Is a Crystal a Rock?
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Contents 1. Crystals are Not Rocks |
Crystals are Not Rocks
The difference between crystals, rocks and minerals causes a great deal of confusion. These three naturally occurring solids are not the same, so this article explains what makes each one different.
If you're looking for a quick answer, here it is:
Crystals are solids with an orderly, repeating internal structure. They're not rocks.
Minerals are naturally occurring solids with a crystalline structure.
Rocks are made from at least one mineral, but most are made from several.
For a more detailed explanation, read on.
What are Crystals?
The following two photos show crystals in the geological sense. The first mineral is quartz and the second is fluorite.
Although crystals are the most obvious feature, these two pieces would usually be described as minerals. A mineral is the material itself, regardless of whether it occurs as one crystal, many crystals or in a form where no crystals are visible.
The words 'mineral' and 'crystal' describe different things. A mineral is a naturally occurring substance defined by its chemical composition and internal structure. A crystal is a solid in which atoms are arranged in an orderly, repeating pattern.
When a mineral has enough space to grow freely, this internal structure can produce visible crystal faces. These can form many different shapes, including cubes, prisms and hexagonal forms.
A mineral can therefore appear in many forms. Quartz can grow as visible crystals, often as elongated six-sided points, but it can also occur in massive form, where individual crystals cannot be distinguished with the naked eye. Rose quartz is an example.
Quartz can also occur as tiny grains within a rock. However it appears, quartz remains the same mineral.
In recent years, the use of the word 'crystals' to describe rocks and minerals used for their healing properties has added to the confusion. I'll return to this later in the article.
What is a Mineral?
A mineral is a naturally occurring, inorganic solid with an internal crystalline structure. Inorganic means that it has not been produced by living matter.
Quartz and fluorite are minerals, but pearls are not. Pearls are produced by marine oysters and freshwater mussels, which are organisms.
Wood is not a mineral because it's organic, but petrified wood is made largely from minerals. During petrification, the original material is gradually replaced or filled by minerals such as quartz, calcite and pyrite, or occasionally by opal, which is a mineraloid.
How Do Minerals Form?
Minerals can form in several ways. Many crystallise as molten rock cools above or below Earth's surface. Others form when dissolved substances separate from water.
The mineral halite forms when salt water evaporates. As the water disappears, dissolved sodium and chlorine crystallise to form cube-shaped crystals of halite, the mineral we know as salt.
Some minerals can form in a few days, while others take millions of years.
All crystals belong to one of seven crystal systems, but they can grow into many different external shapes depending on the conditions and the space available.
Fluorite often forms easily recognisable cube-shaped crystals. In other minerals, the crystals are far too small to see. Red jasper is made from microscopic crystals and is described as microcrystalline or cryptocrystalline.
Crystals described as macrocrystalline are large enough to be seen with the naked eye.
Crystals are Made Up of Atoms
A crystal is a highly ordered, repeating, three-dimensional arrangement of atoms.
It can take billions, trillions or even quadrillions of atoms to form a single crystal. The way these atoms are arranged is known as the crystal structure.
Crystals exhibit symmetry because their atoms are arranged in repeating geometric patterns. The external shape produced as a crystal grows is known as its crystal habit.
Fluorite commonly forms cube-shaped crystals, while snowflakes are composed of hexagonal, six-sided ice crystals.
The following image shows an example of a highly ordered, repeating arrangement of atoms.
How Does a Crystal Structure Form?
Crystals often form through a process called crystallisation. As a material crystallises, its atoms or molecules arrange themselves into an orderly, repeating pattern.
The smallest repeating part of this structure is known as a unit cell. Repetition of these units in three dimensions produces the crystal lattice.
The structure and symmetry of the lattice influence the external shape of a crystal as it grows.
What Does Amorphous Mean?
All matter is composed of atoms, but the atoms in a solid are not always arranged in an orderly, repeating pattern. A solid without this long-range crystalline structure is described as amorphous or non-crystalline.
The word 'mineraloid' is used for certain naturally occurring materials that do not meet all the requirements of a mineral. Obsidian and opal are familiar examples. Glass is also amorphous, but because ordinary glass is manufactured, it's not a mineraloid.
Is a Mineral a Rock?
No. A mineral is not a rock. A rock is a naturally occurring solid made up of one or more minerals.
Most rocks contain several minerals, but some consist predominantly of just one. These are known as monomineralic rocks.
Examples include marble, which consists predominantly of calcite, and quartzite, which consists predominantly of quartz. Some types of limestone are also composed almost entirely of calcite.
There are thousands of recognised mineral species, but the vast majority of rocks are made primarily from a relatively small number of common rock-forming minerals. These include feldspars, quartz, amphiboles, micas, olivine, pyroxenes and calcite.
Minerals can also contain small amounts of other elements or materials. These can affect their colour. The purple colour of amethyst, for example, is associated with trace amounts of iron within its crystal structure.
An impurity is not the same as an inclusion.
What Makes Something a Rock?
A rock is a naturally occurring solid aggregate of one or more minerals or other geological materials. The minerals present and the way they're arranged help geologists identify and classify the rock.
In geology, the word 'texture' describes features such as the size, shape and arrangement of the grains or crystals. It doesn't mean how the rock feels when you touch it.
When identifying a rock, some of the features geologists look at include:
- Which minerals are present?
- How are they arranged?
- Are the crystals or grains large or small?
- Are they all roughly the same size?
- Are the grains rounded or angular?
- Are layers visible?
- Are the mineral crystals interlocking?
The Three Main Types of Rock
The three main types of rock are sedimentary, igneous and metamorphic.
Sedimentary rocks form from sediment that accumulates and eventually becomes compacted and cemented together. Some sedimentary rocks contain material that originated from living organisms. Coal, for example, formed from plant material, while some limestones contain large quantities of shells and skeletal remains from marine organisms.
Igneous rocks form when molten rock cools and solidifies. Magma that cools beneath Earth's surface forms intrusive igneous rocks, while lava that cools at the surface forms extrusive igneous rocks.
The rate of cooling affects the size of the crystals that form. Magma cooling slowly beneath Earth's surface gives crystals more time to grow, so they can become relatively large. Lava cooling rapidly at the surface produces much smaller crystals.
If molten rock cools extremely quickly, there may not be enough time for an orderly crystalline structure to form. This can produce volcanic glass.
Obsidian is one type of volcanic glass. Although it's a naturally occurring geological material, it lacks a crystalline structure and is therefore classed as a mineraloid rather than a mineral.
Metamorphic rocks form when existing rocks are altered by heat, pressure and chemically active fluids without completely melting. I discuss metamorphic rocks in more detail here.
How to Identify Rocks
Why are Rocks and Minerals Called 'Crystals'?
In recent years, the word 'crystals' has become a blanket term for rocks, minerals and other decorative materials, which may or may not be natural.
I've explored how the use of this word developed, the influence of the New Age movement and the origins of ideas about crystal vibrations in Why are Rocks and Minerals Called 'Crystals'?
Article Pictures
Quartz crystals var. amethyst, fluorite, halite - Courtesy of Stan Celestian.
Red Jasper - Courtesy of James St.John.
Photos are clickable and redirect to the original image.
"How to Identify Rocks" courtesy of Bill Langer, U.S Geological Survey.
Pop-up photos: Quartz, amphibole (on calcite) - Courtesy of Ron Wolf. Fluorite - Courtesy of Steve Blyskal.
Calcite, pyrite, opal, marble, limestone, olivine - Courtesy of Stan Celestian.
These links redirect to the original images on the photographic website Flickr. You may only be able to view them if you join. Joining Flickr is free, with no additional charges.
The remaining pieces in the pop-up photos come from our collection.
