Please note that this is an Earthcache; a find will only be accepted if the Logging Requirements have been met.
Gz is at the junction of Piccadilly and Paton Street. Originally it was one of those classic tall Victorian building boasting the affluence of a great city of the industrial revolution, a memorial to King Cotton and those who grafted to create the wealth. During the C20 the most austere parts of the building were covered by more decorative facades , which divert the passer-by's gaze to the more fashionable frontages. It is that frontage, so easily ignored, which is the subject of this EarthCache. For the benefit of those cachers who find the logging requirements of an EarthCache just too onerous, the shop is a well known seller of pasties, so this visit will not be in vain.
The ground floor facades are covered in a light coloured material; this is the rock Larvikite and is visible both in Piccadilly and Paton Street. A popular name for Larvikite is 'Blue Granite', but this is a misnomer as Larvikite is not real granite at all. 'Granite' is often a quarryman's term for a hard rock which takes a good polish and is pretty resistant to scratching, whereas true Granite is a coarse grained igneous rock with considerable amounts of free quartz. Larvikite is also a course grained igneous rock but with a very different composition with only small amounts of free quartz. It is quarried from a 300 million year old igneous batholith intrusion near the town of Larvik in Norway.
Although the major constituents of Larvikite are feldspars there are other silicate minerals present composed of Magnerium and Iron which are dark coloured; these are known as Mafic minerals
Analysis has shown Larvikite to be be composed of nearly all Feldspar, about the commonest rock mineral on the planet. Feldspar is an alumino-silicate of either potassium, sodium or calcium. With dominant potassium we have Orthoclase feldspar and with sodium and or calcium we have Plagioclase feldspar; (The sodium content can vary from 100-0%, when is completely replaced by calcium) . This Larvikite has Plagioclase feldspar approx 60% calcium which is called Labradorite taking its name from Labrador in Canada where this composition of Plagioclase was first described. So how come that a rock composed of an extremely common mineral, quarried in Norway should appear in Manchester?!
Lamellar twinning and the Schiller effect
In simple terms Labradorite is ornamental. As you face the building you will realse that the cladding is light coloured in contrast to the more sombre appearance of the King Cotton legacy, however as you move about the light reflected from the surface crystals shows a striking blue iridescence. This is due to interference of light called the Schiller effect. Focussing on one of the larger crystals you will see that it is composed of a series of parallel sheets, lamellae. This so called lamellar twinning is common in Labradorite feldspar, and the thickness of the lamellae is responsible for the light interference phenomenon. (For the benefit of those cachers who are already bloated by comfort pasty eating we will simplify matters by stating simply that the lamellae were as a result in some way of the cooling of the magma (molten rock), during the intrusive process). Those wishing to learn more about light intereference are pointed in the direction of another pasty and the internet!
1. To show that you have been to this cache what is the thickness of the Larvikite cladding?
2. Find another name appropriate to this earthcache for the Schiller effect. The internet should help here otherwise pasty time?
3. Larvikite nowadays has a fashionable domestic use. Where in the home?
4. Look at those bits of the rock cladding not displaying the Schiller effect. What colour are they.
5. Look at the mineral grains showing the lamellar twinning. Do you think that thay crstallised from the molten magma at the same time or sequentially