Geochemical Testing Explanation

Most of the time geochemical testing is done it is in a laboratory where chemists use highly sophisticated equipment to determine not only the elemental make up of the sample, but also the exact concentrations of each element desired. A sample can be analyzed for 30 elements for less than $20. Normal lab turn-around time usually ranges from 1 – 3 weeks (although most labs have express service for an extra fee which can process a sample in a few days if necessary. Modern geochemical exploration relies on comparing the relative concentrations of elements in samples so the results from different samples can be compared in a precise manner and plotted on maps to evaluate their significance. The results are often compared using geostatistics.

Several simple, chemical tests can be performed using the flame of a propane torch. These tests only identify the presence or absence of specific elements (not the amount), but when combined with other diagnostic tests, unknown minerals can be identified with some degree of confidence. The chemical tests should be performed on pure minerals. If the unknown mineral is within a rock, then careful crushing and isolating of the mineral into a small amount of pure substance is required. Use tweezers if necessary. In some cases rock material can be tested in the same manner to get a general idea of the presence or absence of a particular element, especially if the rock is composed of only a couple different minerals.

Equipment for these tests is relatively inexpensive and supplies are readily available and portable. However, field geologists rarely carry a propane torch in the field. Instead, the tests are performed at home or in a safe, appropriate setting after the specimen is brought back from the field. Below is a list of simple flame tests requiring minimal equipment and reagents:

Fusibility: Fusibility is a measure of a minerals melting temperature. The test is performed by using a pair of forceps to hold a very thin splinter, or sharp edge of a larger grain, in a position just beyond the tip of the oxidizing flame (Figure F7) of a propane torch. Observe the edge of the grain or splinter before and after heating and determine the change (if any). The fusibility can be judged qualitatively as follows:

*Fusibility*	*Description*	*Examples*	*Approximate Melting Temp.* (^oC)
Fuses Easily	Melts quickly into globule, or edges of grain quickly become rounded by heating a low to medium size flame.	Stibnite	525
		Chalcopyrite	800
Fuses w/ Difficulty	A thin edge or grain becomes rounded after heating with a very hot flame for several minutes.	Garnet	1050
		Actinolite	1200
		Orthoclase	1300
		Pyroxene	1400
Infusuible	No part of the mineral becomes rounded after a heating with a very hot flame for several minutes.	Quartz	1710

Flame Color Test: This test is a simplified version of laboratory testing techniques called “spectroscopy”. When the material is heated, ions are excited to a higher energy state. When they relax, some ions emit visible light with characteristic wavelengths, which can be indicative of the particular ions.

The unknown mineral is ground into a fine powder using a mortar and pestle or other means.
Obtain or make a tiny loop in a nichrome or platinum wire (available commercially). Clean the wire by dipping it in dilute HCl and then heating the wet loop in the propane flame. Repeat until the flame color is consistently the same color, ie, there is no change.
After cooling the wire loop, dip it in the HCl again, and then immediately dip the wet loop into the powdered mineral, allowing a small amount of sample to stick to it.
Insert the loop into the oxidizing flame of the propane torch.
Observe the color.

Reaction on Charcoal: The oxidizing flame is used to heat the sample as it rests in a depression on a charcoal block. The test can be performed on either 1) a small grain of the mineral approximately ¼ inch across, 2) a small amount of powdered mineral, or 3) a small amount of a mixture of powdered mineral and sodium carbonate (in a ratio of about 1 part mineral to 3 parts sodium carbonate).

Important Note !!!: Stabilize the charcoal block before heating the sample so the force of the flame does not cause the block to fall over. Use a clamp if possible. Position the block at an angle of about 30^o so the force of the flame pushes the sample into the depression, instead of blowing it out of the depression.

Place the sample in the depression.
Heat the sample with the oxidizing flame.
Note the behavior during heating and the appearance of the material after heating. Note characteristics like:

a) the formation of clouds of dense smoke

b) the formation of a melted “globule”

c) distinct odors

d) the formation of a sublimate (coating of powder/oxide material) on the charcoal around the sample, and the color of this sublimate.

Bead Test: This test is an evaluation of the color formed in a clear glass bead by a powdered mineral sample when the bead is heated. The test should be done using both oxidizing and reducing flames, as there can be some variation.

First, prepare the sample, as follows:

Crush and grind the mineral in a mortar and pestle, or by other means.
Place powdered mineral in charcoal block depression.
Heat the mineral with the oxidizing flame for a few minutes.
Place the sample back into the mortar and pestle and grind again.

Next, prepare the glass bead as follows:

Clean the wire loop as explained in the flame coloration testing method above.
Heat the wire loop until red hot, and dip it into borax flux.
Heat the loop (with flux attached) until the flux forms a glass.
Dip the hot glass bead back into the borax flux to attach more flux.
Melt the flux again.
Repeat the procedure a couple more times until a clear, glass bead fills the loop.
Heat the bead until all bubbles disappear.

Next, test the mineral as follows:

Heat the bead until molten.
Touch the red hot bead against the powdered mineral sample so as to attach a very, very small amount to the bead. Note: If too much sample is added to the bead, it will become opaque, rendering it useless.
Heat the bead in the oxidizing flame for a few minutes. Observe the color of the bead when hot.
Let the bead cool. Observe the color of the bead when cool
Heat the bead in the reducing flame for a few minutes. Observe the color of the bead when hot.
Let the bead cool. Observe the color of the bead when cool.

Discard the bead after testing by heating again to red hot, and quickly tapping the wire loop onto a hard surface.

Magnetism on Heating: This is a test to determine if a mineral becomes magnetic after heating.

Prepare the unknown mineral sample by heating and grinding, as described above for the bead test.
After cooling, use a pencil magnet or other type of magnet to test for magnetism. Note: To prevent the sample from becoming stuck to the magnet, place the powdered sample on a piece of paper which extends over the edge of a surface (such as a table), and slide the magnet along the underneath side. Note movement of the sample caused by magnetism.

Odor: To conduct this test simply use tweezers to hold a large grain of the unknown mineral in the flame for a couple minutes. Alternatively, the powdered mineral can be placed on the charcoal block and heated. Upon heating, sulfide minerals produce hydrogen sulfide gas, which smells like rotten eggs. If the mineral contains arsenic, a garlic odor is produced.

Decrepitation: Decrepitation is the violent, noisy breaking up of a mineral when it is heated. Decrepitation indicates that water is part of the chemical composition of the mineral.

Pyroelectricity: Upon gentle heating, the mineral tourmaline develops a weak but detectable electrical charge which can be measured using an ohm meter.

Thermoluminescence: Upon gentle heating, the mineral fluorite will luminesce (glow).

Figure F7: Cross section of propane torch flame.

Field Methods | Geochemical Methods | Geophyscial Methods | Drilling Methods | Petroleum Exploration

DMTC Home | ISM Home