EM 1110-2-1100 (Part III)
30 Apr 02
immediately to the shore. Quartz sand and clay are more common on shores far from mountains where
weathering has had more time to reduce the relative proportions of feldspars and related silicates.
(2) Most carbonate sands owe their formation to organisms (both animal and vegetable) that precipitate
calcium carbonate by modifying the very local chemical environment of the organism to favor carbonate
deposition. Calcium carbonate may be deposited as calcite or aragonite, but aragonite is unstable and changes
with time to either calcite or solutions. Calcite or limestone, the rock made from calcite, may, under some
conditions, be altered to dolomite by the partial replacement of the calcium with magnesium. Carbonate
sands can contribute up to 100 percent of the beach material, particularly in situations where it is produced
in the local marine environment and there are limited terrestrial sediment supplies, such as reef-fringed,
tropical island beaches. These sands are generally composed of a combination of shell and shell fragments,
oolites, coral fragments, and algal fragments (Halimeda, foraminiferans, etc.). When carbonate shell is mixed
with quartz sand, it may be necessary to dissolve out the shell to get a meaningful representation of each size
fraction. ASTM Standard 4373 describes how to determine the calcium carbonate content of a beach.
(3) Other minerals that frequently form a small percentage of beach sands are normally referred to as
heavy minerals, because their specific gravities are usually greater than 2.87. These minerals are frequently
black or reddish and may, in sufficient concentrations, color the entire beach. The most common of these
minerals (from Pettijohn (1957)) are andalusite, apatite, augite, biotite, chlorite, diopside, epidote, garnet,
hornblende, hypersthene-enstatite, ilmenite, kyanite, leucoxene, magnetite, muscovite, rutile, sphene,
staurolite, tourmaline, zircon, and zoisite. Their relative abundance is a function of their distribution in the
source rocks of the littoral sediments and the weathering process. Heavy minerals have been occasionally
used as natural tracers to identify sediment pathways from the parent rocks (Trask 1952; McMaster 1954;
Giles and Pilkey 1965; Judge 1970).
In coastal engineering work, knowledge of the sediment composition is not normally important in its own
right, but it is closely related to other important parameters such as sediment density and fall velocity.
b. Density.
(1) Density is the mass per unit volume of a material, which, in SI units, is measured in kilograms per
cubic meter (kg/m3). Sediment density is a function of its composition. Minerals commonly encountered
in coastal engineering include quartz, feldspar, clay minerals, and carbonates. Densities for these minerals
are given in Table III-1-4.
(2) Quartz is composed of the mineral silicon dioxide. Feldspar refers to a closely related group of metal
aluminium silicate minerals. The most common clay minerals are illinite, montmorillonite, and kaolinite.
Common carbonate minerals include calcite, aragonite, and dolomite. However, carbonate sands are usually
not simple, dense solids; but rather the complex products of organisms which produce gaps, pores, and holes
within the structure, all of which tend to lower the effective density of carbonate sand grains. Thus, carbonate
sands frequently have densities less than quartz.
(3) The density of a sediment sample may be calculated by adding a known weight of dry sediment to
a known volume of water. The change in volume is measured; this is the volume of the sediment. The sedi-
ment mass (= weight / acceleration of gravity) divided by its volume is the density. A complicating factor
is that small pockets of air will stay in the pores and cling to the surfaces of almost all sediments. To obtain
an accurate volume reading, this air must be removed by drawing a strong vacuum over the sand-water mix-
ture. ASTM Volume 4.08 gives standards for measuring the density of soils and rocks, respectively (ASTM
1994).
Coastal Sediment Properties
III-1-15
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