determines which of the minerals is created. The Pressure- Temperature phase diagram, shown in Figure 2.4 When heated, the sillimanite group of minerals undergo a phase transformation to mullite plus excess silica. This reaction is shown in Equation 1.
KYANITE
SILLIMANITE ANDALUSITE
Temperature of Full Conversion 2550°F (1400°C) 3090°F (1700°C) 2910°F (1600°C)
Theoretical Expansion 17% 6.2% 4.1%
Table 1: Conversion characteristics of the sillimanite group minerals. The temperature of this phase
transformation differs in each of the three sillimanite minerals.5 is applied, the Al3+
When heat cations change
coordination number. This causes the rearranging of the atoms to form the more open crystal structure of mullite. The kyanite crystal structure is the least ordered of the three and thus has the weakest chemical bonding. This means it requires the least heat energy for the phase transformation to begin. Table 1 shows the conversion temperatures and theoretical expansion rates of the three minerals as they convert to mullite.6 The lower temperature requirement for conversion makes kyanite an ideal candidate for the production of mullite.
Virginia Mullite™: How It’s Made The Deposit In order to understand how to make Virginia Mullite™, we must first explore how its precursor mineral, Virginia Kyanite™ is made. The first requirement for mining and refining industrial grade kyanite is to find a source mine. Silica and alumina are the two most abundant
oxides on planet Earth,
making up 59% and 15% of the earth’s crust respectively.7
Yet, finding these
two oxides in the correct ratios (that have been subjected to the necessary heat and pressure regimes) to form the sillimanite family of minerals is more uncommon. However, that is not to say that the sillimanite family of minerals are rare. They can be found on almost every continent on the Earth. What is rare, is a deposit that is not only large enough to be economically viable, but one that has a high purity ore body. Pure in this sense typically means not only lacking in impurities but also the other sillimanite minerals. Once such a deposit has been
22 ❘ February 2020 ®
found, the real testing begins. Even if a deposit is large and pure, it does not mean that it will be able to produce industrial grade material. The geology of the deposit and the other minerals present will determine how difficult it is to remove the kyanite from the ore body and its associated minerals- even in trace amounts. When it comes to kyanite, Willis Mountain in Dillwyn, VA is a world- class deposit that is suitable for the production of industrial grade kyanite. The mining and production of industrial grade kyanite (and mullite from kyanite) has taken place at Willis Mountain for over 60 years. Willis Mountain is located in
Central Virginia about seventy miles west of Richmond and fifty miles south of Charlottesville. The Willis Mountain deposit resides in the geological occurrence known as the Whispering Creek Anticline. The deposit is made up of two halves: Willis Mountain itself and East Ridge, which is the other half of the anticline. The deposit was formed around 465 million years ago during the Middle Ordovician Period.8 The predominant theory is that venting of mineral rich fluids deposited clay minerals. That material was subducted to a depth conducive to form the minerals seen today. The deposit was then thrust upwards to the surface and then truncated which created the exposed east and west legs of the formation.8
The ore body is made up of
kyanite-quartzite rocks. Willis Mountain rises 475 feet above the surrounding countryside and runs for about 1.75 miles. The ore body contains 20-30% kyanite. The main impurities are quartz, several varieties of mica, iron oxides, iron sulphides, and a multitude of minor minerals. The deposit runs near vertical through the entire height of the mountain, and is, generally, 120 yards thick through its entire length.
Willis Mountain has been continuously worked since the late 1950’s, but the majority of this large orebody remains untouched to this day. It is estimated that well over 50% of the Willis Mountain ore remains. The East Ridge deposit, adjacent
to Willis Mountain, also runs for 1.75 miles. The ridge is shorter than the mountain, rising up from the surrounding countryside about 300 feet. The East Ridge deposit contains more impurities than the deposit on Willis Mountain. Higher amounts of iron sulphides and clay minerals are common, and a wider variety of iron oxides and micas are seen. The vein at East Ridge is also thinner than the Willis Mountain vein and runs at a 30° angle instead of vertically. Mining at East Ridge began in the late 1970s, and substantial reserves remain in this part of the orebody also. The lifetime of these two Virginia
deposits is difficult to assess and has never been definitively determined. It is clear that there is enough exploitable ore to continue mining for many generations at current production rates.
Mining and Comminution Mining is done from both the above- mentioned deposits on a single daylight shift, five days a week. Blasting occurs as needed but typically happens once or twice a month at each deposit. Blasting is done by filling drilled holes (usually roughly 30 feet deep) with a liquid explosive. Rocks that remain too large to move after the blast are broken down by a rock breaker. Loaders pick the material up and put it in the back of haul trucks that bring the material to a single jaw crusher where the ore from various parts of the two quarries is blended together. The average kyanite percentage of the ore runs around 20-25%, which means that roughly 650,000 tons of ore must be mined and processed each year to meet
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