In a scientific experiment, the controlled variable is a factor that the experimenter holds constant and does not allow to change. Other variables are changed by the participants in the study, by the experimenter or by natural circumstances.
The Dana system divides minerals into nine basic classes: native elements, silicates, oxides, sulfides, sulfates, halides, carbonates, phosphates, organics and mineraloids. The Dana system of classification was devised by Professor James Dana of Yale University in 1848. His system classified minerals according to their chemical composition. For example, a halide has Cl (chlorine), Br (bromine), F (fluorine) or I (iodine) at the end of its chemical formula.
A new Dana system of classification contains 78 different classes of minerals based on composition and then further classified by type and group. To be considered a mineral, a substance must be an inorganic, naturally formed solid, with a specific chemical formula and a fixed internal structure. To test whether something is a mineral, there are several identification tests to which the substance is subjected, including its resistance to scratching, its density in comparison to water, its color, the degree of light it reflects, the color of the powdered mineral, its breakage pattern and its crystalline form.
There are over 4,900 different types of minerals known in the world.