Glossary of Rock and Mineral Terms - I

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ice age

A period during which the Earth is substantially cooler than usual and a significant portion of its land surface is covered by glaciers. Ice ages generally last tens of millions of years.

ice cap

An alpine glacier that covers the peak of a mountain.

icosahedron

Twenty sided polyhedron; all sides are equidimensional. Minerals shaped as icosahedrons belong to the isometric system.

icosi-tetrahedron

Synonym of trisoctahedron

idiochromatic

Exhibiting only a single color. Minerals that are monochromatic occur in only one color, no matter what specimen. Same as monochromatic.

igneous

Of volcanic origin.

igneous rock

A rock made from molten (melted) or partly molten material that has cooled and solidified.

impurity

An item present in a mineral which is not part of its integral structure, and may change its optical properties, such as color.

inclusion

Materials that are locked inside a mineral as it is forming.

incrust

To form a crust over.

incrustation

1. A pseudomorph occurring when a mineral forms a coating over another mineral, and the coated mineral dissolves. This leaves a hollow cast of the mineral that coated the dissolved mineral.
2. A crusty coating.

index of refraction

Synonym of refractive index

index fossil

The fossil of an organism known to have existed for a relatively short period of time, used to date the rock in which it is found.

indiscernable

Not noticeable.

infiltration

A pseudomorph occurring when some atoms of a mineral are replaced by different atoms forming another mineral.

inorganic

Not containing any organic substances.

inorganic chemical sediment

Sedimentary rocks - such as limestone, chert, and evaporates - that forms when the dissolved products of chemical weathering precipitate from solution. The ornate columns of travertine in caves is one example of an inorganic chemical sedimentary rock.

inosilicates

Group of silicate minerals that have their tetrahedrons form single or multiple chains, with two oxygen atoms of each tetrahedron part of its neighboring tetrahedron forming long, thin, chains.

inselberg

A steep ridge or hill left when a mountain has eroded and found in an otherwise flat, typically desert plain.

irridium

A rare earth metal often alloyed with Platinum for jewelry at either 5 or 10 percent to raise the its hardness. Interestingly, Iridium is thought to be more common in extraterrestrial matter than on Earth, and is the most important piece of evidence linking the extinction of the dinosaurs with an asteroid strike because of its high concentration in the K-T boundary which occurs in the rock formations from the end of the Cretaceous Period.

insoluble

Not able to be dissolved.

intaglio

Gemstone carved into a cameo, but instead of the engraving raised above the background, as in the cameo, the engraving is etched into the background. 

intermediary

Describing a mineral midway between two mineral of a series. For example, the mineral Olivine - (Mg,Fe)₂SiO₄ - is midway between Forsterite - Mg₂SiO₄ - and Fayalite - Fe₂SiO₄ - in the Olivine Group.

intermolecular bonding

The act or process by which two or more groups of atoms or molecules combine due to weak positive or negative charges that develop at various points within each group of atoms due to uneven distribution of their electrons. The side of the molecule where electrons are more likely to be found will have a slight negative charge, and the side where they are less likely to be found will have a slight positive charge. Such charged regions attract oppositely charged regions of nearby molecules, forming relatively weak bonds.

internal deformation

The rearrangement of the planes within ice crystals, due to pressure from overlying ice and snow, that causes the downward or outward flow of a glacier.

intrusive rock

An igneous rock formed by the entrance of magma into preexisting rock.

ion

An atom that has lost or gained one or more electrons, thereby becoming electrically charged.

ionic bond

The combination of an atom that has a strong tendency to lose electrons with an atom that has a strong tendency to gain electrons, such that the former transfers one or more electrons to the latter and each achieves chemical stability under the octet rule. The atom that loses electrons acquires a positive electric charge and the atom that gains electrons acquires a negative electric charge, so that the resulting compound is electrically neutral.

ionic bonding

The act or process of forming of an ionic bond.

ionic substitution

The replacement of one type of ion in a mineral by another that is similar to the first in size and charge.

iridesce

The present term describing something displaying iridescence.

iridescence

Light effect causing a mineral to display a play of colors on an apparently monocolored surface. Iridescence is many times the result of pearly luster, seen around an area where pressure occurred, displaying a similar image to that of fresh oil rising to the surface of a road at the beginning of a rain. Iridescence is also the result of mild tarnishing of a few metallic lustered minerals, such as Chalcopyrite and Hematite.

irradiation

The act of being exposed to radiation. This may have an effect on several gemstones by altering their color.

isometric crystal system

Any mineral that falls under the following specifications belongs to the isometric crystal system: Three axes, all of them are equal in length and lie at 90° from the other.

isomorphism

Having the same crystal form, meaning that the molecular arrangement is identical, except for the fact that different elements are present. If two minerals are isomorphous to each other, than they contain different elements arranged in the same arrangement and number, such as Calcite (CaCO₃) and Siderite (FeCO₃).

isostasy

The equilibrium maintained between the gravity tending to depress and the buoyancy tending to raise a given segment of the lithosphere as it floats above the asthenosphere.

isotope

One of two or more forms of a single element; the atoms of each isotope have the same number of protons but different numbers of neutrons in their nuclei. Thus isotopes have the same atomic number but differ in atomic mass.

isotope dating

The process of using relative proportions of parent to daughter isotopes in radioactive decay to determine the age of a given rock or rock stratum.

isotropic

Synonym of monotropic.

isotropization

Radioactive decaying that causes destruction to a minerals crystal lattice.

isotype

Synonym of isomorph.

isotypous

Synonym of isomorphous.

isotypism

Synonym of isomorphism.

ivory

Ornamental material made out of the calcareous teeth of many mammals, such as elephants, sea lion, hippopotamus, narwhal, and boar.