The oviline series of minerals consists of crystals in which $F e$ and $M g$ ions may substitute for each other causing substitutional impurity defect without changing the volume of the unit cell. In oviline series of minerals, oxide ion exists as FCC with $S i^{4 +}$ occupying $1 / 4 t h$ of octahedral voids and divalent ions occupying $1 / 4 t h$ of tetrahedral voids. The density of foresite (magnesium silicate) is $3.21 g / c c$ and that of fayalite (ferrous silicate) is $4.34 g / c c$ . Find the formula of forsterite and fayalite minerals and the percentage of fayalite in an olivine with a density of $3.88 g / c c$ .

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Solution

$Z_{e f f}$ of $O_{2} = 4$
$Z_{e f f}$ of $S i^{+ 4} = \frac{1}{8} \times 4 = 1$
$Z_{e f f}$ of divalent ion ( $F e$ or $M g$ )= $\frac{1}{4} \times 8 = 2$
General Formula $M_{2} S i O_{4}$
Forsterite= $M g_{2} S i O_{4}$ Fayalite: $F e_{2} S i O_{4}$

(B) Let fraction of fayalite be $x$ and fraction of forsterite be $1 - x$
$\Rightarrow 3.88 = 4.34 x + 3.21 (1 - x)$
$0.67 = 1.13 x \Rightarrow x = 0.593$
Therefore, percentage of fayalite = $59.3$ %

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Q. The olivine series of minerals consists of crystal in which

F e^{2 +}

and

M g^{2 +}

ions may substitute for each-other causing substitutional impurity defect without changing the volume of the unit cell. In olivine series of mineral, oxide ion exists as fcc with

S i^{4 +}

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Q. The olivine series of minerals consists of crystal in which

F e^{2 +}

and

M g^{2 +}

ions may substitute for each-other causing substitutional impurity defect without changing the volume of the unit cell. In olivine series of mineral, oxide ion exist as fcc with

S i^{4 +}

occupying

\frac{1}{4}

th of octahedral void and divalent ion occupying

\frac{1}{4}

th of tetrahedral void. The density of forsterite (Magnesium silicate) is

3.21

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4.34

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3.88

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A series of minerals consists of a crystal in which $F e^{2 +}$ and $M g^{2 +}$ ions may substitute for each other causing a substitutional impurity defect without changing the volume of the unit cell. In the mineral, the oxide ions exist as $f c c$ with $S i^{2 +}$ occupying $\frac{1}{4} t h$ of the octahedral voids and divalent ions occupying $\frac{1}{4} t h$ of the tetrahedral voids. The density of forsterite (Magnesium silicate) is 3.21 $g / c c$ and that of fayalite (Ferrous silicate) is 4.34 $g / c c$ . If the density of mineral is 3.88 g/cc, then which of the following statements is/are CORRECT.