[meteorite-list] Thermoluminescence - Part 2

From: bernd.pauli_at_paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:52 2004
Message-ID: <DIIE.0000004000001C91_at_paulinet.de>

S.W.S. McKEEVER and D.W. SEARS (1980) Meteorites
That Glow (Sky and Telescope, July 1980, pp. 14-16):

Thermoluminescence

A glow curve is the graph of TL intensity as it varies with temperature. Two
examples are shown on the next page. Ordinary chondrites, the most common
meteorites, luminesce brightly with a maximum at about 200=B0 C; a second peak
occurs at 350=B0C, and the color at both peaks is blue-green. Aubrites are a small
class of meteorites with an entirely different glow curve, with several peaks and
colors ranging from blue to red.It takes several steps to produce TL. The crystals
in meteoroids orbiting in space become energized by ionizing radiation from cosmic
rays and, to a lesser degree, from decay of the bodies' own radioactive isotopes
of potassium, uranium, and thorium. Some of the energy causes electrons to wander
randomly through the solids. Eventually they be come trapped at imperfections in the
crystal structure. Heating the sample shakes the electrons out of these traps. They then
return to their original energy levels; the excess energy converts to photons of visible
light. In ordinary chondrites the mineral feldspar produces the TL; in aubrites, enstatite
is primarily responsible.

Cathodoluminescence

Most thermoluminescent minerals also turn out to be cathodoluminescent. A beam of
electrons striking a polished section of the meteorite generates CL that can be seen
through a microscope. Basically, the electrons bypass the lattice traps and emit light
in a single-step process. CL is bright, so luminescent grains in the slice of meteorite
are easy to locate, and they can be photographed through the microscope. This provides
a powerful bridge between TL and the many techniques geologists use to examine meteoritic
minerals. As in TL, feldspar produces most of the light, a distinctive blue-green. Feldspar
is an important component of ordinary chondrites and the chondrules in them. These chondrules
are curious solidi-fied droplets found only in meteorites and some lunar rocks. Their intricate,
often beautiful structures are strikingly revealed by CL.
The first scientists who examined chon drules microscopically thought they were solidified droplets
of "fiery rain" splashed out from the sun. Modern theories are scarcely less spectacular- some think
they are solidified droplets of dust which were melted in the primordial nebula by lightning flashes
or by meteorite impacts.

.. more to follow

Best wishes,

Bernd
Received on Mon 22 Mar 2004 10:30:15 AM PST