 50 µm to several millimeters in size.

In this article, the foraminiferan Baclogypsina sphaerulata will be characterized.

Baclogypsina sphaerulata is found on beaches in the Indo-Pacific. On some beaches, Baclogypsina sphaerulata exoskeletons occur in enormous numbers, and are commonly referred to as star sand due to their characteristic shape. Baclogypsina sphaerulata is an ancient creature, with remains found in Precambrian rocks.

Polarized light microscopy, fluorescence microscopy, and scanning electron microscopy may be used to identify and examine the beautiful exoskeletons of Baclogypsina sphaerulata. Other analytical techniques useful for characterization of the exoskeletons are energy dispersive X-ray spectrometry, Fourier-transform infrared spectroscopy, and X-ray diffraction.

Polarized Light Microscopy

Baculogypsina sphaerulata exoskeletons can be colorless, off-white, tan, or dark brown. The majority of the time, the lighter-colored particles have dark specks or blotches of darker color present on the surface. The surface of the shells range from smooth to rough, and in B. sphaerulata and similar species the shell is perforated with many holes, or fenestrations. Crushed B. sphaerulata has an irregular morphology, with fragments appearing to be platy, amorphous, or both. In transmitted light, the lighter-colored shells are ~50% transparent, while the darker-colored shells are slightly transparent. The shells may appear opaque if the chambers are filled with air. The fragmented shell particles show incomplete extinction. All of the particles appear to be anisotropic. There is a moderate to high birefringence measurement, but due to the extinction characteristics, no refractive indices were determined. The anisotropic character is indicated in the view between slightly uncrossed polarizers (P-102, P-107, and P-112), and confirmed between crossed polarizers (P-103, P-108, and P-113). Figure P-114 is the 400X view with the first-order red compensator added between the crossed polarizers; the high-order whites are obvious. Due to the morphology of B. sphaerulata, an interference figure cannot be gathered.

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Historical Information

The foraminifera are so beautiful that, like the radiolaria, they have been favorite objects for mounting from their discovery. So-called “type slides” have been made by individually mounting selected forams onto a microphotograph giving the species name, so that both the specimen and its name can be seen when viewed through a microscope. Figures P-116, P-117, P-118, and P-119 are 200X views of different portions of such a type slide, demonstrating the extraordinary beauty of these microscopic forms.

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In life, there are long threadlike reticulate pseudopodia that stream in and out through the openings in the test (shell), by which they attach to plants, hydroids, or the ocean bottom; other forms creep, or are pelagic (living in the open ocean).

The three pyramids at Giza, Egypt were constructed from limestone blocks that contain the tests (shells) of an Early Tertiary foraminiferan, called nummulites.

  • Petroleum geologists study the forams obtained in drilling test wells to aid in identifying oil-bearing strata.
  • Recent surveys of planktonic foraminifera of the California Current indicate a twentieth-century warming trend through increasing abundance of tropical and subtropical species (Field).
  • Ernst Haeckel thought of foraminifera as amongst the most beautiful forms from nature. He had lithographic plates prepared from his original drawings and gathered them in book form at the turn of the twentieth century (Figures P-120–121 ).

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Fluorescence Microscopy

When illuminated with a narrow-band 365 nm ultraviolet light, Baculogypsina sphaerulata exoskeletons fluoresce with moderate intensity, showing colors of green and blue (P-105, P-110, and P-115).

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Solubility Tests

  • Insoluble in deionized water
  • Soluble in 1M HCl; when subjected to acid the particle began to effervesce, then dissolved rapidly. This test indicates presence of carbonates in the sample.

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Cushman, Joseph A. (1948) Foraminifera; Their Classification and Economic Use. Fourth Edition. Harvard University Press, Cambridge, MA. Reprinted 1976, 1968, 1980.

Field, David B, et. al. (2006) Planktonic Foraminifera of the California Current Reflect 20th Century Warming. Science. 311 63-66. Full text available online:

SEM Characterization

SEM Images show a variety of sizes and forms of Baculogypsina sphaerulata exoskeletons. The images S-101–S-114 show unaltered foraminifera. The sizes of B. sphaerulata range from 500 µm to 3.5 mm. S-101, S-103, S-105, S-107, S-1089, S-111, and S-113 are low magnification images of particles showing typical ranges of particle sizes and shapes. B. sphaerulata are found in several unique shapes; some are found in star-like shapes with between four and seven points that can be rounded (S-107) or very pointed (S-109 and S-113). Others are oval/disk shaped (S-101 and S-111).

S-102, S-104, S-106, S-108, S-110, S-112, and S-114 are higher magnification views of the preceding particles. Many of the B. sphaerulata shells have surface striations and fractured edges. Smaller particles can also be found on the surface of the larger particles (S-102, S-104, and S-106).

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Energy Dispersive X-Ray Spectrometry (EDS)

Energy dispersive X-ray spectrometry was used to determine the elemental chemistry and distribution on Baculogypsina sphaerulata exoskeletons. Analyses on exterior and interior surfaces of B. sphaerulata were performed using an accelerating voltage of 15kV and a pressure of 30 Pa. Figure SE-101 shows a comparison of three different locations: body, arm, and interior pore. The summary table of the elemental weight percentages can be found below.

Trace elements may vary depending on impurities in source material.

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Fourier Transform Infrared Spectroscopy

Particles from Baculogypsina sphaerulata exoskeletons show a very strong, broad band (F-101) in the region of 1420 cm-1 that is due to the CO3 out-of-phase stretch. The sharp band in the 876 cm-1 is the CO3 out-of-phase deformation, and the band at 717 cm-1 is attributed to the CO3 in-plane deformation. The spectrum also has a band at 2522 cm-1. These bands are consistent with calcium carbonate.

Particles from B. sphaerulata exoskeletons may also show strong, sharp bands in the region from 1036 cm-1 to 913 cm-1 (F-102) that are due to silicate (Si-O-Si) bonds, typically from clay minerals. There may also be sharp bands in the region of 3700 cm-1 to 3600 cm-1 that are likely due to hydroxyl groups from water and Si-OH groups.

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Raman Spectroscopy

Baculogypsina sphaerulata exoskeletons were analyzed by Raman spectroscopy using 514 nm and 785 nm lasers. A peak was observed at 1089 cm-1 in the spectra from both lasers. This peak is identified as the CO3 symmetric stretch. The primary spectral feature of calcite, a form of calcium carbonate (CaCO3), is this peak at 1089 cm-1. The sloping baseline of both of the spectrums are due to background fluorescence, which can most likely be attributed to clay or other silicates.

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X-Ray Diffraction

A sample of Baculogypsina sphaerulata was attached to a glass fiber and analyzed using X-ray diffraction (XRD). The powder diffraction pattern was compared to the International Center for Diffraction Data database of diffraction patterns, and an excellent match was made to calcite pattern #01-086-2336.

X-101. XRD spectrum.


Martin L. Scott

I am amazed at this wealth of information.

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Leslie Bolin

Thank you for the compliment. We're glad you enjoyed this article.

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