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What about the variations on color / habit of a rock though? Would you have to get multiple samples for each variant, or is just one sufficient.
What traits do you guys normally use for ID?

You only really need one sample of a rock, but it always helps to have more. If you only have one sample, you will just have to research the rock on google. Wikipedia is also really helpful, because they usually have articles that outline every variation. Plus, the rocks that you will get grilled on the most (fluorite, calcite, etc.) generally have long articles, because they're complicated or very common.

For ID, I use crystal structure the most, followed by texture, and then density. It also helps to know what shades of colors they are (dark vs. light), and their translucency. Some you can tell automatically just from one or two traits. Galena (the heaviest for its size,) for instance, or graphite (rubs off on your skin).

On the side, I think next year they might be introducing the history of geologic discovery to the possible topics. The nationals test had some questions about Mohs that I think most people weren't expecting. We had the answer in our field guide, but that doesn't always happen. I wouldn't count on it, but if you have some extra time, go Wikipedia hopping and get a paragraph or so on the major discoveries and geologists.

Hey, anybody know the "official" chemical formula for the mineral apatite? There are so many different varieties of the chemical formula i don't even know which one is "real"....

hotchocolate123 wrote:Hey, anybody know the "official" chemical formula for the mineral apatite? There are so many different varieties of the chemical formula i don't even know which one is "real"....

I think Apatite is a mineral group, so its individual members will have different formulas (e.g. fluorapatite and chlorapatite). What kinds of formulas are you finding?

Paleofreakazoid wrote:

hotchocolate123 wrote:Hey, anybody know the "official" chemical formula for the mineral apatite? There are so many different varieties of the chemical formula i don't even know which one is "real"....

I think Apatite is a mineral group, so its individual members will have different formulas (e.g. fluorapatite and chlorapatite). What kinds of formulas are you finding?

^That's right. I think that there are only individual formulas; I don't know about the group as a whole though.
For example, (CaF) or Flourapatite is Ca5(PO4)3F, while (CaCl) or Chlorapatite is Ca5(PO4)3Cl.

aim4me26 wrote:

Paleofreakazoid wrote:

hotchocolate123 wrote:Hey, anybody know the "official" chemical formula for the mineral apatite? There are so many different varieties of the chemical formula i don't even know which one is "real"....

I think Apatite is a mineral group, so its individual members will have different formulas (e.g. fluorapatite and chlorapatite). What kinds of formulas are you finding?

^That's right. I think that there are only individual formulas; I don't know about the group as a whole though.
For example, (CaF) or Flourapatite is Ca5(PO4)3F, while (CaCl) or Chlorapatite is Ca5(PO4)3Cl.

Ohh.... thanks, but like I've been using the one on the Audubon field guide which is like the group formula --> [Ca5(PO4)3(F,Cl,OH)] then there's one on wikipedia that says the group is Ca10(PO4)6(OH,F,Cl,Br)2 and then when you purchase the science olympiad rock kit it comes with like a mini field guide which then says that the mineral apatite is Ca5(Cl,F)(PO4)3 ... I am sticking to the Audubon because it's an "official" published resource, and then most teachers tell you to NEVER use wikipedia... so...

hotchocolate123 wrote:Ohh.... thanks, but like I've been using the one on the Audubon field guide which is like the group formula --> [Ca5(PO4)3(F,Cl,OH)] then there's one on wikipedia that says the group is Ca10(PO4)6(OH,F,Cl,Br)2 and then when you purchase the science olympiad rock kit it comes with like a mini field guide which then says that the mineral apatite is Ca5(Cl,F)(PO4)3 ... I am sticking to the Audubon because it's an "official" published resource, and then most teachers tell you to NEVER use wikipedia... so...

Someone please correct me if I'm wrong, I'm still new to this kinda. I think the Audubon formula is the empirical formula; it is simplified, if you notice on your wikipedia formula, you can simplify all the subscripts; they are divisible by two. It also includes Bromine, which I think is a rare form of Apatite that is often omitted. Also, on wikipedia they have the Audubon formula on the side bar. The mini field guide just has the elements switched around. I don't have enough chemistry knowledge to tell you about the rules on that, since there are like, more than two ions or something, but essentially all the formulas have the same constituents. I think it is smart to go with the Audubon one, although somewhere in your notes you can include that Bromine can also be in there. Again, don't take me word for word, my science knowledge is limited and unspecialized...

Paleofreakazoid wrote:

hotchocolate123 wrote:Ohh.... thanks, but like I've been using the one on the Audubon field guide which is like the group formula --> [Ca5(PO4)3(F,Cl,OH)] then there's one on wikipedia that says the group is Ca10(PO4)6(OH,F,Cl,Br)2 and then when you purchase the science olympiad rock kit it comes with like a mini field guide which then says that the mineral apatite is Ca5(Cl,F)(PO4)3 ... I am sticking to the Audubon because it's an "official" published resource, and then most teachers tell you to NEVER use wikipedia... so...

Someone please correct me if I'm wrong, I'm still new to this kinda. I think the Audubon formula is the empirical formula; it is simplified, if you notice on your wikipedia formula, you can simplify all the subscripts; they are divisible by two. It also includes Bromine, which I think is a rare form of Apatite that is often omitted. Also, on wikipedia they have the Audubon formula on the side bar. The mini field guide just has the elements switched around. I don't have enough chemistry knowledge to tell you about the rules on that, since there are like, more than two ions or something, but essentially all the formulas have the same constituents. I think it is smart to go with the Audubon one, although somewhere in your notes you can include that Bromine can also be in there. Again, don't take me word for word, my science knowledge is limited and unspecialized...

Wow!! Thanks! It's awesome you know so much about chemistry ! Meanwhile.... idk anything about chemistry ... But thanks so much! I noticed that it also happens alot with other minerals...sigh.. I have forgotten quite alot over the summer... Back to work I guess