XRD Studies of Home-grown Crystals

A scene-by-scene text narrative of the movie.

by
J. W. Mies
Department of Physics, Geology & Astronomy
The University of Tennessee at Chattanooga
Chattanooga, TN 37403-2598

This movie was inspired by 3rd-grade students at Thrasher Elementary School, 
Signal Mountain, TN, who grew crystals like those featured here.

At the conclusion of the crystal-growth experiment, the crystals were collected and 
were studied using X-ray diffraction in the XRD lab of the geology program at UTC 
(http://www.utc.edu/~jmies/xrd/xrd.html). This X-ray diffractometer was funded by 
the National Science Foundation (NSF DUE-9952345) and UTC's Center of 
Excellence for Computer Applications (CECA).

Scenes 1 through 12 are time-lapse footage of crystal growth. The hours and 
minutes indicated for each of these scenes are time elapsed since the beginning of 
the experiment. The experiment began on Sunday, March 10, 2002, at 7:00 AM and 
concluded on Tuesday, March 12, 2002, at 5:13 PM. The crystals featured in these 
scenes grew on the top edge of a triangular piece of sponge that protruded from a 
shallow bath of water, laundry bluing, and salt.

Scenes 13 through 25 show the various steps involved in X-ray diffraction of the 
crystals, from sample preparation through analysis.

Scene 1 (behind title), 15 hours, 23 minutes

Scene 2, 19 hours, 13 minutes

Scene 3, 23 hours, 33 minutes

Scene 4, 26 hours, 48 minutes

Scene 5, 30 hours, 59 minutes

Scene 6, 34 hours, 30 minutes

Scene 7, 36 hours, 28 minutes

Scene 8, 39 hours, 20 minutes

Scene 9, 45 hours, 52 minutes

Scene 10, 48 hours, 47 minutes

Scene 11, 53 hours, 46 minutes

Scene 12, 58 hours, 13 minutes

Scenes 13 and 14

Crystals are scraped from the sponge substrate and from the sides of the plastic 
tray.

Scene 15

The accelerating potential and current on the X-ray generator are slowly turned up.

Scene 16

While the X-ray generator is warming up, the sample is prepared for analysis.
Here, the sample holder is taped to a glass plate.

Scene 17

The crystal powder is loaded into the sample holder.
The sample is "back loaded". Eventually, the glass plate will be removed to expose 
the top (front) side of the sample holder.

Scene 18

The powder is gently pressed into the sample holder with a second glass plate.

Scene 19

A spring-clip backing plate is pressed onto what will become the back (bottom) side 
of the sample holder.

Scene 20

The sample holder and glass plate are turned over; the tape is removed; and the 
sample holder with the pressed powder is carefully separated from the glass plate.

Scene 21

The accelerating potential and current on the X-ray generator are slowly turned up to 
operating values of 40 kV and 40 mA.

Scene 22

The sample, in the sample holder (scenes 16 to 20), are placed in the sample 
chamber.

Scene 23

A reconnaissance scan is performed to determine optimal analytical parameters 
(counting time, step size, and scan range).

Scene 24

During the scan, the sample and goniometer rotate very slowly and the sample 
powder is struck by X-rays at various angles. (During this scan the goniometer 
swung from 25 degrees to 60 degrees, 2 theta). X-ray diffraction occurs in the 
crystal lattice of the randomly oriented powder fragments. The detector, on the 
goniometer arm, counts diffracted X-rays.

Scene 25

The diffractogram is a chart of 2-theta angle vs X-ray counts (or counts per second). 
This pattern of peaks is characteristic of the mineral halite (NaCl), table salt.
Note, for example, the prominent peak at 45.5 degrees.