Logging tasks for this EarthCache:
On the southern edge of Market Square, check for the initial red granite wall. Go to the cache coordinates and choose the correct answers:
1- On site, what is the average length of the feldspar ovoids? I- 1 cm; II- 3 cm; III- 6 cm; IV- 10 cm; V- >15 cm
2- On site, what is the colour of the central part of the feldspar ovoids? I- White; II- Purple; III- Green; IV- Beige; V- Pink
3- A- On site, what surrounds the feldspar ovoids? {If you can't identify the mineral, go by colour: I)- White (Feldspar), II) - Black (Biotite)} and B- how wide is the rim thickness?
Please don't let your reading stop here. The real lesson continues below - read on!
What is alkali FELDSPAR?
The alkali feldspars include monoclinic feldspars (orthoclase and sanidine) and triclinic feldspars (microcline and anorthoclase).
What is BIOTITE?
Biotite is a silicate mineral in the common mica group. It is abundant in metamorphic rocks (both regional and contact), in pegmatites, and also in granites and other intrusive igneous rocks with the approximate chemical formula K(Mg, Fe)₃AlSi₃O₁₀(F, OH)₂ . Is is a mineral that is commonly black or very dark in colour.
What is HORNBLENDE?
Hornblende is an inosilicate (chain silicate) mineral, built around double chains of silica tetrahedra. These chains extend the length of the crystal and are bonded to their neighbors by additional metal ions to form the complete crystal structure. Amphibole is calcium-rich and monoclinic in crystal structure. Hornblende’s generalised chemical formula is (Ca,Na)2(Mg,Fe,Al)5(Al,Si)8O22 (OH)2. The four end-members and the cation content of their respective compositions are as follows: hornblende, Ca2(Mg4Al) (Si7Al); tschermakite, Ca2(Mg3Al2)(Si6Al2); edenite, NaCa2(Mg)5(Si7Al); pargasite, NaCa2 (Mg4Al)(Si6Al2).
Local setting
The feldspar, biotite and hornblende, which are the focus of this EarthCache is an integral part of the mineralogy of Rapakivi Granite.
Rapakivi granite is a hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). The name has come to be used most frequently as a textural term where it implies plagioclase rims around orthoclase in plutonic rocks. Rapakivi is a Finnish compound of "rapa" (meaning "mud" or "sand", while rapautua means "to erode") and "kivi" (meaning "rock"), because the different heat expansion coefficients of the component minerals make exposed rapakivi crumble easily into sand.
Rapakivi was first described by Finnish petrologist Jakob Sederholm in 1891. Since then, southern Finland's rapakivi granite intrusions have been the type locality of this variety of granite.
He described rapakivis as granites containing ovoid phenocrysts (these are 1 to 5 cm diameter crystals set in a finergroundmass) of pink or brown alkali feldspar (potassium-rich feldspar) that are entirely mantled bygrey plagioclase (calcium-rich feldspar, commonlyoligoclase) forming the rapakivi feldspar, or rapakivitexture.The colour of rapakivi feldspar is rather variable. The rapakivi feldspars areembedded in finer-grained groundmass, usually consisting of quartz, alkali feldspar, plagioclase,amphibole and biotite. Mantled feldspars occur side-by-side with non-mantled alkali feldspar phenocrysts.In continuation of this work, rapakivi texture was characterized sensu stricto by Atso Vorma in 1976 as: 1. an ovoidal shape of the alkali feldspar phenocrysts; 2. mantling of part of the ovoids by plagioclase shells; and 3. the presence of two generations of alkali feldsparand quartz. The age of rapakivi granites varies from LateArchaean to the Tertiary.
Photographs of rapakivi feldspars. A. Beige alkalifeldspar mantled by darkgreenish grey plagioclase,Wiborgite, SE Finland. B. White/brown alkalifeldspar with carmineplagioclase mantle, Altenberg-Frauenstein microgranite, EGermany. C.Broken pink alkali feldsparmantled by white plagioclase,Hammarudda porphyry, SWFinland. D. whitealkali feldspar with greenishyellow plagioclase mantle,Land’s End granite, SW England. (Source: Muller, A., 2007. Geology Today,Volume 23, Issue 3,May 2007,P.82-120;https://doi.org/10.1111/j.1365-2451.2007.00616.x)
Rapakivi granites, especially the famous rapakivis from southern Finland, are some of the mostattractive building and decorative stones, used world-wide because of their striking textures andbeautiful colours. But rapakivi granites are more than just building material, they have also beenstudied scientifically for over 100 years to try to explain the formation of their plagioclase-mantledalkali feldspar ovoids, the rapakivi feldspars which are responsible for the unique texture of the rock. Despite all the work which has been done on them, the formation of this distinctive texture is still not fully understood.
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