When many different crystals grow near each other, they mesh together to form a conglomerated mass. This is the case with most rocks, such as granite mentioned above, which is made up of many tiny mineral crystals. The museum-quality specimens shown in the images here grew in roomy environments that allowed the geometric shapes to form uninhibited. Light interacts with different atoms to create different colors.
Many minerals are colorless in their pure state; however, impurities of the atomic structure cause color. Quartz, for example, is normally colorless, but occurs in a range of colors from pink to brown to the deep purple of amethyst, depending on the number and type of impurities in its structure.
In its colorless state, quartz resembles ice. In fact, the root for crystal comes from the Greek word krystallos-ice-because the ancient Greeks believed clear quartz was ice frozen so hard it could not melt. In subterranean gardens, they branch and bristle as trillions of atoms connect in regular three-dimensional patterns. Each crystal starts small and grows as more atoms are added. Many grow from water rich in dissolved minerals, but they also grow from melted rock and even vapor.
Under the influence of different temperatures and pressures, atoms combine in an amazing array of crystal shapes. It is this variety and perfection of form and symmetry that has long drawn scientists to the study of minerals.
Symmetry is a regular, repeated pattern of component parts. Symmetry is everywhere in nature-the paired wings of a butterfly, the whorls and petals in a sunflower, the pattern of a snowflake, the legs of a spider-and minerals are no exception. In crystals, these repeated patterns occur within the basic atomic structure and reflect the pattern of faces of the crystal. You often can see the characteristic symmetry of a mineral crystal with the naked eye, but if the crystal is tiny, then you may need to look at it with a magnifying glass or microscope as will be demonstrated in Lesson Plan 2.
Recognizing symmetrical patterns in crystals may be difficult at first, but experience helps: the more specimens you look at, the more symmetry-and crystals-you will recognize. However, some specimens do not have well-formed crystals and are difficult even for experts to classify. A crystal is a solid where the atoms form a periodic arrangement. Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure.
Most macroscopic inorganic solids are polycrystalline, including almost all metals, ceramics, ice, rocks, etc. Solids that are neither crystalline nor polycrystalline, such as glass, are called amorphous solids, also called glassy, vitreous, or noncrystalline. These have no periodic order, even microscopically.
There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does. A crystal structure an arrangement of atoms in a crystal is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement.
The unit cells are stacked in three-dimensional space to form the crystal. The symmetry of a crystal is constrained by the requirement that the unit cells stack perfectly with no gaps. There are possible crystal symmetries, called crystallographic space groups. These are grouped into 7 crystal systems, such as cubic crystal system where the crystals may form cubes or rectangular boxes, such as halite shown at right or hexagonal crystal system where the crystals may form hexagons, such as ordinary water ice.
Crystals are commonly recognized by their shape, consisting of flat faces with sharp angles. These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is often present and easy to see. Euhedral crystals are those with obvious, well-formed flat faces.
Anhedral crystals do not, usually because the crystal is one grain in a polycrystalline solid. The flat faces also called facets of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They are planes of relatively low Miller index. This occurs because some surface orientations are more stable than others lower surface energy.
As a crystal grows, new atoms attach easily to the rougher and less stable parts of the surface, but less easily to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces.
See diagram on right. Different minerals in the same crystal can happen when the cavity closes and reopens. This is where you get gorgeous examples of layered colors and textures like in watermelon tourmaline or fluorite. Gemstone inclusions can also happen with the opening and closing of cavities. Sometimes, a new crystal will start to grow on an already established stone and then the growing process will be halted. Then, if the environment supports it, the older crystal will start to regrow and essentially swallow up the new crystal.
Two different minerals may also start to grow at the same time but flourish at different speeds which also ends up in one crystal overtaking and engulfing the other.
You can see this in emeralds for example which have pieces of pyrite inside. Temperature and environmental changes can also cause some minerals to have impurities which then go on to crystallize themselves. This is when the main mineral acts like a cavity itself and the impurities crystallize and grow within the walls and space of that mineral. For example, this is how you get rutile stashed inside of a quartz crystal or corundum. Impurities can make all the difference within crystals.
For example, a perfect diamond would only contain carbon atoms and maybe a few boron atoms , but if impurities or the 'wrong' atom is found, it can change the diamond and these impurities can even be one of the only things that separates a ruby from a sapphire. Another natural phenomenon that can occur is something called phantoms.
Ghostly and glorious, this is when a transparent crystal gets a new layer from another crystal growing over it. This setup can be quite rare as the old transparent crystal may also continue growing and can form a new layer of quartz, leaving a ghostly or shadowy specter of the other crystal sitting beneath the surface. Fire, energy, cracks, collapses — it all adds up to a tumultuous place to be.
This also makes for a challenging place for crystals to grow and means that many crystals end up broken or cracked in the process. Sometimes, materials can seep into these cracks and fissures which essentially heal the crystal and bond it once more. It creates a binding space for the crystal to fuse and grow together again. These healing fractures etched across the crystal are also called fingerprints by geologists.
After studying the secrets that sit beneath the earth, geologists have a pretty good handle on how gems form. This is super useful for recreating certain environments within the lab so that conditions can be mimicked when creating lab-grown gems.
Traditionally, are three different processes for a rock formation. What could be a seed of a crystal can branch and stack and stretch and bloom as all the atoms connect and grow. This process can come from fire but also the depths of water, the steam of gas and vapor, and melted rock. Some crystals need both heat and water to form such as quartz, which is one of the most well-known and beloved kinds of crystals out there. Quartz is often one of the last minerals to crystallize and it kind of fills in the gaps in spaces where other minerals have formed.
When formed in rock cavities it tends to create hexagonal crystals but when formed deep within the earth it will be smaller and in rounder masses. Quartz is often colorless but takes its color from reflecting the surrounding minerals.
For example, Amethyst takes its beautiful purple hue from the inclusion of iron oxide or maybe manganese and the citrine stone gets its golden sunshine shades from Amethyst being overheated. Another interesting element of crystal growth is that there are no restrictions on how big a single crystal can grow.
One of the largest crystals in the world can be found sitting beneath a Mexican town. The huge hunk of Selenite is 12m long and has an eye-watering diameter of 4m too. It is believed to be half a million years old, a reminder that with enough space and time, large crystals can flourish without restrictions.
Crystals are minerals that have grown into regular, geometric solids bounded by flat faces! Crystals are unusual!
In most cases, minerals grow into irregular, random shapes! Many substances are composed of atoms that are randomly arranged glass is an example. Such substances are called 'amorphous'! But in minerals, the atoms are arranged in an orderly fashion! They form a regular, repeated pattern, like old-fashioned wallpaper! Such substances are called 'crystalline'!
But if all minerals are crystalline, why aren't they all crystals? I mean, look at the mineral grains in this enlarged picture of a rock! Most of them don't have regular shapes.
They are highly irregular!
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