Hey, Earth scientists, Bea here, and... I might have
made a tiny mistake. In my rock collection, these three were all labeled as different minerals:
quartz, calcite, and selenite. But, I mean, come on, they're practically identical. I
thought there's no way they're all different. So, I took them out to take a closer look,
and, well... you can totally understand how I got them all mixed up, right? Right?
Wait a minute. There must be some way Earth scientists figured out that these were
three different minerals in the first place. Maybe they looked at them under microscopes or
traveled back in time to see how they were formed? [sigh] Well, to get my rock collection back
in order, we'll need to figure out how Earth scientists tell the difference between the
thousands of different minerals found on Earth. By the end of this lesson, you'll be
able to explain how Earth scientists can use the physical properties of minerals to
identify and compare them. Let's get started. Before we start identifying these minerals, let's
take a moment to review just what a mineral is. A mineral is a naturally occurring solid with atoms
organized in a pattern. Minerals are like the ingredients that make up rocks and crystals, so to
identify a rock or a crystal, scientists need to know what minerals are in it. But how do you think
they identify different minerals, especially when they look so similar? Pause the video here and
record your predictions in your guided notes. Minerals have lots of different physical
characteristics, like their color, luster, crystal form, streak, cleavage and fracture,
and hardness. Scientists use these physical characteristics to identify minerals. So, let's
take a closer look at what each of these mean. Some characteristics of minerals are pretty
straightforward, like their color. Minerals can be every color of the rainbow. We can
also describe a mineral by its luster, which is the sheen or appearance
of its surface. It might be glassy, dull, metallic, pearlescent, waxy, silky,
or even greasy-looking. Unfortunately, all my minerals are the same clear color and
have the same glassy luster, so let's keep going. As we know, the atoms in minerals are organized
into unique patterns. These patterns cause each mineral to form crystals of a certain
shape. This shape is called a mineral's crystal form. It looks like two of my minerals'
crystal forms are in the shape of a rhombus, but one is hexagonal. That might help us identify
it. Let's look at their other characteristics. In some minerals, the way their crystal shape or
luster reflects light can change the way their color looks. So, to more accurately identify
minerals, scientists often use their streak. A mineral streak is the color it has when ground
into a powder. To test a mineral streak, we can rub them against a very hard surface called
a streak plate. It's the same basic idea as drawing with chalk. A thin layer rubs off onto the
surface underneath, which lets us see its color more accurately. Streak can be really helpful
for identifying minerals. For example, check out these minerals: hematite and magnetite. They look
almost identical, right? Except magnetite has a blackish-gray streak, while, surprise, hematite's
streak is bright red. The crystal form and luster of hematite can sometimes make it look black,
which makes it much harder to identify by color alone. Checking its streak is the best way to
tell the difference between it and magnetite. But all of our mystery minerals have a white
streak, so let's check some other properties. All minerals have a property called fracture.
Fracture is when a mineral breaks in random places when a force is applied, like if it's hit
with a hammer. Some minerals also have cleavage, which is when they break along flat planes because
of patterns in their crystal form. For example, a mineral's cleavage might make it break
into smooth sheets or into small cubes. So, both cleavage and fracture describe how a mineral
will break apart. All minerals have fracture, but only some have cleavage. A mineral
with cleavage might show fracture if just a tiny piece of it breaks off, but it isn't a big
enough piece to break along the cleavage planes. So, that means all of my minerals have fracture,
but what about cleavage? The first one is cleaving into thin, flat sheets; the second is
cleaving into rhombuses, and the third one... hmm, it isn't cleaving at all.
It must only have fracture. I think we're getting close to identifying them.
Let's check out one last property: their hardness. A mineral's hardness is how resistant it is to
being scratched. Scientists use a tool called the Mohs hardness scale to rate the hardness of
minerals from 1 to 10. The higher a mineral is on this scale, the harder it is to scratch. So,
for example, a mineral with a hardness of three couldn't scratch one with a hardness of five, but
a mineral with a hardness of seven could. In fact, it could scratch any mineral with
a hardness level lower than seven. Wow, one of our mystery minerals is so soft
I can scratch it with my fingernail. So, we've gathered a ton of information by checking
out each mineral's physical characteristics, but what now? How do you think Earth scientists use
all these little details to identify minerals? Pause the video here and record
your thinking in your guided notes. Earth scientists use all the characteristics
of minerals together to identify them. They compare their observations of a mineral's unique
combination of characteristics with identification guides that describe tons of minerals to figure
it out. And now, I think we finally have enough info to identify our minerals here, too! The soft one
that cleaved into thin flat sheets is selenite. This one that's a bit harder with
rhombus-shaped crystal form is calcite. And the last one that's way harder with no
cleavage and hexagonal crystals is quartz. I'm glad to have my rock collection back in
order. Let's review what we learned along the way. Now you know that minerals have many different
physical characteristics, including their color, luster, crystal forms, streak, cleavage
and fracture, and hardness. And that Earth scientists use each mineral's unique combination
of these characteristics to identify them. Be sure to complete the practice questions and extension
activities that go with this lesson to try out identifying some minerals on your own. And always
remember, in Earth science, as in life, you rock!
[electric guitar plays] [electric guitar plays]
See you next time!
