Please see response attached, which is also presented below. I hope this helps and best of luck!

RESPONSE:

Let's take a closer look through discussion and example, which you can draw on for your final copy.

1. Overall, the physical properties of minerals provide a reliable means to identify common minerals. However, certain properties can exhibit a range of characteristics or values making them less useful for identification purposes.

Minerals are defined as solid, inorganic substances that occur naturally and have a definite atomic structure and chemical composition. They exhibit several properties that are useful for identification. Color, crystal shape, hardness, and luster are a few of the properties that can be used to determine a mineral's identity. Color is one of the least reliable tests for identifying a mineral sample, but may provide some clues depending on the specimen. Various physical properties, in addition to crystal habit, can be used to identify different minerals. The properties include:

· Luster is the appearance of light reflected from fresh mineral surface, and includes two major subdivisions:

- metallic
- non-metallic - vitreous (glassy), waxy, pearly, earthy, resinous, etc.

· Cleavage is the tendency to break along the direction of weak atomic bonds and reflects relative strength of bonds within a mineral's atomic structure. Some minerals have cleavage, whereas others do not. Micas provide a good example of the tendency to break along direction of weak atomic bonds. Like all sheet silicates, a single cleavage is well-developed between the sheets. Minerals that do not have cleavage (e.g., quartz) tend to fracture or break irregularly.

Conchoidal fracture is developed where a mineral breaks with smooth curved surfaces (like broken glass). Cleavage may be the best means of identifying between minerals that otherwise look identical, e.g.,
pyroxene ? 90°
amphibole ? 120°

· Hardness is the resistance to scratching, and reflects differences in the relative strength of chemical bonds in different mineral structures. Mineral hardness is described in terms of Mohs' Hardness Scale (from softest to hardest):

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase (K-feldspar)
7. Quartz
8. Topaz
9. Corundum
10. Diamond

· Streak is the color of the powdered mineral (is highly reliable).
· Specific gravity or density is a measure of the weight and packing of atoms involved in the mineral's atomic structure. It equals the weight of the mineral divided by the weight of an equal volume of water.
· Color is the least reliable property.
· Double refraction involves generation of two images of single object when viewed through a mineral. The two images are caused by non-uniform velocities of light transmission in different directions (e.g., calcite).
· Striations are parallel lines or bands across the surface of a mineral's crystal or cleavage face. They are caused by non-uniform growth rates or multiple individuals inter-grown in the crystal (e.g., feldspar).
· Magnetism is a characteristic feature of magnetite.
· Taste is a characteristic feature of rock salt. (1)

Many mineral properties (e.g., cleavage) may be seen best when a wafer-thin slice of the mineral (or rock) mounted on a glass slide (a thin section) is viewed under a microscope.

Now let's look at three of these physical properties that vary between samples.

2. Choose three physical properties that might vary considerably between samples of the same mineral and explain why such variability might exist.

The properties and the identity of a mineral are determined by its chemical composition and the pattern in which the atoms are arranged. So, how is it possible for a particular mineral species, such as quartz, to have a wide range of appearances and still be the same mineral? Some of the properties (color, hardness, luster, streak, crystal shape) of a given mineral may vary without the name

Color is a striking feature of minerals. Some minerals have characteristic colors. For instance, sulfur (yellow), gold (golden), Amethyst (purple). Minerals with metallic luster usually have characteristic colors. For examples, sulfides are relatively easy to diagnose from their colors. However, the colors of minerals with non-metallic luster can vary within the same mineral, as their colors are often controlled by minor or trace element concentrations. For example, Corundum can be sapphire (blue) or ruby (red). (3) The color of a larger chunks of mineral can really vary, depending on what trace element impurities may be present, for example, in Calcite, which can be any color of the rainbow (and a few that aren't on any rainbow). But calcite always has a white streak.

Hardness - A good property in mineral identification is one that does not vary from specimen to specimen. In terms of reliability, hardness is one of the better physical properties for minerals. Specimens of the same mineral may vary slightly from one to another, but generally they are quite consistent. Inconsistencies occur when the specimen is impure, poorly crystallized, or actually an aggregate and not an individual crystal. In other words, the same mineral can vary in hardness due mostly to impurities or substitution of certain ions. (4)

Also, the hardness of a particular mineral may vary with direction within the same grain. Kyanite is a good example. Kyanite generally occurs in long bladed crystals. The hardness taken the short way across the blade has a hardness of 7 the hardness taken the long way along the same grain will be 4.0. Muscovite is another good example of this. Its hardness is 2.5 when taken across the flat surface of a cleavage sheet, but 4 when taken across the grain of a book. The reason hardness varies in this way is that the phenomenon depends on the strength of the bonds holding the mineral together. The bond strength can be significantly different in different directions in the mineral, giving the different hardness. In most minerals this difference with direction is minor and doesn't affect the test. In the case of kyanite, this difference in hardness is a confirming test by itself. (6)

Some minerals' hardness may vary from sample to sample depending on that mineral's exact chemical composition. Hornblende's hardness can vary from 5 to 6, meaning some hornblende is softer than glass, some harder. This reflects the fact that hornblende can accommodate varying amounts of sodium, calcium, iron and magnesium in its structure, which affect the details of its chemical bonding, hence its hardness. (6)

Luster: The quantity and quality of light reflected from the surface. Most identification schemes begin with a simple classification based on luster. The luster of a mineral is the way that it shines or doesn't shine. There are two main categories: Metallic and Non- Metallic. The metallic luster does not have any sub-categories. Non-metallic luster has many sub-categories. Because of this, luster is the first fundamental test to be made when identifying any mineral. Most are relatively obvious, but some minerals can exhibit a range of luster's (ex. Hematite). (5)