Q. How do you tell the difference between-
1. Quartz crystal, gypsum selenite, albite plagioclase, calcite, and other colorless minerals with the same crystal structure
2. Fluorite, beryl and topaz
3. Diorite, gabbro, and granite
4. Kaolinite, diatomite, and chalk
5. Sphalerite, magnetite, and galena
A.
1. They have VERY different crystal structures.
Quartz forms six-sided crystals that almost always terminate at one end (come to a point). It has a hardness of 7, which means that it scratches glass.
Selenite can look similar to calcite, but the main identifier is that selenite is very soft. Gypsum is a 2 and calcite is a 3. It's only a difference of 1 on the Mohs scale, but it's a huge difference because a fingernail has a hardness of 2.5. You can scratch selenite with your fingernail but not calcite.
Calcite has a ton of unique identifiers. Its rhombohedral crystal shape is a dead giveaway. Remember, it looks like a prism made from parallelograms, it is NOT a cube (some people mix them up). Calcite also fizzes in acid. You might not always get acid at a competition, but some event writers will tell you if the sample fizzes or not. On that note, any time they tell you that a mineral fizzes in acid, that narrows down the choice significantly. Fizzing in HCl means that it must be a carbonate mineral, and of those, the only ones on your list are calcite, aragonite, dolomite, azurite, and malachite, and they're all easily distinguishable. Calcite also exhibits double refraction. If you get a transparent sample, put it on top of your test and look through it. You'll see that it splits the words into two because it is birefringent.
These three minerals tend to be transparent or translucent, but albite is typically opaque. You can get opaque calcite though, but albite is much harder, with a hardness of 6 (it should just scratch glass). Albite, as a feldspar, is most easily identified by its cleavage. In any feldspar sample, you should see two planes of cleavage that make a 90 degree angle. In practical terms, you should see two smooth, flat, shiny sides that meet together while the other sides are more rough.
2. Out of these three, fluorite is by far the softest. With a hardness of 4, it won't scratch glass. Fluorite also has 4 planes of cleavage, and while you don't always get nice samples that show it on the sides, if you look closely in a translucent/transparent sample, you should see the cleavage planes inside the mineral. Fluorite is often green or purple, but it can be other colors too. And of course, fluorite glows under UV light.
Beryl and topaz can be a bit tricky. Both have a very high hardness, so they scratch glass with ease. Beryl has a hexagonal crystal structure and will generally appear as a six-sided column. Topaz is very lustrous and if you get a transparent sample, you can see kind of a rainbow, shimmery effect sometimes. It's hard to describe, but that's how I tell the difference.
3. As for the igneous rocks, all igneous rocks are classified based on two criteria: their composition (color) and texture (crystal size). Diorite, gabbro, and granite all have a coarse (large crystals) texture, but they vary based on composition. Granite is felsic, which means that it contains light colored minerals like potassium feldspar and quartz. Pink or clear minerals are a dead giveaway that the sample is granite. Diorite is intermediate. It will have a mix of light and dark minerals, mostly gray with spots of white or black. Gabbro is mafic, so it will be black, possibly with some green in it.
4. Kaolinite is a mineral while diatomite and chalk are rocks. They can be difficult to tell apart. Kaolinite is a clay mineral, and often has some brown in it, but not always. Diatomite and chalk are almost impossible to distinguish in hand sample. Luckily, the difference isn't too bad. Both are made up of the skeletons of once-living organisms, but the composition differs greatly. Diatomite is made from diatoms, which have a silica skeleton, so it is mostly made up of SiO2. Chalk is made of coccolithophores, which have a calcite skeleton, so it is made of CaCO3. That means that chalk (being a form of limestone) will fizz in acid and diatomite won't. If the event writer tells you that the sample fizzes in acid, it is definitely chalk.
5. Sphalerite is light brown to yellowish brown color and has more cleavage directions than the three cubic directions than galena has. It is also not as dense as galena, although it is also quite dense.
This link might help:
http://www.janrasmussen.com/physical%20geology/2_mineral_characteristics_Lab%20ppt.pdf
1. Quartz crystal, gypsum selenite, albite plagioclase, calcite, and other colorless minerals with the same crystal structure
2. Fluorite, beryl and topaz
3. Diorite, gabbro, and granite
4. Kaolinite, diatomite, and chalk
5. Sphalerite, magnetite, and galena
A.
1. They have VERY different crystal structures.
Quartz forms six-sided crystals that almost always terminate at one end (come to a point). It has a hardness of 7, which means that it scratches glass.
Selenite can look similar to calcite, but the main identifier is that selenite is very soft. Gypsum is a 2 and calcite is a 3. It's only a difference of 1 on the Mohs scale, but it's a huge difference because a fingernail has a hardness of 2.5. You can scratch selenite with your fingernail but not calcite.
Calcite has a ton of unique identifiers. Its rhombohedral crystal shape is a dead giveaway. Remember, it looks like a prism made from parallelograms, it is NOT a cube (some people mix them up). Calcite also fizzes in acid. You might not always get acid at a competition, but some event writers will tell you if the sample fizzes or not. On that note, any time they tell you that a mineral fizzes in acid, that narrows down the choice significantly. Fizzing in HCl means that it must be a carbonate mineral, and of those, the only ones on your list are calcite, aragonite, dolomite, azurite, and malachite, and they're all easily distinguishable. Calcite also exhibits double refraction. If you get a transparent sample, put it on top of your test and look through it. You'll see that it splits the words into two because it is birefringent.
These three minerals tend to be transparent or translucent, but albite is typically opaque. You can get opaque calcite though, but albite is much harder, with a hardness of 6 (it should just scratch glass). Albite, as a feldspar, is most easily identified by its cleavage. In any feldspar sample, you should see two planes of cleavage that make a 90 degree angle. In practical terms, you should see two smooth, flat, shiny sides that meet together while the other sides are more rough.
2. Out of these three, fluorite is by far the softest. With a hardness of 4, it won't scratch glass. Fluorite also has 4 planes of cleavage, and while you don't always get nice samples that show it on the sides, if you look closely in a translucent/transparent sample, you should see the cleavage planes inside the mineral. Fluorite is often green or purple, but it can be other colors too. And of course, fluorite glows under UV light.
Beryl and topaz can be a bit tricky. Both have a very high hardness, so they scratch glass with ease. Beryl has a hexagonal crystal structure and will generally appear as a six-sided column. Topaz is very lustrous and if you get a transparent sample, you can see kind of a rainbow, shimmery effect sometimes. It's hard to describe, but that's how I tell the difference.
3. As for the igneous rocks, all igneous rocks are classified based on two criteria: their composition (color) and texture (crystal size). Diorite, gabbro, and granite all have a coarse (large crystals) texture, but they vary based on composition. Granite is felsic, which means that it contains light colored minerals like potassium feldspar and quartz. Pink or clear minerals are a dead giveaway that the sample is granite. Diorite is intermediate. It will have a mix of light and dark minerals, mostly gray with spots of white or black. Gabbro is mafic, so it will be black, possibly with some green in it.
4. Kaolinite is a mineral while diatomite and chalk are rocks. They can be difficult to tell apart. Kaolinite is a clay mineral, and often has some brown in it, but not always. Diatomite and chalk are almost impossible to distinguish in hand sample. Luckily, the difference isn't too bad. Both are made up of the skeletons of once-living organisms, but the composition differs greatly. Diatomite is made from diatoms, which have a silica skeleton, so it is mostly made up of SiO2. Chalk is made of coccolithophores, which have a calcite skeleton, so it is made of CaCO3. That means that chalk (being a form of limestone) will fizz in acid and diatomite won't. If the event writer tells you that the sample fizzes in acid, it is definitely chalk.
5. Sphalerite is light brown to yellowish brown color and has more cleavage directions than the three cubic directions than galena has. It is also not as dense as galena, although it is also quite dense.
This link might help:
http://www.janrasmussen.com/physical%20geology/2_mineral_characteristics_Lab%20ppt.pdf