Definitions

anisotropic (1) the velocity of light varies depending on direction through the mineral and (2) double refraction is visible

birefringence The quantity of the maximum index of refraction minus the minimum index of refraction. The maximum birefringence is listed for each mineral.

cleavage The tendency to split along crystallographic planes, yielding smooth surfaces. If the cleavage is "poor" this is listed as "no."

cleavage angle The angle between cleavages which is < 90 degrees. This convention is used to reduce ambiguity. If you measure an angle of 87 degrees, simply subtract this from 180 degrees (93 degrees) and input this number.

color Color in thin section, including pleochroic colors. If a mineral's colors are extremely pale, or if there is no color, the term "colorless" is used. Also, in order that all blue and bluish minerals may be found when you search for blue, bluish was spelled incorrectly as "bluish." The words used to describe color are: blue, blueish, brown, brownish, colorless, gray, grayish, green, greenish, orange, pink, pinkish, purple, red, reddish, yellow, yellowish

crystal system Groups of minerals with common symmetry characteristics. NA would include non-crystalline minerals. The groups are: isometric, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, triclinic, NA.

dispersion The separation of polychromatic light into its component wavelengths, causing the magnitude of the 2V angle to vary with wavelength.

elongation A measure of the sign of elongation. Length-slow is positive elongation; length-fast is negative elongation. The word "cleavage" is used when the elongation is measured relative to the cleavage rather than the crystal form.

extinction Orientation of the crystal when it becomes dark between crossed polars

habit The relative development of forms and faces in a crystal.

indices of refraction The degree to which ligh is bent when passing through a crystal. The indices of refraction were taken from Fleischer et al. (1980). If a mineral had more than one entry in this reference, the maximum and minimum indices were all averaged, and that value was used in the database.

mineral group Name for structurally and/or chemically related minerals. The mineral groups are specific, however some groups (like the amphiboles) have a series within the group (the hornblende series).

name The accepted mineral name (other commonly used names might appear in parentheses): rarely a group name (such as serpentine) might also be used.

occurence The possible genesis of the mineral in general terms. Options include: igneous, metamorphic, sedimentary, secondary, hydrothermal.

The possible genesis of the mineral in more specific terms. The general term is listed first (igneous), with the more specific term listed in parentheses after it (felsic), as follows:

igneous (accessory, alkalic, felsic, intermediate,

mafic, ultramafic)

metamorphic (contact, high, medium, low grade,

schist, metasomatic, skarn)

sedimentary (authigenic, detrital, evaporite,

carbonate)

secondary (cavities, oxidized zones of ore deposits,

vugs, bauxite, alteration)

hydrothermal (high, moderate, low T, pegmatite)

optic character Description of the manner in which light travels through the crystal. Options include: biaxial, uniaxial, isotropic.

optic sign Description of which ray of light passing through the crystal is faster, the ordinary ray or the extraordinary ray. If the optic sign can be either positive or negative, both are listed.

orientation diagram A change in color of a mineral when the stage is rotated in plane polarized light, due to varying absorption in the ordinary and extraordinary rays.

pleochroism The average index of refraction of the mineral [(highest+lowest)/2] minus 1.537, the index of refraction of Canada balsam. For minerals with high birefringence, like the carbonates, relief will be difficult to use in a search because of this averaging. Using a wide value range when searching this field.

relief The following are words used to describe the average relief of a mineral:

>0.363 extreme

0.243 - 0.363 very high

0.143 - 0.243 high

0.033 - 0.143 moderate

- 0.047 - 0.033 low

<-0.047 moderate negative

system see crystal system

wave front A surface that connects similar points in adjacent waves.

wave normal A line constructed at right angles to the wave front.