3.2 How to Identify Minerals

Charlene Estrada

Identifying Minerals

Given the narrow criteria, you might think that s are pretty rare on Earth. However, our planet (and the greater universe) has thousands of unique ways to combine s into solids. Scientists have discovered over 5,600 minerals on Earth so far! We also think there are at least another thousand or more that have yet to be discovered by mineral scientists, s. And we are just beginning to explore the mineral composition of Mars and other planets in our solar system.

If there are so many minerals on Earth, how does a scientist tell one apart from another? If you found the crystal cluster pictured in Fig. 3.2.1. in a cave, how would you know whether it was and not ?

Image of a colorless cluster of crystals
Figure 3.2.1.. A cluster of colorless crystals, each with well-defined faces. What might this mineral be?

Physical Properties of Minerals

We categorize minerals based on their chemical composition (see table 3.2.1). Some minerals within the same compositional group will have similar physical characteristics, such as color or shape. These properties are usually related to the type of s within a mineral or the types of chemical bonds holding the s together.

Table 3.2.1 Main chemical groups of rock-forming minerals.
Group Name Chemical Composition Examples
Single Elements , ,
SiO2 Family , s, s
CO3 Family
SO4 Family
Oxygen ,
Sulfur Anion (S) ,
Chloride or Fluoride Anion ,

s have identified some simple physical tests (described in the above video) to determine the identity of different minerals. These tests are ideal because, prior to their use, scientists had to rely on complex (and expensive!) instruments such as s. With these physical identification tests, most minerals can be identified in the field by scientists and nonscientists alike. We will break the code for you. The physical properties of minerals include:

  • Color
  • Breakage
  • Unique Properties

Video 3.2.1. Overview of how to identify and classify minerals (7:31 min).

Below, we will explore each of these tests in more detail. You will find that some tests will be more useful than others. Which tests will you find to be the most reliable when identifying an unknown mineral?

Color

Color is one of the first things we notice about minerals, and for good reason. Some minerals are bright with reds, greens, blues, and yellows. These colors are determined by the main s in a mineral’s chemical formula. For example, copper (Cu), will cause to be blue and to be green.

Green Malachite and Blue Azurite
Figure 3.2.2. Rock containing green malachite and blue azurite.

Sometimes, minerals have trace elements within their structure that cause them to take on unique colors. These trace elements are often metals such as chromium (Cr), manganese (Mn), or titanium (Ti), and because they are present in such limited amounts, they do not appear in the mineral’s chemical formula.

There are certain minerals that are reliably a specific color. Malachite is green. Azurite is blue. Sulfur is yellow. Knowing these patterns makes it easier to identify certain minerals. However, there are a lot of minerals that take on a wide variety of colors. is a good example–depending on the trace elements hiding within its structure, it can be clear, white, pink, gray, black, yellow, or brown!

The many different colors of quartz
Figure 3.2.3. Quartz can be nearly any color as shown in these pebbles.

Streak

Both gray and red hematite leave a diagnostic reddish-brown streak.
Figure 3.2.4. Both samples of hematite (Fe2O3, gray on the left, red on the right) leave a diagnostic red-brown streak on a porcelain streak plate.

is the color of a mineral’s powder. Streak is a more reliable property than color because streak does not vary between two samples of a mineral, even when those samples are different colors (see above example). Minerals that are the same color may have a different colored streak. Many minerals, such as quartz, do not have a streak.

To check the streak, scrape the mineral across an unglazed porcelain plate. Yellow-gold pyrite has a blackish streak, another indicator that pyrite is not gold, which always has a golden-yellow streak.

Brassy yellow pyrite leaves a dark gray streak and pink rhodochrosite leaves a white streak.
Figure 3.2.5. Some minerals leave a different streak color than their bulk mineral would indicate. Left: Pyrite, Right: Rhodochrosite.

Luster

describes the reflection of light off a mineral’s surface. s have specific terms to describe luster. One straightforward way to classify luster is based on whether the mineral is metallic or non-metallic. Minerals that are opaque and shiny, such as , have a luster.

Minerals such as have a non-metallic luster, but there are still a variety of ways to describe how the light reflects off the mineral. Let’s look below:

​Table 3.2.2. Visual examples of common luster types.
LUSTER TYPE EXAMPLE
Diamond with Adamantine Luster
Figure 3.2.6. Polished diamond.
Quartz demonstrating vitreous luster
Figure 3.2.7. Vitreous quartz.
Selenite Gypsum demonstrating silky luster
Figure 3.2.8. Selenite with silky luster.
Greasy luster demonstrated by Graphite
Figure 3.2.9. Greasy graphite.
Waxy luster shown by Mimetite
Figure 3.2.10. Mimetite with waxy luster.
Dull/
Earthy, Dull luster as shown by Kaolinite
Figure 3.2.11. Kaolinite with dull luster.
Metallic luster as shown by pyrite
Figure 3.2.12. Pyrite with metallic luster.

Habit

A mineral’s is the shape or texture in a specimen. It can refer to the expression of a crystal shape or the shape of multiple crystals aggregated or bunched together. Besides color, it is often the first thing you might notice about a mineral. Crystal shapes are usually determined by the arrangement of the atoms within the crystal structure. For instance, minerals with a cubic atomic structure will have a tendency to grow into cube shapes. Table 3.2.3 displays some common mineral habits.

Table 3.2.3 Common mineral habits
HABIT TYPE DESCRIPTION EXAMPLE
Column-like with visible crystal faces
Quartz with Prismatic Habit
Figure 3.2.13. Quartz with prismatic habit.
Thin, needle-like or in clusters
Acicular Crystal Habit shown by Rutile
Figure 3.2.14. Rutile displays acicular habit.
Flat and flaky, peels apart into layers
Micaeous habit shown by muscovite
Figure 3.2.15. Micaceous habit in muscovite.
Botryoidal Bubbling or globular, circular crystals
Botryoidal habit demonstrated with hemimorphite
Figure 3.2.16. Botryoidal habit in hemimorphite.
Boxy or round with roughly equal dimensions
Equant habit shown with rounded zircon crystals
Figure 3.2.17. Round zircons showing equant habit.
Elongated but flattened crystals or clusters
Bladed crystal habit shown in Diopside
Figure 3.2.18. Bladed crystal habit in diopside.
Grainy, with no distinct crystals
Massive/Granular habit shown by cobaltite
Figure 3.2.19. Cobaltite showing massive habit.

Breakage

What does a mineral look like when it is broken? and/or describes the appearance of a mineral when a is broken from an external force such as or when you strike it with a hammer.

Video 3.2.2. This video shows what happens when you break minerals with planes of weakness (cleavage) and a mineral with equally strong bonds (fracture) (3:25).

is the tendency for crystals to break along planar surfaces that are parallel to atomic planes. It is common to observe:

Table 3.2.4 Common types of cleavage.
CLEAVAGE TYPE EXAMPLE

: 1 direction of cleavage

Muscovite demonstrating basal cleavage.
Figure 3.2.20. Muscovite demonstrating basal cleavage.
2 directions of cleavage, perpendicular
Orthoclase feldspar demonstrating 2-directional cleavage at 90 angles from one another.
Figure 3.2.21. Orthoclase feldspar showing 2-directional cleavage at 90° angles from one another.
2 directions of cleavage, non-perpendicular
Albite feldspar demonstrating 2-directional cleavage planes at non-90 angles
Figure 3.2.22. Albite feldspar displaying 2-directional cleavage planes at non-90° angles.
: 3 directions of cleavage, perpendicular
Halite demonstrating cubic cleavage
Figure 3.2.23. Halite showing cubic cleavage.
: 3 directions of cleavage, rhombohedral
Calcite demonstrating rhombohedral cleavage.
Figure 3.2.24. Calcite demonstrating rhombohedral cleavage.
: 4 directions of cleavage (rare)
Fluorite demonstrating octahedral cleavage.
Figure 3.2.25. Fluorite showing octahedral cleavage.

Some minerals do not break along smooth planes at all! Such a tendency for a mineral to break unevenly is called Metals usually fracture into jagged edges. Some minerals, such as , form smooth curved surfaces when they fracture. This special type of breakage is called , and it is also seen in rocks such as and .

Conchoidal fracture demonstrated by quartz.
Figure 3.2.26. Conchoidal fracture in quartz.

Hardness

Mineral is the mineral’s resistance to being scratched. The hardness of the mineral can be defined by a scale of relative hardness, called the , it goes from 0 (softest) to 10 (hardest). You can test the hardness of a mineral using everyday objects like a penny, fingernail, nail, glass, or file to find out where on the hardness scale the mineral lies.

Mohs Hardness Scale of Minerals: From 1 (softest) to 10 (hardest), the order goes: talc, gypsum, fingernail (2.5), calcite, penny (3.5), fluorite, apatite, glass plate (5.5), feldspar, steel file (6.5), quartz, topaz, corundum, diamond.
Figure 3.2.27. Mohs hardness scale of minerals. Friedrich Mohs created and named the hardness scale.

If you can scratch the mineral with the object, then the object is harder than the mineral. If the mineral can scratch the object, then the mineral is harder than the object. You can also use other minerals to assign a hardness number to a mineral. In other words, we assign a hardness number to a mineral by making relative observations!

Unique Properties

Some minerals have unique properties that make them much more easily identifiable. Some glow brightly under UV light; others are magnetic. Some fizz when dilute acid is dripped onto their surfaces, and some are explosive in contact with water! There’s a wide range of unusual and interesting things that these minerals can do!

Magnetism

Certain iron-bearing minerals can be strongly or weakly magnetic. The most well-known of these is the mineral for which this property is named: .

Magnetite, or lodestone, attracting iron filings
Figure 3.2.28. Magnetite, or lodestone, attracting iron filings

Fluorescence

When specific minerals are exposed to high-energy light, the atoms within them are excited to a higher energy state. As those atoms relax back down to their default state, they’ll emit light at a new wavelength, which can mean that we’ll see beautiful and bright neon colors! This neat visual property is called .

Minerals glowing bright, neon colors under UV light, or "fluorescing"
Figure 3.2.29. Minerals glowing bright, neon colors under UV light, or “fluorescing.”

Effervescence

is a mineral’s reaction to acid. Geologists can often identify certain minerals by dripping small amounts of dilute hydrochloric acid (HCl) on them. This is called the “acid test”. We often see the strongest reaction from the mineral .

Video 3.2.3. Watch the fizzing reaction between a drop of HCl with the calcium carbonate in calcite (0:20).

[Video Description: Hydrochloric acid is dripped onto the white calcite mineral and it begins to loudly fizz, foam, and bubble. Some smoke also begins to rise from the mineral as a very tiny amount of it dissolves.]

The reaction in the above video occurs because the carbonate group in (CaCO3) easily dissolves into carbon dioxide (CO2) gas, which you can see by the fizzing and bubbling at the surface. Sometimes, depending on the purity of the mineral, you might even see vapor or smoke rising from the fizzing mineral!

This test is extremely useful for geologists because only carbonate materials will strongly react to the acid. Geologists can also identify some and rocks that contain these very same minerals based on this test.

Optical Properties

Some minerals can be completely transparent, but even if we can view images through certain crystals, they can appear different or distorted! Calcite is well-known to “double” images if you look through clear, transparent samples of this crystal. This property is called ““.

Doubling of images with transparent calcite, called double refraction
Figure 3.2.30. Doubling of images with transparent calcite, called double refraction.

ulexite can transmit an entire image when polished. This unique ability to transmit images has given ulexite the nickname “TV Stone”.

Polished ulexite demonstrating image transmission
Figure 3.2.31. Polished ulexite showing image transmission.