Answer the following me questions on feldspar group minerals. Explain how you \'
ID: 161432 • Letter: A
Question
Answer the following me questions on feldspar group minerals. Explain how you 'build' the different feldspars (give formulas) given Si_4O_8 as the starting material. Using the triangular feldspar diagram below, give the names and formulas for the three end-member feldspars labeled A. B. and C. Then, give the exact name of the feldspar (i e.. labradonte) with the following composition Use the T-X dayam given below to explain the origin of exsolution phenomena (i.e.. pert hit ic texture) In alkali feldspars. What happens to a feldspar with a composition given by 'X' on the figure as it coots from 900degree C to 600degree C? What would be the temperature at which exsolution would first begin to occur? What would be the approxmate composition of the exsolution lamellae and the host crystal (e.g . expressed as Ab_10O_r90 and Ab_95O_rs) at a temperature of 600degree C?Explanation / Answer
(a) Al can proxy for Si at the centre of Sio4 tetrahedron,but substitution of Si4+ by Al3+ need to invite other cations to maintain charge balance so that formation of a varity allminosilicates is favoured. The most widesspred aluminosilicates ids the feldspar group which constitutes more than sixty percent of the crust. If 20% of the structure Si4O8 is replaced by Al the we get a complex(Si3Alo8) which will will invite a monovalent cation either Na or K gain charge balance & produce e.g sodium feldspar NaAlSi3O8 (Albite or Ab) & potassium feldspar KAlSi3O8 (genaralised as orthoclase or Or). 50% Si-Al substitution invites a single divalent cation e.g Ca2+ in the structure to produce CaAl2Si2O8 (Anorthite or An). No further substitution of Si by Al under natural condition is possible
(b)
A= Anorthite (CaAl2Si2O8 )
B= Albite (NaAlSi3O8 )
C= orthoclase (KAlSi3O8 )
The name of the feldspar is - albitize plagioclase
(c) As the magmas cool from 9000c to 6000c the calcium ions are depleted, the feldspar formation becomes predominantly sodium feldspar (Na-feldspar, NaAlSiO). At 600°C, the feldspar formation is nearly 100% sodium feldspar (Na-feldspar, NaAlSiO).
Exsolution would first begain to occur near 7000C
NaAlSi3O8 is approx composition of exsolution lamellae & Host crystls is KAlSi3O8.