Geology Reference
In-Depth Information
Aside from halite, licking is not a useful way of identifying minerals. In fact,
licking unknown minerals may expose you to dangerous (poisonous) metals such
as arsenic and is not recommended.
Another unique mineral property is the
effervescence,
or fizzing, of certain minerals
when sprayed with weak hydrochloric acid. If you do not have a taste for rocks, this
method is another way to distinguish calcite from halite: If you spray diluted hydro-
chloric acid on the mineral and it fizzes and bubbles, you have calcite; if nothing hap-
pens, you have halite. (After you have put acid on the mineral, you definitely don't want
to lick it to confirm it is halite!)
Measuring properties in the lab
Some mineral properties can be measured only in a laboratory. To examine minerals un-
der a microscope, geologists make
thin-section
slides, which are slices of the rock they
want to examine.
A thin-section slide is made by attaching a small piece of the rock to a glass slide and
grinding down the rock until it is thin enough for light to shine through it. Then the thin-
section can be examined under a special geologic microscope, called a
petrographic
mi-
croscope, that shines light through the slide. The petrographic microscope has settings
that change the angle of the light shining through the minerals — a process called
polar-
ization.
When light passes through a crystal, it is separated and different colors appear depend-
ing on the crystal shape and composition. The
polarized light
highlights the internal
structure of the crystal, which can indicate the types of minerals in the rock.
Other mineral properties measured in a lab include:
Fluorescence:
Fluorescent
minerals in a rock glow when you shine ultraviolet light
on them.
X-ray diffraction:
Each mineral displays a particular pattern when x-rays pass
through it and are separated, or
diffracted.
The distinct
x-ray diffraction
patterns in-
dicate which minerals are in a rock.
