Access
Please only visit this EarthCache during the normal opening times of the museum which can be found here
Free parking is available at N50°21.861 W004°48.329
Enter the main museum entrance and say that you have come to do the EarthCache…Entrance for this will be free, but if you wish to see any of the other exhibits, you will need to pay – They are well-worth seeing though!
Leave the buildings and head uphill to N50° 21.914 W004° 48.600 where you will find a small pit and a boulder park of rocks from other nearby sites that you will need to study.
The walk is steep but mostly even and not muddy.
Geology
Around 270-295 million years ago, a huge body of granitic rock (called a batholith) was being formed a few kilometres beneath much of southwest England.
Millions of years of weathering and other earth processes have changed the nature of the rock and now brought patches of it to the surface allowing geologists to study it in more detail.
Within the St Austell (Hensbarrow) mass of the granite intrusion, there are significant differences in the composition, texture, crystal size and economic importance of the rock.
The Hensbarrow intrusion is quite complex, having been formed in at least two major phases - approximately 285 million years ago on the eastern side and 274 million years ago on the western side. However, geologically and economically speaking, the most significant change that has affected the granite has been chemical alteration, principally kaolinisation, forming the kaolin (china clay) deposits.
The main bulk of the older eastern granite has not suffered much alteration. It yielded great blocks for building and as such was extensively quarried around Luxulyan. The stone was used locally and also further afield in the construction of many naval dockyards around the country until the middle of the 19th Century.
The western granite on the other hand has been subjected to significant chemical alteration, changing the rock from tough granite into soft china clay deposits.
The small pit beside the boulder park is a SSSI (Site of Special Scientific Interest) as it reveals relatively unaltered granite, unlike that which occurs immediately to the west in the active china clay pits.
The rock exposed here is described as a megacrystic lithium-mica granite. This means that it contains some large alkali feldspar crystals (not uncommon in granite!) but zinnwaldite mica instead of the original biotite mica. This altered mineralogy provides evidence that the original granite has been affected by two events that changed the chemistry within the rock. Geologists describe these as metasomatic, rather than metamorphic events because the actions changed the bulk chemistry of the resulting rock each time. The first event introduced potassium, giving rise to the alkali-feldspar megacrysts. The second event involved an influx of fluid carrying lithium, sodium, fluorine and boron into the rock system. The resultant recrystallisation saw the development of a new suite of minerals (zinnwaldite, albite, fluorite and tourmaline) that incorporated these elements. It is this type of granite that has undergone the most severe kaolinisation further west and produced the best quality china clay.
History
Wheal Martyn was just one of many pits opened to extract china clay from the altered granite. The china clay was (and still is!) used in the production of porcelain, pottery, paper, pharmaceuticals and as a filler in many other everyday items.
When the feldspar crystals in granite got altered, they turned into the hydrated aluminium silicate mineral kaolinite or kaolin. The first Cornish china clay deposits were found by William Cookworthy in 1746, much further west at Tregonning Hill.
You can find out much more about china clay by visiting the museum!
In order to claim this EarthCache you need to visit the site and study the boulders from different china clay pits to find the answers to the following questions, then email them to me via my profile:
1) Find the boulder of Wheal Remfry Breccia – What event created it and when did it happen?
2) Find the boulder of china stone – Which pit did it come from?
3) Although both fluorite and topaz can be found in the St Austell Granite, explain why topaz occurs more commonly than fluorite. (Hint - Study the boulder with fluorite from Wheal Martyn).
In keeping with the recent changes to EarthCache rules, photographs are now optional and I cannot require these to be posted. A photograph says a thousand words though, so I hope you will choose to post one to the cache page anyway!
Thanks for taking the time to visit this EarthCache and I hope you enjoy your visit.