# Laboratory Synthesis of the Mineral Hedenbergite

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Mineralogists often synthesize minerals to verify the composition of naturally occurring minerals by preparing them synthetically. Sometimes minerals are first synthesized, and only later, when their properties are known, are they found in nature. Synthesizing related minerals in the laboratory can also help understand how the formations came to be in nature. For example, the mineral Hedenbergite

is rarely found in nature, but it has been synthesized ^{[1]}^{[2]} in the laboratory.

The pyroxene family to which it belongs the variation in composition as shown in the composition diagram below. Hedenbergite has one Fe^{2+} ion and one Ca^{2+} ion associated with a chain of tetrahedral silicate groups [~O-Si(O2)-O-Si(O2)-O~]. Other pyroxenes include Fe_{2}Si_{2}O_{6} and MgCaSi_{2}O_{6}, but Wollastonite (Ca_{2}Si_{2}O_{6}) has a different structure and is not classified as a pyroxene. Some pyroxenes have Al^{3+} in place of the Si in the tetrahedral chains. This variation is typical of minerals with chains of silicate tetrahedra ^{[3]}, and results in mineral samples from adjacent locations that are slightly different in composition and appearance. Mixtures of the various minerals are common. A synthesis therefore has to combine the elements in the proper proportions to prepare Hedenbergite and not another pyroxene.

The chemical equation

- CaO + FeO + 2 SiO
_{2}→ CaFeSi_{2}O_{6}(1)

tells us that we might be able to make hedenbergite by grinding a mixture of CaO (lime), FeO, and SiO_{2} (sand) together in a mortar or mill. The mixture has to be heated to 1000^{o}C for a week or so at 1 Atm pressure to complete the reaction. In reality, CaCO_{3} (limestone) is used in place of CaO (which is unstable with respect to reaction with humid air to form Ca(OH)_{2}, H_{2}SiO_{2} is used in place of SiO_{2}and Fe_{2}O_{3} is used in place of the unstable FeO (which oxidizes slowly in air to Fe_{2}O_{3}). Nonetheless, the simplified equation above can be used^{[4]} ^{[5]}.

We can use the balanced chemical equation (1) to calculate the optimal ratio of reactants. It not only tells how many atoms, molecules, or "formula units" ^{[6]} of each kind are involved in a reaction, it also indicates the *amount* of each substance that is involved. Equation (1) says that 2 SiO_{2} *formula units* can react with 1 CaO *formula units* and 1 FeO *formula units* to give 1 CaFeSi_{2}O_{6} *formula unit*. It also says that 2 *mol* SiO_{2} would react with 1 *mol* CaO and 1 *mol* FeO, yielding 1 *mol* CaFeSi_{2}O_{6}.

The balanced equation does more than this, though. It also tells us that 2 × 2 = 4 mol SiO_{2} will react with 2 × 1 = 2 mol CaO, and that ½ × 2 = 1 mol SiO_{2} requires only ½ × 1 = 0.5 mol CaO. In other words, the equation indicates that exactly 1 CaO must react *for every* 2 mol SiO_{2} consumed. For the purpose of calculating how much CaO is required to react with a certain amount of SiO_{2} therefore, the significant information contained in Eq. (1) is the *ratio*

We'll call such a ratio derived from a balanced chemical equation a **stoichiometric ratio** and give it the symbol *S*. Thus, for Eq. (1),

The word *stoichiometric* comes from the Greek words *stoicheion*, “element,“ and *metron*, “measure.“ Hence the stoichiometric ratio measures one element (or compound) against another.

## Contents |

## EXAMPLE 1

Derive other possible stoichiometric ratios from Eq. (1)

**Solution** Any ratio of amounts of substance given by coefficients in the equation may be used:

*When any chemical reaction occurs, the amounts of substances consumed or produced are related by the appropriate stoichiometric ratios*. Using Eq. (1) as an example, this means that the ratio of the amount of SiO_{2} consumed to the amount of CaO consumed must be the stoichiometric ratio S(SiO_{2}/CaO):

Similarly, the ratio of the amount of CaFeSi_{2}O_{6} produced to the amount of SiO_{2} consumed must be

S(K_{2}S/SiO_{2}):

In general we can say that

or, in symbols,

Note that in the word Eq. (3a) and the symbolic Eq. (3b), X and Y may represent *any* reactant or *any* product in the balanced chemical equation from which the stoichiometric ratio was derived. No matter how much of each reactant we have, the amounts of reactants *consumed* and the amounts of products *produced* will be in appropriate stoichiometric ratios.

## EXAMPLE 2

Find the amount of CaFeSi_{2}O_{6} that will be produced when 0.00806 mol SiO_{2} is consumed according to Eq. (1).

**Solution** The amount of CaFeSi_{2}O_{6} produced must be in the stoichiometric ratio S(CaFeSi_{2}O_{6}/SiO_{2}) to the amount of potassium nitrate consumed:

Multiplying both sides by *n*_{KNO3 consumed}, by we have

This is a typical illustration of the use of a stoichiometric ratio as a conversion factor. Example 2 is analogous to Examples 1 and 2 from Conversion Factors and Functions, where density was employed as a conversion factor between mass and volume. Example 2 is also analogous to Examples 2.4 and 2.6, in which the Avogadro constant and molar mass were used as conversion factors. As in these previous cases, there is no need to memorize or do algebraic manipulations with Eq. (3) when using the stoichiometric ratio. Simply remember that the coefficients in a balanced chemical equation give stoichiometric ratios, and that the proper choice results in cancellation of units. In road-map form

or symbolically.

When using stoichiometric ratios, be sure you *always* indicate moles *of what*. You can only cancel moles of the same substance. In other words, 1 mol KNO_{3} cancels 1 mol KNO_{3} but does not cancel 1 mol CO_{2}.

The next example shows that stoichiometric ratios are also useful in problems involving the mass of a reactant or product.

## EXAMPLE 3

Calculate the masses of CaO and FeO that will react with 0.484 g of SiO_{2} according to equation (1), and the masses of all products that will form. This will solve the problem of which recipe for hedenbergite is optimal^{[7]}.

**Solution** The problem asks that we calculate the masses of reactants and products. As we learned in Example 2 of The Molar Mass, the molar mass can be used to convert from the amount of any substance to its mass. So we can calculate the amount of SiO_{2} available:

Now we need to calculate the amounts of reactants and products from the amount of SiO_{2} consumed. This is similar to Example 2. It requires stoichiometric ratios like

The *amount* of CaO consumed is then

The *mass* of CaO is

With practice this kind of problem can be solved in one step by concentrating on the units. The appropriate stoichiometric ratio will convert moles of O_{2} to moles of SO_{2} and the molar mass will convert moles of SO_{2} to grams of SO_{2}. A schematic road map for the one-step calculation can be written as

Thus

These calculations can be organized as a table, with entries below the respective reactants and products in the chemical equation.

- + 2 SiO
_{2}

CaO | + FeO | + 2 SiO_{2} →
| CaFeSi_{2}O_{6}
| |
---|---|---|---|---|

m (g) | 0.226 | 0.289 | 0.484 | 0.999 |

M (g/mol) | 56.1 | 71.8 | 60.1 | 248.1 |

n (mol) | 0.00403 | 0.00403 | 0.00806 | 0.00403 |

The masses of products are calculated in a similar way, for example:

## EXAMPLE 4

What mass of Fe_{2}O_{3} would be consumed when 0.626 g of H_{2}SiO_{3} reacts (with stoichiometric CaCO_{3} present) to produce CaFeSi_{2}O_{6}, CO_{2}, and H_{2}O?

**Solution** First, write a balanced equation

2 CaCO_{3} + Fe_{2}O_{3} + 2 H_{2}SiO_{3} → CaFeSi_{2}O_{6} + 2 CO_{2} + 2.5 O_{2}

The problem gives the mass of metasilicic acid, H_{2}SiO_{3} and asks for the mass of Fe_{2}O_{3} required to combine with it. Thinking the problem through before trying to solve it, we realize that the molar mass of metasilicic acid could be used to calculate the amount of metasilicic acid consumed. Then we need a stoichiometric ratio to get the amount of Fe_{2}O_{3} consumed. Finally, the molar mass of Fe_{2}O_{3} permits calculation of the mass of Fe_{2}O_{3}. Symbolically

## References

- ↑ Bowen, N.L.; Schairer, J. F.; Posnjak, E. The Systeme CaO-FeO-SiO2. Am. J. Sci., 5th ser., V. 26, p. 193-284.
- ↑ Lindsley D. H. and Munoz J. L. (1969) Solidus Relations Along The Join Hedenbergite – Ferrosilite. American Journal of Science. Vol. 267-A, pp. 295-324
- ↑ http://en.wikipedia.org/wiki/Pyroxene
- ↑ . The equation
2 CaCO
_{3}+ Fe_{2}O_{3}+ 2 H_{2}SiO_{3}→ CaFeSi_{2}O_{6}+ 2 CO_{2}+ 2.5 O_{2}might be preferred by chemists, but geologists often represent syntheses in terms of simple oxides - ↑ This synthesis is the subject of an student laboratory experiment, "Mineral Synthesis and X-Ray Diffraction Experiments" Perkins, D.; Sorensen, P. in
*Teaching Mineralogy*, Brady, J.B. and Mogk, D.W. Eds., Mineralogy Society of America, Washington, DC, 1997, pp. 81-90. - ↑ "formula units" is the term used to describe the composition of ionic or network compounds which are not made up of molecules. For example, SiO
_{2}(quartz or sand) is a network crystal made up of Si atoms each with four bridging oxygen atoms to other Si atoms. The ratio is 1 Si: 2 O, even though the structure is made up of SiO4 tetrahedra linked through their vertices. - ↑ This synthesis is the subject of an student laboratory experiment, "Mineral Synthesis and X-Ray Diffraction Experiments" Perkins, D.; Sorensen, P. in
*Teaching Mineralogy*, Brady, J.B. and Mogk, D.W. Eds., Mineralogy Society of America, Washington, DC, 1997, pp. 81-90.