Page 119 - Laboratory Manual in Physical Geology

A CTIVIT Y 3.3 Determining Specific Gravity (SG)

Name: ______________________________________ Course/Section: ______________________ Date: ___________

A. Imagine that you want to buy a box of breakfast cereal and get the most cereal for your money. You have narrowed your
search to two brands of cereal that are sold in boxes of the exact same size and price. The boxes are made of opaque cardboard
and have no labeling of weight. Without opening them, how can you tell which box contains the most cereal?

B. Like the cereal boxes above, equal-sized samples of different minerals often have different weights. If you hold a mineral

sample in one hand and an equal-sized sample of a different mineral in the other hand, then it is possible to act like a human
balance and detect that one may be heavier than the other. This is called hefting , and it is used to estimate the relative
densities of two objects. Heft the three mineral samples provided to you, then write sample numbers/letters on the lines below
to indicate the sample densities from least dense to most dense.
(Least dense) ______________ _______________ _______________ (Most dense)

C. In more exact terms, density is a measure of an object’s mass (weighed in grams, g)
3
divided by its volume (how much space it takes up in cubic centimeters, cm ).
Scientists use the Greek character rho (ρ) to represent density, which is always
3
expressed in g/cm . What is the density of a box of cereal that is 20 cm by
25 cm by 5 cm and weighs 0.453 kg? Show your work.
D. Mineralogists compare the relative densities of minerals according to their specific gravity (SG) : the ratio of the density of

3
a mineral divided by the density of water. Since water has a density of 1 g/cm , and the units cancel out, specific gravity is
3
the same number as density but without any units. For example, the density of quartz is 2.6 g/cm , so the specific gravity of
quartz is 2.6.
Return to the three mineral samples that you hefted above, and do the following:

1. First (while they are still dry), determine and record the mass (weight) of each sample
in grams.

2. Use the water displacement method to measure and record the volume of each sample

( FIGURE 3.15 ). Recall that one fluid milliliter (mL or ml on the graduated cylinder) SG OF SOME MINERALS

equals one cubic centimeter.
3. Calculate the specific gravity of each sample. 2.1 Sulfur

4. Identify each sample based on the list of specific gravities of some common minerals.
2.6–2.7 Quartz
3.0–3.3 Fluorite
Sample Mass in Volume in Specific Mineral Name
Grams (g) Cubic cm Gravity
3
(cm ) (SG) 3.5–4.3 Garnet
4.4–4.6 Barite
4.9–5.2 Pyrite
7.4–7.6 Galena
8.8–9.0 Native copper
10.5 Native silver
19.3 Native gold

D. REFLECT & DISCUSS Were your data and calculations accurate enough to be useful in identifying the samples? If not,
how could they be made more accurate?

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