The location:
This cache is located in Magdalen Street in Oxford, close to the busy heart of the city. You are required to visit two stages but they are very close together and should present no difficulty. This EarthCache invites you to study the rock used for two shop fronts.
Oxford offers many possibilities for public transport but car parking is scarce and expensive in the city centre.
The EarthCache lesson:
Larvikite has, for more than a hundred years, been appreciated as one of the world's most attractive dimension stones and, at present, its production and use are more extensive than ever. The main reason for the continuous success of larvikite on the world market is the blue iridescence displayed on polished surfaces, which is caused by optical interference in microscopic lamellae within the ternary feldspars. The larvikite complex consists of different intrusions, defining several ring-shaped structures, emplaced during a period of approximately five million years. Following this pattern, several commercial subtypes of larvikite, characterised by their colour and iridescence, have been mapped. Four of these subtypes are being exploited at the present time and define the most important reserves in the short run. Some other subtypes are less attractive in the present market situation, but may provide an interesting potential for the future. However, the industrial value of the larvikite also depends on other geological features, such as various types of dykes, faults and fractures, ductile deformation zones, late-stage magmatic and hydrothermal alteration and deep weathering. When combining the distribution pattern of such features with the map of the larvikite subtypes, it is possible to delineate various types of larvikite deposit that are considered to have commercial value in the short or long term.
Heldal, T. & Kjølle, Idunn & Meyer, Gurli & Dahlgren, Sven. National treasure of global significance. Dimension-stone deposits in larvikite, Oslo igneous province, Norway.
The characteristic iridescence of larvikite goes under several names, including labradorescence and the Schiller effect. The colours visible to the naked eye vary according to the lighting conditions and also seem to vary from one person to another. They appear here as a silvery or bluish glow, appearing to move as the stone is turned (or as the light source is moved) and originating from the reflection of light between layers of mineral beneath the surface of the stone. It is greatly enhanced when the surface of the stone has been polished, as in these shop fronts.
Larvikite can be found is a wide range of colours, typically found only in very specific locations or quarries in the region of Norway around the town of Larvik. The map below (courtesy of https://geogleblog.com/2016/08/22/blue_pearl/) shows the most important quarries and the colour of larvikite sourced from each.
The EarthCache:
At Stage 1, you will see two adjacent, vertical pillars with stone facings. The very bottom stone of the left-hand pillar and most of the right-hand pillar are of Larvikite (ignore the two brown granite features on the right-hand pillar). Examine the polished surface of the stone. In particular, observe the size, shape and colour of the different grains and then answer the following questions. Your answers should be sent to the CO via the Geocaching message service or by email.
1) Describe the iridescent grains you see, noting their colour and size
2) What is the overall appearance of the colour of the stone?
Now proceed a few steps north to Stage 2. Here, there is a single pillar between two shop windows. At the time of placing this cache, there is a black letterbox low down on this pillar. Again, examine the polished surface of the stone making up most of the surface of this pillar, ignoring the brown granite features, and then answer the following questions.
3) Describe the iridescent grains you see, noting their colour and size
4) What is the overall appearance of the colour of the stone?
5) Finally, based upon your observations of the stone in the two locations and using the map and pictures above, identify the varieties of larvikite you see. From which quarry or quarries do you consider the rock to have been mined?