Is a Crystal a Rock?
| Contents 1. Crystals are Not Rocks 2. So What are Crystals? 3. Crystals Made Up of Atoms 4. Is a Mineral a Rock? 5. What is a Rock? 6. Metamorphic Rocks 7. The Word 'Crystals' 8. Article Pictures |
Crystals are Not Rocks
There is so much confusion about the difference between crystals, rocks and minerals. These three naturally occurring solids are not all the same, and I hope this article will help you understand each one.
If you're looking for the answer to one of these questions you'll find it here;
- Are crystals rocks?
- What are minerals?
- What are rocks?
If you don't want to read the entire article and want a quick answer here it is;
Crystals are made up of atoms so they're not rocks.
Minerals are made up of crystals so they're not rocks either.
Rocks are made up of several different minerals.
For a more detailed explanation, read on.
If Crystals are Not Rocks What are They?
The following two photos are 'crystals' in the geological sense of the word. The mineral in the first photo is quartz, the second is fluorite.
Although the crystals are the most obvious feature, these two naturally occurring solids would still be called minerals. 'Mineral' is the object as a whole.
While the crystals in this quartz and fluorite mineral are large enough to be seen, they may not be in other minerals.
The words 'mineral' and 'crystal' describe different parts of what is essentially the same thing.
What has made matters more confusing in recent years is the use of the word 'crystals' by those who use rocks and minerals for their healing properties. I'll talk more about that later.
A mineral is a naturally occurring solid that forms over millions of years. Quartz exhibits a crystalline structure, which means it's made up of crystals.
A crystal refers to the highly ordered repeating arrangement of atoms within a mineral. The crystal is the external shape of the internal arrangement of atoms.
Quartz commonly forms hexagonal prismatic crystals with pointed terminations. These can be seen in the first photo.
Fluorite often exhibits cube-shaped crystals which can be seen in the second photo.
The crystals in these two minerals are clearly visible.
Red Jasper is one of many minerals whose crystals can only be seen when viewed through a powerful microscope. The term used to describe these kinds of minerals is either microcrystalline or cryptocrystalline.
Crystals Made Up of Atoms
For a naturally occurring solid to be crystalline (made up of crystals), it must have a highly ordered arrangement of atoms that repeat in a three-dimensional pattern.
It takes billions of atoms to form one single crystal. The way atoms come together is known as the crystal structure.
There are seven crystal structures. The external shape of the crystal is defined by the internal arrangement of atoms or the pattern of the crystal structure.
In the photos above, you can see crystals in two different shapes. This is because of the internal arrangement of atoms.
All crystals exhibit symmetry because they're made up of repeating geometric (regular lines and shapes) patterns.
The following image is an example of a highly ordered repeating arrangement of atoms.
The final form a crystal takes is known as its habit. Cube-shaped crystals also known as isometric are one of the most common and simplest shapes.
Salt is made up of isometric crystals. A snowflake is made up of a hexagonal or six-sided crystal.
Crystals often (but not always) form when liquids cool and solidify. This process is called crystallisation. As the arrangement of atoms grows, the crystal structure forms. The mass then becomes a crystalline solid.
Crystals are not rocks because they're made up of individual units of atoms all of which are identical. These structures are called unit cells. A single crystal can be made up of just a few or billions of unit cells.
The cell is reproduced over and over again in all directions. This is the internal structure of the crystal. It's known as the crystal lattice.
The shape of the unit cell and the symmetry of the lattice determine the external shape of crystal faces.
All states of matter which includes virtually everything in the universe are made up of atoms. The three states of matter are gases, liquids and solids.
For a solid to be crystalline the atoms must come together in a highly ordered repeating pattern. Where this doesn't happen the substance is non-crystalline. The term mineraloid or amorphous is then used instead.
In an amorphous solid atoms do not come together in a highly ordered arrangement.
Is a Mineral a Rock?
A mineral is an inorganic solid with a crystalline structure (made of crystals). Inorganic means it does not come from or has been formed by living matter.
Quartz and fluorite are minerals but pearl is not. Pearl is produced by marine oysters and freshwater mussels, which are organisms. An organism is an animal, plant or single-celled life form.
Wood is not a mineral because it's organic, but petrified wood is. That's because the organic matter in the original structure has been replaced with quartz, calcite, pyrite or occasionally opal. With the exception of opal, these materials are crystalline so they're minerals.
Opal has no crystalline structure so it's classified as a mineraloid or amorphous solid.
Minerals form when molten rock (magma or lava) cools above or below Earth's surface. They can also form by separating from water rich in dissolved minerals.
The mineral halite forms when salt water evaporates. The dissolved minerals then crystallise to form a substance used to flavour food.
The cube-shaped crystals of halite are soft and dissolve quickly. They're better known as salt.
Another well-known mineraloid is obsidian.
Obsidian forms when a specific type of lava cools on the surface of the earth or beneath the ocean. Obsidian is non-crystalline because the lava cools too fast for crystals to grow.
A mineral is not a rock because rocks are made up of several minerals. Rocks made up purely of one mineral do not exist. Some contain about 90% of just one mineral, but this type of rock is relatively rare. Known as monomineralic rocks, the most common are marble, quartzite and limestone.
There are approximately five thousand minerals on Earth yet the vast majority of rocks are formed from a combination of some of the most common.
Known as rock-forming minerals, they include feldspars, quartz, amphiboles, micas, olivine, garnet, calcite and pyroxenes.
Minerals are naturally pure but can contain traces of other minerals or substances. An impurity will often cause a material to change colour.
An impurity is different to an inclusion.
So, What Exactly is a Rock?
Rocks are a combination of different minerals that come together through various geological processes. The specific chemical composition determines the type of rock that forms.
Unlike minerals, some rocks can be organic meaning they're made of materials that were once part of a living organism. Only sedimentary rocks can be organic. An example is coal which formed over millions of years from compressed plants.
Sedimentary rocks often contain shells and skeletons of marine organisms whose remains accumulate as sediment. Over time, the sediment becomes buried, compaction squeezes out any water and cementation gradually binds layers together. The hardened mass then slowly transforms into sedimentary rock.
The three main rock types are sedimentary, igneous and metamorphic. Sedimentary rocks form from sediments and particles which can be organic or inorganic.
Igneous rocks form from molten rock that cools and solidifies above or below Earth's surface. Molten rock is a mixture of minerals and dissolved gases. Because of immense heat and pressure, it remains fluid.
Rocks formed as magma cooled and solidified beneath the Earth's surface are known as intrusive igneous rocks. Those formed from lava expelled from the vent of a volcano are extrusive rocks.
The size of crystals that form is determined by the length of time it takes for the magma or lava to cool and solidify. Where this happens over thousands to millions of years, crystals have plenty of time to grow so will be very large.
When lava cools and solidifies rapidly there is less time for crystal growth. Therefore, crystals will either be very small or non-existent.
When magma or lava cools and solidifies the minerals in the molten rock crystallise.
Crystallisation is the process by which atoms arrange themselves in a repeating, ordered pattern to form a crystal structure.
This is a fundamental process in the formation of minerals.
When minerals crystallise as magma or lava cools, the rock that forms is composed of various minerals in their crystalline form. This is why rocks are made of minerals rather than crystals.
When magma or lava cools and solidifies too quickly, crystals do not have time to grow. The result is a naturally occurring solid that lacks a crystalline structure. This material may be known as volcanic glass or an amorphous solid.
Obsidian is one type of volcanic glass, but there are many. The type that forms depends on various factors.
An amorphous solid (aka mineraloid) is a naturally occurring non-crystalline solid. It will not always be a type of volcanic glass.
An example is opal. This material, known for its 'play of colour' is classed as an amorphous solid (mineraloid) because it's non-crystalline. Opal is not volcanic glass.
Metamorphic Rocks
The rock type I haven't yet mentioned is metamorphic. This material forms from existing rocks that have undergone a substantial change caused by intense heat and pressure.
The process which mostly takes place beneath the surface is known as metamorphism. The word comes from Latin for 'changed form'.
Marble is a metamorphic rock that was originally limestone. It metamorphosed when the calcite within the limestone recrystallised and formed a much denser material. The different colours in marble are the result of impurities introduced during this process.
Lapis lazuli is often referred to as a mineral but is actually a metamorphic rock. It is made up of several different minerals and occurs when limestone goes through the process of metamorphism. Marble forms when limestone recrystallises.
Lapis lazuli forms in association with metamorphosed limestone but retains its own unique composition and characteristics.
The rich blue colour comes from the mineral lazurite but other minerals present can include pyrite, calcite, sodalite, diopside, amphibole, feldspar, mica plus others.
The Word Crystals
In recent years, the word 'crystals' has become a generic term for rocks and minerals used for their metaphysical healing properties.
It's only used by those who use these materials for healing purposes.
The word 'crystals' was popularised by the New Age movement of the 1970s and 80s. It's now widely used around the world in what has become a multi-billion dollar industry.
In the vast majority of cases the word 'crystals' is a misnomer and from a geological perspective, is often incorrect.
When used to describe rock and mineral-like solids it's not being used literally. However, it has created a considerable amount of confusion.
The word 'crystals' describes a whole host of materials most of which are visually attractive. They may or may not be crystalline in the true sense of the word. In some cases, they're even man-made. Examples include goldstone and opalite.
Whilst the rarity of these materials varies widely, it should be remembered that crystals in the true sense of the word are extremely common. They're present in ice, which is crystallised water, in sugar which is crystallised sucrose and in sand which is crystals of silicon dioxide.
Jewellery is often made of crystals. Gypsum plaster, which is used on walls, is crystalline. Tableware such as cups and plates is also produced from crystalline materials.
You may be surprised to learn that lead crystal is not crystalline. Despite the name, this material is made of glass that contains lead. It's not quartz crystal, so is not a mineral. Due to its lack of a crystalline structure, glass is an amorphous solid.
Article Pictures
The first picture in our article is the mineral quartz, variety amethyst. The second is fluorite from New Mexico. Both photos, along with the mineral halite, are courtesy of Stan Celestian.
The red jasper is courtesy of James St.John.
The obsidian was once part of our collection.
The lapis lazuli included with calcite and pyrite is a stock image.
The quartz, fluorite, obsidian and halite photos are clickable and redirect to the original image.
