Heavy Minerals

Abstract

Heavy minerals are dense grains found not only in rocks, but also in different

types of sand. After being collected from different areas along the beach at Fort Fisher,

the sand grains were brought back to the lab to be examined. Each sample was cleaned,

and placed in a dense liquid. The separation of the heavy minerals from the rest of the

sand then took place and was later examined under a petrographic microscope. The

purpose of the investigation was to recognize the different minerals, their percentage

within the sample and to determine each minerals origin. Results varied with each soil

sample. There was a greater abundance of some minerals than others in the different

mineral samples. This was due to location of where the sand was collected. Many of the

heavy minerals found were known to be familiar to the beach area.

Background

Heavy minerals are minor, high-density sandstone minerals, which have a higher

average of specific gravity than other minerals. These minerals are present in the parent

rocks as either rock forming minerals, like mica, or as accessory minerals, such as zircon

and tourmaline ( Mange, Maria A. and Heinz F.W Maurer, 1992). Heavy minerals

usually make up only one percent of the parent rock from which it came from.

The number and the type of heavy minerals vary within wide limits. Only a few

species are common in sands. Some of these minerals include kyanite, garnet,

tourmaline, magnite, and others. The heavy minerals are often useful guides to the

source rock where the sediments came from. Quartz, which is considered to be a heavy

mineral, is an exception because it is found almost in all parent rocks ( Pettijohn, F.J.,

P.E. Potter, R. Siever, 1987).

In order to separate the heavy minerals from the other sand grains, they must be

concentrated by gravity separation using a liquid with a density of 2.89 or 2.96. The

heavy minerals, because they are more dense than the liquid, will then sink to the bottom

of the funnel. There are certain factors that affect the reliability of heavy mineral studies.

They include such factors as the climate of the source area. This controls the pre-

selection of mineral grains during weathering. The weathering of the minerals

determines the original input of the heavy minerals into the sedimentary system. Another

factor deals with hydraulics. It affects the sorting of grains with the size, form, and

densities of the mineral grains. The heavy minerals have a direct relationship to the grain

size. It helps reflect the initial size of the source rock.

Petrologists use heavy minerals to find provenance (Mange, Maria A. and Heinz

F.W. Maurer, 1992). The word provenance means origin or birth, which deals with the

source rock the mineral came from. Petrologists also use the minerals to trace sediment

transport paths, map sediment transport paths, and to outline sand bodies (Pettijohn, F.J.,

P.E. Potter, R. Siever, 1987).

Research Question

Heavy minerals are dense sediments, which are found in rocks and sands.

Research on these minerals has tapered off considerably due to more advanced

technology on rock fragments and minerals. The investigation is prepared to answer

what type and the percent composition of the minerals found in the sand samples. Also,

the experiment will help determine where the minerals originated.

Methods

During our trip to Fort Fisher, sand samples were collected from three different

areas: sand located between the seawall, the sand along the shoreline, and the protected

sand located near the sand dunes. The sand was then brought back to the lab for further

investigation.

The first step was to measure out 30 grams of each sample on an electronic scale,

which measured up to a hundredth of the number. Each sample was then washed in order

to extract the salt from the sand. The salt, from the beach, might have affected the

density of the liquid, allowing the results to be inaccurate. The sand was then set a side

to dry.

In order to set up for the main portion of the experiment, several items were used.

A funnel with a small rubber tube attached at the bottom was held by a clamp, which was

attached to a stand. A small beaker was placed under the funnel and tube, to catch any

excess waste. A pinch clip was clamped to the rubber tube to catch the heavy minerals

and liquid.

The liquid, sodium polytongstate, which has a density of 2.86, was poured ¾ full

into the funnel. The first sand sample was then poured into the liquid and began the

process of separating, the heavy minerals sank and the light minerals stayed afloat. The

entire separating process took around 4 hours.

In order to extract the heavy minerals, a number of steps were used. First, filter

paper was placed into a second funnel and then into a small beaker. Then the first funnel

was placed over the second, and the pinch clip was slowly released, allowing only the

heavy minerals to be released. The pinch clip was then closed, trapping the light

fragments in the first funnel. The heavy minerals collected were washed thoroughly to

filter out all the polytongstate liquid. The sample was then set a side to dry.

Before the minerals could be mounted onto the slide, they were first measured on

the electronic scale. This was done in order to find the percent composition of the

individual minerals found later in the experiment. In order to mount the mineral grains

onto a slide, a glue mixture, known as epoxy, was mixed until it became sticky. It was

then spread carefully, eliminating air bubbles, onto the slide. The minerals from the sand

were sprinkled onto the slide and set aside to dry.

Using a petrographic microscope at 3x, the heavy minerals were examined to

determine their type. The book was used to compare the minerals under the microscope

with the pictures of the minerals in the book (Mange, Maria A. and Heinz F.W. Maurer,

1992). Sections of the slide were viewed, drawn, and counted. The process continued

until 100 grains were counted, drawn, and each mineral had been recognized. Each

heavy mineral type was then counted individually, and the percent composition for each

was determined. The experiment was then repeated for the last two samples. On the third

sand sample, the mineral grains were viewed at 10x because the grains were finer and

smaller than the minerals in the other two samples. Data charts and graphs were then

composed to show the results.

Results

The results within the experiment varied with each sand sample. The mineral

distinctions within each sample helped categorize each sample with its on area. Each

sample was carefully drawn and identified to make sure the results were as accurate as

possible.

The first heavy mineral sample was from the sand found between the sea wall at

Fort Fisher. The heavy minerals extracted from the sand weighed a total of 0.074 grams.

The percent composition was then calculated to compare its mass to the rest of the 30

gram sand sample. The heavy minerals in the first sample made up only 0.247 percent of

the total sand sample. Eight different heavy minerals were found within this sample.

The majority of the heavy mineral sample was made up of monazite, making up 37

percent, and magnetite, which made up 33 percent of the sample. The other minerals

included garnet, tourmaline, apatite, epidote, chloritoid, and calcite. For a description of

the previous minerals, turn to A4. The pie chart located on A1 shows a comparison of

the percentages of the individual minerals. Each mineral found in this sample is

commonly found in beach sands. This means the source rocks from which the minerals

came, are located at or near the beach area.

The second sand sample was collected from the shoreline on the beach at Fort

Fisher. This area is where the water washes over the sand. The heavy minerals extracted

from this area weighed a total of 0.085 grams. The percent composition of sample two

was then taken to compare its mass with the rest of the sand. The heavy minerals in this

sample made up 0.285 of the total sand sample from the beach. There were a total of

nine different minerals identified in the sample. Again, monazite, making up 25 percent

of the sample, and magnetite, which made up 29 percent, were the dominant minerals

found. The other minerals identified were garnet, tourmaline, apatite, epidote, chloritiod,

calcite, and quartz. The quartz is usually considered to be a light mineral, so it may have

got caught up with the heavy minerals during the separation. To look at percentages of

the minerals within the second sand sample, refer to the chart on A2. The same heavy

minerals that were located in the first sample were found in the second sample. This

means the heavy minerals also came from source rocks located at or near the beach.

The third sand sample was taken from the protected area of the beach at Fort

Fisher. This is the area on the beach close to the sand dunes. The water is unable to

reach this type of sand. After separation occurred, the isolated heavy minerals weighed

3.003 grams. There was a bigger sample of heavy minerals in this sand due to the

increase in the abundance of magnetite. The percent composition of the mineral sample

was taken to compare its mass with the rest of the sand fragments. The heavy minerals in

sample 3 made up 10.01 percent of the sand. This is a greater increase in percent

composition compared with the first two samples. Magnetite was the dominant mineral,

making up 58 percent of the mass of the heavy mineral sample. The other minerals

identified in this sample included garnet, monazite, apatite, epidote, quartz, calcite,

muscovite, and kyanite. The last two minerals, muscovite and kyanite, were found only

in the third sample. The reason for this discovery is unknown. Kyanite is usually found

in the mountain region. There is a possibility that it was swept down by the water

hundreds of years ago, or its source rock could have been brought to the beach area from

an outside factor. The graph on A3 shows a comparison of the percent compositions of

the heavy minerals. Excluding the kyanite and muscovite, the minerals found in this lab

experiment are commonly found within the beach sands.

Conclusion

In this investigation, each heavy mineral was examined under the microscope to

determine its type, its percentage within the mineral sample and the origin of each

mineral. In each sample, the minerals proved to be similar. There was an exception in

the third sample when kyanite and muscovite were found. These two heavy minerals

were not common to the area. The investigation also proved that two minerals, magnetite

and monazite were the two dominant minerals found in all three samples. In the third

sample, magnetite made up over half of the composition of the heavy minerals. This was

the reason the sand tested had a black tint. Each mineral was studied and read about to

determine the origin. Almost all the minerals were found to have originated from beach

sands (Pellant, Chris and Roger Phillips, 1990). It would be interesting to further

investigate heavy minerals by collecting samples of sand on the different beaches around

North Carolina. Then one would be able to compare the different types of heavy

minerals at each location and compare the differences and similarities. This would enable

a person to obtain a clearer understanding of the heavy mineral composition along the

coast of North Carolina.

References

Mange, Maria A. and Heinz F. W. Maurer. Heavy Minerals in Colour. London: Chapman and Hall, 1992.

Pellant, Chris and Roger Phillips. Rocks, Minerals, and Fossils of the world. Boston:Little, Brown and Company, 1990.

Pettijohn, F.J., P.E. Potter, R. Siever. Sand and Sandstone. New York: Springer-Verlag, 1987.