Page 101 - Laboratory Manual in Physical Geology

Why Are Density and Specific Gravity
Important?
Have you ever considered buying silver coins as an
investment? If so, then you should be wary of decep-
tive sales. For example, there have been reports of less
valuable silver-plated copper coins marketed as pure
silver coins. Copper has a specific gravity of 8.94,
which is very close to silver’s specific gravity of 9.32.
So, even experienced buyers cannot tell a solid silver
coin from a silver-plated copper coin just by hefting it
to approximate its specific gravity. They must deter-
mine the coin’s exact specific gravity as one method
of ensuring its authenticity. Mineral identification is
also aided by knowledge of specific gravity. If you heft
same-sized pieces of the minerals galena (lead sulfide,
an ore of lead) and quartz, you can easily tell that one
has a much higher specific gravity than the other. But
x1 the difference in specific gravities of different minerals is
not always so obvious. In this activity you will learn how
FIGURE 3.16 Acid test. Place a drop of weak hydrochloric acid to measure the volume and mass of mineral samples,
(HCl) on the sample. If it effervesces (reacts, bubbles), then it is a calculate their specific gravities, and use the results to
carbonate ( CO 3 - containing) mineral. Please wipe the sample dry identify them.
with a paper towel after doing this test! Note that the mineral in this Before you begin , read the following background
example occurs in several different colors and can be scratched by a information and be sure you have a pencil, eraser, and
wire (iron) nail. The yellow sample is a crystal of this mineral, but the
other samples are broken pieces of crystals that reveal the mineral’s Worksheet 3.3 (p. 102 ). Then complete the activity and
characteristic cleavage angles. Worksheet 3.3.

How to Determine Volume. Recall that volume is the
amount of space that an object takes up. Most mineral
ACTIVITY samples have odd shapes, so their volumes cannot be
calculated from linear measurements. Their volumes
3.3 Determining Specific must be determined by measuring the volume of water
Gravity (SG) they displace. This is done in the laboratory with a

graduated cylinder ( FIGURE 3.17 ), an instrument used to
measure volumes of fluid (fluid volume). Most graduated
THINK How and why do people study
cylinders are graduated in metric units called milliliters
About It minerals?
(mL or ml), which are thousandths of a liter. You should
also note that 1 mL (1 ml) of fluid volume is exactly the

OBJECTIVE Measure the volume and mass of 3
same as 1 cm of linear volume .

minerals, calculate their specific gravities, and use the
Procedures for determining the volume of a
results to identify them.
mineral sample are provided in FIGURE 3.17 . Note

PROCEDURES that when you pour water into a glass graduated
1. Before you begin, read the following background cylinder, the surface of the liquid is usually a curved
information. Your instructor will provide meniscus , and the volume is read at the bottom of its
laboratory equipment, but this is what you will concave surface. In most plastic graduated cylinders,
need to bring to lab: however, there is no meniscus. The water level is flat
and easy to read.
___ Activity 3.3 Worksheet (p. 104 ) and pencil
If you slide a mineral sample into a graduated cylinder
___ calculator
full of water (so no water splashes out), then it takes up
2. Then follow your instructor’s directions about space previously occupied by water at the bottom of the
where to obtain laboratory equipment and graduated cylinder. This displaced water has nowhere to
mineral samples, how to work safely, and how to go except higher into the graduated cylinder. Therefore,
complete the worksheet. the volume of the mineral sample is exactly the same as the
volume of fluid (water) that it displaces.

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