X-ray diffraction

A beam of x-rays is foccussed on a powdered sample where crystals have varied and random orientation. Rotating the sample through about 120 degrees and using a graphite monochromator to give an incident beam of only the k alpha cr line, x-rays will be reflected (not fluoresced) at the appropriate bragg angle. The 2d spacings between the denser layers in the crystal lattice can be measured. Xrd can be used to identify minerals, but also to analyse one element in one mineral by selecting a known 2d spacing and counting the intensity. (this ought to make sense to those who know a little about xrd).

X-ray diffraction maxima

The periodic coherent scattering of x-rays that arise from crystalline materials. The data used to determine the coordinates from which the space group and unit cell of the compound can be determined.

X-ray electron diffraction

The method employed to examine the crystallinity and crystal structure of materials.

X-ray fluorescence

[exploration] the x-ray fluorescence spectrometer or spectrometry. Like the probe, the xrf measures the intensity of emitted radiation, in this case from a fused glass or pressed powder sample an inch or more across, but the excitation energy is provided, not by an electron beam, but by a primary x-ray source of very high energy produced by focussing an electon beam inside a sealed tube onto a metallic target which may be cr, w, rh, etc. These primary tubes operate at 2 ? 3 kilo-watts and a 100 kv. Xrf is still one of the best analytical tools for determination of major elements as well as the te cr, co, ni, cu, zn, ga, rb, sr, zr, nb, th, ba in bulk samples. Detection limits can be as good as 0.2 ppm, more commonly about 1 3 ppm.

X wave

See extraordinary wave.

Xedit

Xedit- is the name of the full-screen text editor which is standard to the vm operating system. Xedit is used to create and edit data files and source-code files.

Xenobiotic

A chemical substance foreign to the body or introduced to the body in higher quantities or by a different pathway than occurs in normal metabolism

Xenocryst

A crystal that resembles a phenocryst in igneous rock, but is a foreign to the body of rock in which it occurs.

Xenolith

A preexisting rock embedded in a newer igneous rock. Xenoliths are formed when a rising magma incorporates the preexisting rock. If the preexisting rock does not melt, it will not be assimilated into the magma and will therefore remain distinct from the new igneous rock that surrounds it.

Xeralf

A suborder of the soil order Alfisol, having good drainage, and found in regions with rainy winters and dry summers in mediterranean climates; the surface horizons tend to become massive and hard during the dry seasons, with some soils having duripans that interfere with root growth.

Xerert

A suborder of the soil order Vertisol, formed in a Mediterranean climate; wide surface cracks open and close once a year.

Xeriscaping

A method of landscaping that uses plants that are well adapted to the local area and are drought-resistant. Xeriscaping is becoming more popular as a way of saving water at home.

Xeroll

A suborder of the soil order Mollisol, formed in a xeric moisture regime; may have a calcic, petrocalcic, or gypsic horizon, or a duripan.

Xerophytes

Drought-resistant vegetation.

Xerothermal period

See xerothermic period.

Xerothermic

Characterized by dryness and heat.

Xerothermic period

A postglacial interval of a warmer, drier climate. Also known as xerothermal period.

Xerult

A suborder of the soil order Ultisol, formed in a xeric moisture regime; brownish or reddish soil with a low to moderate organic-carbon content.

XGA

Extended Graphics Adapter

Xrays

Electromagnetic radiations of a shorter wavelength than visible light. Usually less than one angstrom.

Xylem

The xylem consists of two types of conducting cells: tracheids and vessels. Elongated cells, with tapered ends and secondary walls, both types lack cytoplasm and are dead at maturity. The walls have pits—areas in which secondary thickening does not occur—through which water moves from cell to cell. Vessels usually are shorter and broader than tracheids, and in addition to pits they have perforation—areas of the cell wall that lack both primary and secondary thickenings and through which water and dissolved nutrients may freely pass.

Xylinite

A variety of provitrinite consisting of xylem or lignified tissue.

Xyloid coal

See bituminous wood.

Xyloid lignite

See bituminous wood.