SPECIFIC GRAVITY

Written By Aubrey Whymark 2017
Specific gravity measures the relative density of a substance with reference to water (at 4°C). The unit used is grams per cubic centimetre (g/cm3 or g/cc), with water being 1 g/cm3. Specific gravity measurements of a tektite can be bulk specific gravity, which measures the whole specimen inclusive of bubbles, or true specific gravity, measuring only the solid material and eliminating the bubbles. Bulk specific gravity tests are commonly carried out to establish the presence and size of bubbles. If this is not the purpose of the experimental procedure then care should be taken to either eliminate or adjust for bubbles.

True specific gravity measures only the solid glassy material, excluding bubbles. The subtle variations in true specific gravity of tektites is related to chemistry. Principally, as silica content increases, the oxides of Fe, Al, Ca, Mg, Na and K decrease. Elevated temperatures results in a loss of heavier oxides and relative enrichment of silica. Tektites with the highest silica content have the lowest specific gravities (Chapman et al., 1964). As such, it is of value in grouping tektites and as patterns are established conclusions can be drawn.

The standard laboratory method for measuring bulk specific gravity is the liquid flotation method as outlined in Chapman et al. (1964). This utilizes crystals of zinc iodide (ZnI2) dissolved in water. The density of the liquid can be varied between 1.0 and 2.6 g/cm3. The tektite is successively immersed in the liquid and if it floats the liquid is of higher specific gravity, if it sinks the liquid is of lower specific gravity than the tektite. The specific gravity of the liquid is incrementally raised by increasing the concentration of zinc iodide until the tektite changes from a ‘sinker’ to a ‘floater’. This method is highly accurate and is sensitive and can detect variations of ±0.001 g/cm3.

In order to calculate the true specific gravity, after bulk specific gravity is measured, a 1 mm thick section was sliced from each specimen. One surface was polished and the specific gravity was measured. The cavities within the section were counted and measured and a correction was computed for void space (Scheiber, 1970). Scheiber (1970) notes that seldom was this correction significant.

The average tektite collector does not have access to such equipment and is usually after a crude measurement to ascertain whether a sizable bubble is present in the specimen and to estimate the bubble volume. There are a couple of ways of doing this.

The first method is described by Norm Lehman of The Tektite Source in 2007. Archimedes discovered that the volume of an irregular solid could be measured by immersion in water. Since the object displaces it’s exact volume and 1 cubic centimeter of water weighs 1 gram, the volume of an object in cubic centimeters will be the gram weight for the solid when dry minus the weight whilst suspended in water. The measured mass in water is reduced by the number of grams equivalent to its volume due to the buoying effect of the displaced water.

Method (with reference to figures below):
1)      Weigh the tektite when dry.
2)      Zero the scale with the scale with the tripod / hook / harness / wire / sling.
3)      Weigh the tektite when 100% immersed in water. Use a piece of very fine thread. Ensure no bubbles adhere to the tektite. Ensure the tektite is not touching the beaker sides.
4)      Calculate volume: Volume (cm3) = dry weight (g) – weight (g) whilst suspended in water.
5)      Calculate specific gravity: Specific Gravity (g/cm3) = dry weight (g) / volume (cm3). For instance 176.2 grams divided by a volume of 71.9 cm3 = 2.45 g/cm3.

Specific Gravity

Specific gravity measures the relative density of a substance with reference to water (at 4°C). The unit used is grams per cubic centimetre (g/cm3 or g/cc), with water being 1 g/cm3.
 
ABOVE: A simple set-up to measure specific gravity. Place the scales on a shelf and tape a wire hook to the scales. Tie on the specimen with very fine thread. Remember to zero the scales before weighing the tektite in water.
ABOVE: A simple set-up to measure specific gravity. Construct a tripod or ‘crane’ with a wire hook attached. Tie on the specimen with very fine thread. Remember to zero the scales before weighing the tektite in water.
An alternative method to measure specific gravity, which involves no thread or hook construction is as follows (with reference to Figure 6.3):
a)      Accurately weigh the specimen whilst dry.
b)      Gather a flat pan and a beaker, placing the beaker in the pan.
c)      Fill the beaker up to the very top with water, taking care to ensure the underlying pan remains dry.
d)      Deposit the specimen into the water. This displaces water into the pan below.
e)      Usually this is not neat and excess water is splashed out from the beaker. After depositing the specimen in the water simply take some of the displaced water out of the flat pan and fill the beaker back up to the top until it flows over (leaving the specimen in the beaker).
f)       Remove the beaker containing the specimen and water, taking care not to spill the water into the underlying pan. Discard this water.
g)      Decant the water in the flat underlying pan into an accurate measuring cylinder to ascertain the volume of water (which equals the volume of the tektite).
h)      Repeat the test to improve/check accuracy. Obviously the accuracy is largely dependent on apparatus used, but basic kitchen supplies will give a good impression if you are careful.
i)       Calculate specific gravity: Specific Gravity (g/cm3) = dry weight (g) / volume (cm3). For instance 124 grams divided by a volume of 59 cm3 = 2.10 g/cm3.
ABOVE: Simple measurement of specific gravity. To ascertain the volume of a specimen, a beaker full of water is placed in a larger empty pan. The tektite is deposited into the water, displacing an equal volume into the empty pan. The volume of water in the pan is measured.
If the specific gravity of the tektite, or tektite group is known and the aim is to calculate the bubble volume and size then the following calculation can be done:

AW= Actual weight of specimen (from above) = 124 g
V = Volume of specimen (from above) = 59 cm3
SG = Known specific gravity of bubble-free tektite glass = 2.45 g/cm3 (value will vary depending on strewn field/ group within strewn field). Note that this is not your calculated specific gravity as that may include a bubble.
EW = Expected bubble-free weight of specimen
DW = Difference between expected and actual weight (actual weight should never be heavier)
BV = Bubble volume
BR= Radius of bubble
BD = Diameter of bubble
π = Pi = 3.14159

Firstly, calculate the expected bubble-free weight from your measured volume.

V x SG = EW
59 cm3 x 2.45 g/cm3 = 144.55g

If the specimen if lighter than the expected bubble-free weight then it contains a bubble (or may be of different composition or incorrectly identified). In this case the specimen actually weighs 124g and the weight lacking is 20.55g.

EW – AW = DW
144.55 g - 124 g = 20.55 g

Now calculate the volume of the bubble(s).

DW ÷ SG = BV
20.55 g ÷ 2.45 g/cm3 = 8.388 cm3

If one wishes to calculate the bubble diameter from volume, proceed with the following steps. Note that only on large medial tektites (over around 75 mm diameter) can one reasonably make an assumption of a single spherical bubble. Smaller tektites (which also includes all distal forms) usually contain multiple small spherical bubbles. Proximal tektites will usually contain distorted ellipsoidal bubbles.

BV ÷ π = Ans
8.388 cm3 ÷ 3.14159 = 2.67

Ans x ¾
2.67 x 0.75 = 2.0025

Cube root of Ans = BR
³√2.0025 = 1.26 cm (12.6 mm)

BR x 2 = BD
1.26 x 2 = 2.52 cm (25.2 mm)

Specific gravity reflects bulk chemistry, principally the silica content. The simple test procedure offers a means to analyse numerous samples in order to establish trends, patterns and groupings.

The silica content will reflect the original source rock composition, which was most likely not totally homogenous. Silica content is then subsequently enhanced during the tektite formation process. One expects bodies heated to the greatest temperatures to have suffered greater loss of volatile material, which includes heavier components such as the oxides of Fe, Al, Ca, Mg, Na and K (Baker and Forster, 1943). These specimens, heated to the greatest temperature, would therefore become enriched in silica and will therefore have a lower specific gravity compared with tektites from a lower thermal regime.

This effect can be demonstrated in ablated, distal, flanged forms. The flange, representing re-entry melted material, should theoretically be enriched in silica, relative to the unmelted core/body of the specimen, and so the flange should have a lower specific gravity. This is borne out in the study by Baker and Forster (1943) which showed flanges to typically have a specific gravity of 2.385 g/cm3 as compared to the central cores having an average specific gravity of 2.426 g/cm3.

Theoretically, the first formed tektites (the most distal tektites) should be enriched with silica relative to the last formed proximal tektites, which formed in a lower thermal regime. In order to establish a pattern, further research would need to be carried out in the Asian (proximal and medial) sections of the strewn field. A pattern, however, certainly seems to emerge in the Australian continent. The data from Baker and Forster (1943), which was in turn taken from numerous sources, if plotted with specific gravity against distance from an assumed crater in the Bay of Tonkin reveals a trend (see figure below). On average the more distal australites appear to have a lower specific gravity. This might be a reflection of the more distal forms having been more ablated and therefore a greater percentage of material is re-melted flange material. It might also be a genuine signature that the primary melt was of a higher temperature and greater distances. Further study, strictly utilizing cored specimens would need to be carried out in order to draw conclusions.

Bubbles

Once you know the typical specific gravity of a tektite type you can check to see if a tektite contains a bubble and even calculate the bubble volume pretty accurately.
 
ABOVE: The specific gravity of tektites plotted against distance from an assumed impact crater in the Gulf of Tonkin. Data taken from Baker and Forster (1943), after various authors.