San Diego Natural History Museum--Your Nature ConnectionSDNHM Field Guide

Glossary

Acicular
Bristly, spiny, needlelike.
Alkali feldspar
(K,Na)AlSi3O8. The variety of feldspar with potassium and sodium It has three common forms with the same chemical composition, sanidine, orthoclase, and microcline. Sanidine is the high-temperature form that occurs only where quenched in volcanic rocks. Orthoclase and microcline occur in successively lower temperature plutonic rocks. See also the field guide entry for feldspar.
Alloy
An alloy is a metal formed by the mixture of two or more metals, or by the mixture of a metal and another substance.
Alluvial
Made of soil and sand left by rivers or floods.
Amphibole
A complex family of silicate minerals with similar physical features and a general formula A0-1 X2Y5 (Si,Al)8O22(OH,F,CL)2 where A= Ca, Na, K; X = Ca, Fe, Mg, Mn, Na; Y = Al, Fe, Mg, Mn, Ti. Amphiboles form in both igneous and metamorphic environments. See hornblende.
Andalusite
Al2SiO5. A metamorphic mineral formed by the reconstitution of clay minerals at low pressures and temperatures. See also chiastolite.
Aquamarine
The blue-green variety of the mineral beryl.
Axis
A straight line through the center of a plane figure or solid, around which the parts are symmetrically arranged.
The plural form of axis is axes and is pronounced ak-seez.
Beryl
Be3Al2Si6O18. A rare igneous mineral usually found in pegmatites. Its colored varieties include aquamarine (pale blue-green), emerald (dark green), heliodor (yellow), and morganite (pink). See also the field guide entry for beryl.
Biotite
K(Mg,Fe)3(AlSi3O10)(OH)2. The dark colored, iron- and magnesium-bearing member of the mica family.
Cabochon
Any precious stone cut in a convex shape, polished but not faceted. This is one of the oldest and simplest cuts. It's often used on opaque and translucent stones, such as opals, or stones with unusual optical properties, such as chrysoberyl cat's-eyes.
Calcite
CaCO3. A common sedimentary mineral that forms by direct precipitation from aqueous solutions or through biological activity. See also the field guide entry for calcite.
Chiastolite
The variety of andalusite with a diagnostic cross pattern of minute dark inclusions when viewed on end.
Chlorite
(Mg,Fe,Al)6Al,Si)4O10(OH)8. A metamorphic mineral that forms at relatively low pressures and temperatures. It can also form through hydrothermal alteration of other iron- and magnesium-bearing minerals.
Clay
A family of aluminum- and silica-rich minerals that commonly form by reaction of feldspars with hydrothermal solutions. See also kaolinite.
Conchoidal
"Shell-like," with a smooth curved surface.
Cordierite
(Fe,Mg)2Al4Si5O8. A metamorphic mineral that is diagnostic of low temperatures and pressures.
Crystal
A crystal is the solid form of a substance in which the atoms or molecules are arranged in a definite, repeating pattern. The formation results in one of three ways: dissolved matter may precipitate out of a solution such as molten magma or sea water; gases may condense into a solid form, and two or more solid crystals under high temperature and pressure may recombine into a new mineral. The external shape of the crystal -- smooth, symmetrically arranged, flat surfaces -- reflects its atomic structure.
Diopside
Ca(Fe,Mg)Si2O6. An common igneous mineral and metamorphic member of the pyroxene family. As a metamorphic mineral, it forms at moderate to high pressures and temperatures.
Elbaite
The dark blue variety of tourmaline.
Epidote
Ca2(Al,Fe)8Si3O12(OH). A common metamorphic mineral formed at low temperatures and pressures, or a mineral formed by the hydrothermal alteration of plagioclase feldspar.
Feldspar
A family of minerals consisting primarily of alkali feldspar and plagioclase. The most common mineral group in the earth’s crust. See also the field guide entry for feldspar.
Garnet
A family of minerals consisting primarily of grossularite (Ca4Al3Si4O12), pyrope (Mg4Al3Si4O12), and almandine (Fe4Al3Si4O12). See also the field guide entry for garnet.
Granular
Containing or consisting of grains.
Hackly
Sharp, jagged surfaces.
Hexagonal
These crystals are usually shaped like six-sided prisms or pyramids. Each crystal has four axes of symmetry. Three lie in the same plane, are the same length, and intersect at 120° angles. The fourth axis is not the same length, and is perpendicular to other three.
Example: beryl
Hornblende
NaCa2(Mg,Fe,Al)5(Al,Si)8O22(OH)2. The most common member of the amphibole family of minerals. It is found in both igneous and metamorphic rocks. See also the field guide entry for hornblende.
Ilmenite
FeTiO3. An oxide of iron and titanium that forms under relatively reducing chemical conditions. See also magnetite.
Isometric
Also called the cubic crystal system. Crystals can form in the shape of a cube (like halite and pyrite), an octahedron (diamond), dodecahedron (garnet), trapezohedron (garnet), and others. The most common and recognizable in this class are the cube, the octahedron, and the dodecahedron. Isometric crystals have three axes of symmetry, all at right angles to each other, and all of the same length.
Examples: pyrite, halite, garnet
Kaolinite
Al4Si4O10(OH)8. A member of the clay mineral group.
Kunzite
The pale purple variety of spodumene. See also the field guide entry for spodumene.
Magnetite
Fe3O4. An oxide of iron that forms under relatively oxidizing chemical conditions. See also ilmenite.
Metamorphic rock
Any rock formed deep within the earth from pre-existing rock material as the result of high temperatures and pressures, or by reaction with chemically active fluids.
Mica
A family of minerals with a common sheet-like internal structure. See also biotite and muscovite.
Microcline
See alkali feldspar. See also the field guide entry for feldspar.
Monoclinic
Crystals are short and stubby with tilted faces at each end. Each crystal has three unequal axes. Two axes lie in the same plane at right angles to each other. The third axis is inclined.
Example: gypsum.
Morganite
See beryl.
Muscovite
KAl2(Al,Si3)O10 (OH)2. The light colored member of the mica family.
Native metal
A metal which occurs in its elemental form, uncombined with any other elements, such as gold, silver, or copper. Other metals, like aluminum, iron, or tin occur only as compound ores.
Olivine
(Mg,Fe)SiO4. A common igneous mineral in peridotite and basalt.
Oolitic
Little spheres, between the size of a pinhead and a pea.
Orthoclase
See alkali feldspar.
Orthorhombic
These crystals are short and stubby. Each crystal has three unequal axes, all at right angles to one another.
Example: topaz
Piezoelectric
A mineral that vibrates regularly when an electric current passes through it.
Pegmatites
Pegmatites are very coarse-grained igneous rocks, usually granite, that form from water-rich magmas. They are important sources of gem material and crystal specimens. For more information, please see our Mineralogy FAQ page.
Plagioclase
A member of the feldspar family consisting of a compositionally continuous series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).
Prism
A prism is a solid geometric shape. It has three or more sides, and ends that are similar in size and shape. A crystal shaped like a prism has faces that are parallel to one another.
Pyrite
FeS2. Referred to as “fool’s gold” because of its brassy color, it is a common accessory mineral in sulfide -rich ore deposits. See also the field guide entry for pyrite.
Pyroxene
A family of silicate minerals with a general formula Y2(Si,Al)2O6 where Y = Ca, Na, Mg, Fe, Mn). Pyroxenes occur in both igneous and metamorphic rocks. See also diopside.
Quartz
SiO2. The second commonest mineral in the earth’s crust (after feldspar).
Rhombohedron
A six-sided prism.
Rubellite
The red to pink variety of tourmaline.
Sanidine
See alkali feldspar.
Schorl
The black variety of tourmaline.
Serpentine
Mg6Si4O10(OH)8. A product of the hydrothermal alteration of olivine and pyroxene.
Sillimanite
Al2SiO5. A metamorphic mineral formed by the recrystallization of clay minerals at moderate to high pressures and temperatures.
Sphene
CaTiSiO5. An common accessory mineral in igneous rocks.
Spinel
MgAl2O4. An accessory mineral in some igneous rocks.
Spodumene
LiAlSi2O6. A rare mineral found in pegmatites. The gemmy green variety is hiddenite, the pink to violet variety is kunzite. See also the field guide entry for spodumene.
Stalactitic
Having a cone shape that tapers to a narrow point, like an icicle or a stalactite.
Tabular
Flat, like a table.
Talc
Mg3Si4O10. A product of the hydrothermal alteration of olivine and pyroxene.
Tetragonal
Typically, the crystals are shaped like four-sided prisms and pyramids. Each crystal has three axes, all perpendicular to one another. Two axes are the same length and lie on a horizontal plane. The third axis is not the same length and is at a right angle to the other two.
Example: zircon
Tourmaline
Na(Mg,Fe)3Al6(BO3)3(Si6O18)(OH)4. A compositionally variable mineral found as an accessory in pegmatites and some hydrothermally altered rocks. See also the field guide entry for tourmaline.
Triclinic
Triclinic crystals are usually flat with sharp edges, but no right angles.
Example: feldspar
Vesuvianite (idocrase)
Ca10Mg2Al4(Si2O7)2(SiO4)5(OH)4. An accessory mineral found in metamorphosed limestones (marble).
Wollastonite
CaSiO3. A metamorphic mineral associated with marble that forms under moderate temperatures and pressures.
Zircon
ZrSiO4. An accessory mineral in igneous rocks that also contains enough uranium to make it useful in radiometric dating.