Hi Mohamed,

A mineral is said to be solid and homogeneous and is said to be having a fixed chemical composition or a composition varying within certain limits. What you have said in the background to the question is acceptable. A mineraloid is not crystalline like glass, and sometimes mineraloids may have short-range (across a few angstroms) internal order of atoms whereas a mineral is having long-range internal order. In mineralogy texts, Opal is said to be one of the best examples of a mineraloid, and some 'gel-minerals' formed during chemical weathering of rocks at low P and T are mineraloids too. Although we would like a mineral to be physically and chemically homogeneous, that can be expected in minerals which happened to be in equilibrium with the environment where it formed. However, many minerals are physically and chemically zoned, the zoning resulting from the mineral striving for equilibrium under changing physical conditions like P, T during formation.

LRK Perera

Dear Dr. El Nady:

The key elements of the definition of mineral are:

1. Inorganic chemical compound or element

2. Homogeneous as to its crystal structure, chemical composition, physical properties

3. Natural occurrence

There are chemical compounds which have not yet achieved fully ordered X structures or show complete homogeneity and as such may be called “mineraloids” a term which is rarely used in practice.

Today there is much more concern about chemical compounds in the grey zone between natural and artificial or man-made , e.g., chemical compounds formed during supergene alteration as post-mining minerals, in tailing ponds during processing of ore minerals and dumps. Not all of them are generally agreed and accepted by geoscientists as minerals sensu stricto (Nickel, 1995).

See: Nickel, E.H., 1995, Definition of a mineral. Mineralogical Magazine, v. 59, p. 767-768.

Using the term “mineraloid” for opal or other pre-stages of minerals such Al-Si compounds in the field of clay minerals I would not recommend.

In this case it is much better to describe the substance as precisely as possible.

Best regards H.G.Dill

The scientifically renowned and globally known internet platform mindat.org gives the following definition of the term "mineraloid" (quotation):

"Definition of mineraloid

A mineral-like naturally-occurring geological material which is non-crystalline and not definite enough in chemical composition or in physical properties to be considered a mineral. Hydrocarbons, volcanic glass, and palagonite are classed as mineraloids."

Also, I strongly advise against using the term "mineraloid". Instead, any unknown substance should be studied and characterized as carefully as possible.

Best regards

Guenter Grundmann

Definition of a mineral and mineraloid is man-made. The nature provides the chemistry, physico-chemical conditions (geological processes) and the geological time for these chemical constituents (atoms) to order themselves to form definite structures. While trying to coin the terms mineral or mineraloid we scientists place more emphasis on the orderly arrangement of atoms knowing the fact that some geological processes lasted only for a very short time (eg. volcanic glass, clays, limonite like stuff), compared to others, hindering long-range ordering of atoms. The mineraloids did not have a fair-share of geological time to put the atoms in order although those are also naturally occurring, solids formed by inorganic geological processes. We have been unfair towards the mineraloids.

The International Mineralogical Association (IMA) decides what a valid mineral species is, and what is not.

The IMA is a non-profit scientific association aiming to support and promote the science of mineralogy. The IMA was founded in 1958 and currently consists of 40 members, national mineralogical companies and groups.

The IMA describes itself as the only truly international organization promoting the worldwide cooperation of the Mineralogical Institutes. The work of mineralogy is one of the oldest branches of science. It is intended to support the ore, gemstone, petroleum, and industrial mineral processing industries, and to bring them increasingly into the public eye.

One of the most important commissions is the Commission on new Minerals, Nomenclature and Classification (abbreviation CNMNC), which aims to normalize and standardize the nomenclature and information on the more than 5380 currently valid mineral species.

Best regards,

Guenter Grundmann

El mineraloide tiene composición química definida pero no tiene cristalización

The difference between a mineral and a mineraloid is simply related to their internal atomic arrangement. Minerals have an ordered long range internal atomic arragement or in other words minerals have a crystalline structure. But, mineraloids do not possess a long range internal atomic arrangement (ex: Volcanic glass- Obsidian). This is mainly due to the rapid cooling of magma or the mineralizing fluids.

Mineral is defined as a naturally occurring, inorganic, solid, crystalline substance which has a fixed structure and a chemical composition which is either fixed or which may vary within certain defined limits.

Mineraloid is defined as a mineral-like naturally-occurring geological material which is non-crystalline and not definite enough in chemical composition or in physical properties to be considered a mineral.

Recommend Harald G. Dill answer

A mineraloid is a mineral-like substance that does not demonstrate crystallinity. Mineraloids possess chemical compositions that vary beyond the generally accepted ranges for specific minerals. For example, obsidian is an amorphous glass and not a crystal.

A mineral is a naturally occurring chemical compound,