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


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.
An alloy is a metal formed by the mixture of two or more metals, or by the mixture of a metal and another substance.
Made of soil and sand left by rivers or floods.
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.
Al2SiO5. A metamorphic mineral formed by the reconstitution of clay minerals at low pressures and temperatures. See also chiastolite.
The blue-green variety of the mineral beryl.
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.
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.
K(Mg,Fe)3(AlSi3O10)(OH)2. The dark colored, iron- and magnesium-bearing member of the mica family.
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.
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.
The variety of andalusite with a diagnostic cross pattern of minute dark inclusions when viewed on end.
(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.
A family of aluminum- and silica-rich minerals that commonly form by reaction of feldspars with hydrothermal solutions. See also kaolinite.
"Shell-like," with a smooth curved surface.
(Fe,Mg)2Al4Si5O8. A metamorphic mineral that is diagnostic of low temperatures and pressures.
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.
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.
The dark blue variety of tourmaline.
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.
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.
A family of minerals consisting primarily of grossularite (Ca4Al3Si4O12), pyrope (Mg4Al3Si4O12), and almandine (Fe4Al3Si4O12). See also the field guide entry for garnet.
Containing or consisting of grains.
Sharp, jagged surfaces.
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
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.
FeTiO3. An oxide of iron and titanium that forms under relatively reducing chemical conditions. See also magnetite.
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
Al4Si4O10(OH)8. A member of the clay mineral group.
The pale purple variety of spodumene. See also the field guide entry for spodumene.
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.
A family of minerals with a common sheet-like internal structure. See also biotite and muscovite.
See alkali feldspar. See also the field guide entry for feldspar.
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.
See beryl.
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.
(Mg,Fe)SiO4. A common igneous mineral in peridotite and basalt.
Little spheres, between the size of a pinhead and a pea.
See alkali feldspar.
These crystals are short and stubby. Each crystal has three unequal axes, all at right angles to one another.
Example: topaz
A mineral that vibrates regularly when an electric current passes through it.
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.
A member of the feldspar family consisting of a compositionally continuous series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).
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.
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.
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.
SiO2. The second commonest mineral in the earth’s crust (after feldspar).
A six-sided prism.
The red to pink variety of tourmaline.
See alkali feldspar.
The black variety of tourmaline.
Mg6Si4O10(OH)8. A product of the hydrothermal alteration of olivine and pyroxene.
Al2SiO5. A metamorphic mineral formed by the recrystallization of clay minerals at moderate to high pressures and temperatures.
CaTiSiO5. An common accessory mineral in igneous rocks.
MgAl2O4. An accessory mineral in some igneous rocks.
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.
Having a cone shape that tapers to a narrow point, like an icicle or a stalactite.
Flat, like a table.
Mg3Si4O10. A product of the hydrothermal alteration of olivine and pyroxene.
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
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 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).
CaSiO3. A metamorphic mineral associated with marble that forms under moderate temperatures and pressures.
ZrSiO4. An accessory mineral in igneous rocks that also contains enough uranium to make it useful in radiometric dating.