FMF - Friends of Minerals Forum, discussion and message board

[Ashleysara]

what is the difference between a mineral and a crystal?

[Jesse Fisher]

A mineral is a naturally occurring crystalline substance. Crystalline meaning that it has a defined internal structure and chemical makeup. A crystal must have the defined structure and chemistry but can be either man-made or natural. Man-made crystals such as the iridescent hopper-growth bismuth commonly seen these days are called "synthetic" to differentiate them from naturally occurring crystals/minerals.

[Roger Warin]

Hello,
The difference between a mineral and a crystal is structural. Most minerals are crystals, even microcrystals of quartz in agate. But all minerals are not crystals, because they remained amorphous. For example, the opal has only very partial internal order, just enough to give opalescence. Opal is not crystalline because its chemical composition varies, an amount of water is variable.
Pure sulfur gives crystals on cooling. If this phenomenon occurs in the vicinity of a volcano, it’s a natural phenomenon. If melting followed by crystallization takes place in a lab, sulfur crystals are identical to natural crystals, but they say they are synthetic.
It is the same for bismuth. In nature there are natural bismuth native crystals. But if in your garage, you melt bismuth and if you plunge a large bolt in the melt, crystallization takes a minute and you get skeletal crystals shaped hopper. Cooling was rapid. It is said that these crystals are synthetic.
In fact, the internal arrangement of atoms is the same in natural and synthetic crystals.
Minerals are often combinations of metals and non metals.
The salt is sodium and chlorine (ionized form). The proportions of the atoms are balanced and consistent. It is a pure product. It’s salt (NaCl).
It is found crystallized in mines in Poland, for example, but also crystallized from seawater. Both salts are chemically identical, but one is natural and the other is artificial.
There are also natural glasses that are not crystals, such as obsidian or feldspar glasses.
Roger.

[Peter Perkins]

Think of a crystal as a manifestation of a mineral. A mineral may take the form of a crystal - in fact, it does so more often than not.

[Riccardo Modanesi]

Hi to everybody!
A synthetic mineral takes all of the physical and chemical properties as its natural correspondent! For example a synthetic emerald IS mineralogically an emerald, the only difference is it's made in a lab and not growm in a rock. Therefore the inclusions (microscopical characters) are different: for example the synthetical gemstones take for example crucible residuals, liquid chemical substance residuals, growth inhomogenities, etc. and the natural ones take for example mineral inclusions, liquid rock inclusions (different from the artificial ones! The latter ones are made by metallic chemical substances!), etc. An exception is given by phenakite crystals which are found sometimes in synthetic beryls, and by some hematite crystals which appear sometimes in synthetic corundums (rubies, sapphires etc.). However there is a big big scientifical research about. try to ask yourself: why did scientist understand "this mineral has grown in these temperature and pressure conditions" and therefore "at this earth depth"? Did anyone see minerals growing at that temperature and depth? Did anyone go deeply inside the earth crust and pick this mineral up? Certainly not! The reason why it was understood is someone tried (and succeeded many a time) to reproduce several kinds of minerals synthetically, history is full of those discoveries!
Greetings from Italy by Riccardo.

[Roger Warin]

Hello,
Thank you Riccardo to clarify and complete my remarks.
In reality, Nature is complex because there is no other process of purification of the material that successive crystallizations from igneous rocks. Hydrothermal waters also play an important role. Minerals crystallized keep track of all this information, even providing us their birth certificate and their age.
A previous post showed how there was a competition between descloizite, duftite (group adelite-descloizite), and mottramite with which descloizite form a series. Thereupon were deposited crystals of Wulfenite.
These crystallization sequences are useful in modern geology to complete the ancient knowledge.
Roger.

[Jesse Fisher]

Naturally occurring amorphous substances such as opal are referred to as "mineraloids." It is my understanding that anything defined as a "mineral" needs to have an ordered crystalline structure, which will give a unique "fingerprint" by x-ray diffraction. Obsidian is a volcanic glass, and is classified as a rock and not a mineral or mineraloid. If allowed to cool slowly enough to crystallize, it would be a mixture of quartz, feldspar and possible a few Fe-bearing minerals.

[Larry Laskowski]

I enjoy reading this post, very informative, great reading.

[Roger Warin]

Nothing is simple in geology. Words varying with time and schools. Mine are rather those of a chemist.
I wanted to emphasize the difference (I see) between a mineral crystal and a mineral species (inorganic compound).
A crystal is a regular 3-D structure of atoms or molecules. This is often a pure mineral species. But the crystalline pattern can also be an organic molecule like sugar, aminoacids, etc. The crystal is characterized by a crystal lattice with its symmetry determined first.
Mineral (and this is how I see it every day) is crystalline. But in fact it can be amorphous or glassy without being a rock. Plagioclase (Ca-Na-feldspar) can be glass under the effect of a shock. This is also a metastable state. Over time it can be transformed into crystal.

[cascaillou]

General definition of a mineral: an homogeneous natural inorganic solid which can be defined by its chemical composition and its crystallographic structure

(with the exception of liquid native mercury and organic minerals, which are still considered minerals for some reasons which we don't need to debate here)

chemical composition: constitutive elements (as atoms, ions or molecules)

crystallographic structure: there are crystalline minerals and amorphous minerals.
Crystalline minerals show a regular (periodic) geometrical arrangement of their constitutive atoms, ions or molecules, that is to say that they have an internal crystalline structure (which is based on any of the seven crystal systems).
On the other hand, amorphous minerals have no crystalline structure (their constitutive elements are randomly arranged).

Only crystalline minerals can express themselves in the form of a crystal, amorphous minerals can't.

In the right environnemental conditions, a crystalline mineral will grow as a crystal (with geometrical forms), but otherwise it will occur in massive form (showing no visible crystal forms, despite of its internal crystalline structure).

Note that 'massive' is not a synonym for 'amorphous'. Massive means that there are no visibly expressed crystal forms, but that doesn't necessarly mean that the mineral doesn't have an internal crystalline structure.

Rock: a rock is a natural solid aggregate of one or more minerals.