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Activity 45 • Mineral Identification
Explain that double refraction is a special property
that is only found in certain minerals, including some
forms of calcite. When the material is placed on
printed text or a printed line, two images of the
printed image will be seen through the sample. This
property of double refraction is not the same as transparency, which simply indicates whether or not the
material can be seen through clearly.
3.
(DI ASSESSMENT) Students design and conduct
an investigation to identify a mineral.
Students are provided with data on seven mineral
properties in Table 1, “Mineral Properties.” Only
five of these properties are useful in helping students identify the unknown mineral: transparency,
luster, hardness, reaction with acid, and double
refraction. The remaining two properties (color and
streak color) do not provide data that would help
distinguish between calcite and quartz.
Have students work in pairs to design their investigations. Depending on your student population,
you may wish to provide additional guidance on
one or more parts of the investigation or use Student Sheet 45.1, “Writing Frame: Investigating Minerals,” to help students structure their writing.
3When approving students’ investigation design,
look for designs that plan to observe or test either
color or streak color, or for plans that require the
streak plate. Such responses indicate a lack of
understanding of the idea that differing properties
are the most useful for identifying a mineral. Guide
students to the understanding that it is not helpful
to test properties that are similar for different materials.
After approving their designs, provide students
with the materials necessary to conduct their investigations. Note that the streak plate should not be
required by any of the student groups, though you
may wish to provide it after students have completed their investigations so students can become
more familiar with that property.
You can use the DESIGNING INVESTIGATIONS (DI) Scoring
Guide to assess students’ work. Note that this is not
a typical DI assessment; however, this activity can
be used to introduce students to designing an investigation and it contains enough components to be
scored with the DI Scoring Guide. A Level 3 response
is shown below.
Sample Student Design
Purpose: To identify an unknown mineral.
Five properties to test: Transparency, luster, hardness, reaction with acid, and double refraction.
Reason for selecting these properties: All of these properties are different for calcite vs. quartz. The sample
will only match one of them, so I will know what it is.
Materials needed: Mineral, copper strip, glass scratch plate, hydrochloric acid bottle, SEPUP tray, cup of
water, dropper, magnifying lens, paper towel, safety goggles.
Data table:
Observations of Unknown Mineral
Property
Observations
Transparency
transparent to translucent
Luster
dull to glassy
Hardness
greater than 5.5
(scratches copper and glass)
Reaction with acid
no reaction
Double refraction
no double image
Conclusion: I will compare my observations of the unknown mineral with the data in Table 1. I will identify
the mineral by seeing whether calcite or quartz has more properties in common with the unknown.
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Mineral Identification • Activity 45
FOLLOW-UP
4.
(RE ASSESSMENT) Students identify the
mineral by comparing their data with data
on two other minerals.
After students have completed their investigations,
use Question 2 to have students share their results.
Reinforce the idea that similarities in the results for
a particular mineral are dependent on the material
itself. The properties are intrinsic to the mineral,
which is why students should have obtained similar
results for most of the properties. (There may be
more variation in data related to properties that
rely on observation or properties that may vary
among the samples, such as transparency or luster.)
Students can be assessed on their ability to recognize evidence with Question 3. Prior to assigning
the question, review your expectations for a Level 3
response. Explain that students must explain how
their laboratory results helped them identify the
unknown mineral. This means explaining the
results of their tests and describing whether these
tests match data for calcite or quartz. A complete
answer means that the writer cannot assume that
the reader knows any of the test results, data on the
properties of either calcite or quartz, or how to
interpret the comparision of these data sets.
EXTENSION
Ultraviolet light can react with the chemicals of a
mineral and causes the mineral to glow; this is
called fluorescence. Not all samples of a particular
mineral demonstrate this property. Calcite from
Franklin, New Jersey, which is enriched with small
amounts of manganese, fluoresces a bright red
under UV light. A certain form of calcite from Mexico can fluoresce purple or blue. Quartz can fluoresce blue, yellow, and red.
SUGGESTED ANSWERS TO QUESTIONS
1.
Which properties did you decide to test?
Why did you choose these properties?
The properties most useful for identifying the
mineral as either calcite or quartz are transparency, luster, hardness, reaction with acid,
and double refraction. This is because these
properties are different for calcite vs. quartz. For
example, calcite has a hardness of 3 and quartz
has a hardness of 7.
2.
Did everyone have the same data for each
property of the unknown mineral? Why or
why not?
Most of the data should be the same or similar
because a property is characteristic of a material. For example, calcite has a particular reaction with acid, and all students who tested the
effect of acid on the mineral should have
observed no formation of small bubbles.
There may have been greater variation in the
data that relied on descriptive observations,
such as luster, or that had some variation
among the samples, such as transparency.
3.
(RE ASSESSMENT) a. What is the unknown
mineral: calcite or quartz?
The unknown mineral is quartz.
b. Support your identification with at least four
pieces of evidence from your investigation. Be
sure to compare your laboratory results to the
properties of both calcite and quartz.
Level 3 Response:
Quartz does not react with acid. The
unknown also had other properties of
quartz. It was transparent and glassy. Quartz
is transparent, glassy, and shows one image.
The unknown had a hardness greater than
5.5. Calcite has a hardness of 3 and quartz
had a hardness of 7, so the unknown matches
quartz.
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