anorthite and albite 10. Anorthite (CaAl,Si Os) and albite (NaAlSi,Os) are end-m

Question

anorthite and albite

10. Anorthite (CaAl,Si Os) and albite (NaAlSi,Os) are end-members of a continuous series of feldspar minerals found in igneous rocks. Although albite and anorthite share the same feldspar crystal structure, anorthite, the Ca-rich end-member, crystallizes at high temperature from magmas deficient in silicon (Si). By contrast, albite, the Na-rich end- member, crystallizes at lower temperature from magmas enriched in Si. Using atomic weights found in the periodic chart, a) find the formula weights b) the weight percentages of Si and c) the Si/O mole ratios for albite and anorthite. Formula weight (g mol') Si/O mole %Si l ratio Feldspar Albite Anorthite Formula NaAlSi Og CaAl Si2Os 11. Olivine, (Mg.Fe) SiO4 and quartz, SiO2 are two silicate minerals also found in igneous rocks. Like the continuous feldspar series, olivine crystallizes at high temperature from Si- deficient magmas while quartz crystallizes from Si-rich, low temperature magmas; however, unlike the continuous feldspar series, olivine and quartz have quite different crystal structures. The crystal structure of olivine is based on Fe and Mg bonded to "isolated" silicate (Sio44) units whereas, quartz can be described as a 3-dimensional network of silicate "building blocks". Using atomic weights found in the periodic chart, a) find the formula weights b) the weight percentages of Si and c) the Si/O mole ratios for olivine and quartz. Formula weight (g mol) Si/O mole %Si l ratio Silicate mineral Formula (Mg,FebSi04 SiO Olivine uartz

Explanation / Answer

Atomic weights ( g/mole): Na=23, Al=27, SI=28 and O=16

Formula weight of NaAlSi3O8= 1* atomic weight of Na+ 1* atomic weight of Al+ 3* atomic weight of Si+ 8* atomic weight of O

=1*23+1*27+3*27+8*16=262 g/mole

Basis: 1 moles of NaAlSi3O8. It contains

NaAlSi3O8-------àNa+Al+3SI+ 8O

Mole of Si in 1 mole NaAlSi3O3= 3, 1 mole of Al, 1 mole of Na and 8 moles of O.

Mole= mass/molar mass, mass= molar mass= 262 gm

262 gm contains 3*28=84 gm of Si

% of SI= 100*84/262=32.1%

Si/O mole ratio = 3/8

2. CaAl2Si2O8.

Atomic weights ( g/mole): Na=23, Al=27, SI=28 and O=16, Ca= 40

Formula weight= 40+2*27+2*28+8*16=278 g/mole

Basis : 1 mole of CaAl2Si2O8 = 278 g

Mass of Si in CaAl2Si2O8= 2*28= 56 gm

Mass % of SI= 100*56/278=20.14%, mole ratio of Si/O= 2/8

2.

2. CaAl2Si2O8.

Atomic weights ( g/mole): Na=23, Al=27, SI=28 and O=16, Ca= 40

Formula weight= 40+2*27+2*28+8*16=278 g/mole

Basis : 1 mole of CaAl2Si2O8 = 278 g

Mass of Si in CaAl2Si2O8= 2*28= 56 gm

Mass % of SI= 100*56/278=20.14%, mole ratio of Si/O= 2/8

3. (MgFe)2SiO4, atomic weight: Mg=24, Fe= 56, Si= 28, O=16

Molar mass of (MgFe)2SiO4= 2*(24+56)+27+4*16= 251 g/mole

251 g/mole contains 28 gm of Si

% of Si =100*28/251 = 11.15%

Si/O ratio = ¼=0.25

4. SiO2, molar mass = 27+2*16= 59, % of SI= 100*27/59 = 45.76%

Si/O ratio = ½= 0.5

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