Identifying Minerals

okay so let's talk about mineral color now you might think mineral color is a good simple characteristic to use for mineral identification after all minerals exist in all different colors why not use that as a jumping off point but the fact is there's a problem there's a problem with using color to identify minerals and let me explain what that problem is take a look at this necklace the beads that make up this necklace are all made of quartz that is to say they are all the same mineral so clearly quartz exists in a variety of colors and so I can't count on quartz being white or blue or green it could be any of those colors that poses a problem for mineral identification similarly take a look at these minerals I think you'd agree they're all purple in color but the fact is each of these is a different type of mineral and so using color is not a reliable characteristic for identification so how do we measure the hardness of a mineral well in order to do this we need a piece of glass now glass has a hardness of 5.5 it falls in the middle of the Mohs hardness scale and I will take a mineral sample this is a piece of potassium feldspar and I will try and scratch the glass with the feldspar in order to do this you have to have the glass flat down on the table and you take your sample and you have to push not only are you looking for scratches on the surface but you're also trying to listen and feel to see if the sample is grinding into the glass or not if it does scratch the glass then this mineral has a hardness greater than 5.5 if it does not scratch the glass then it's less than 5.5 lustre refers to how light reflects off of the surface of a mineral for our class we will be using two types of luster metallic and nonmetallic metallic minerals have a metallic appearance what I mean is that they are gold or silver and actually look like a piece of metal like this chunk of Galena or this chunk of pyrite notice how they look like they are metal so those have a metallic luster then there are other minerals like this olivine which does not look like metal and this has a nonmetallic luster as does this gypsum it does not look like metal so when you're doing a luster test all you have to do is ask yourself do the samples look like metal if so they are metallic or do they not look like metal if so they are nonmetallic breakage is probably the hardest property to identify minerals break in one of two ways cleavage or fracture fracture is when a mineral breaks randomly if you look at this piece of olivine you'll notice it has a jagged irregular shape it looks like it was just smashed into a random shape this is referred to as fracture this mineral displays fracture this red hematite also displays fracture you notice the random irregular shape as does this quartz sample random and irregular and this pyrite sample random and irregular that is fracture however there are some minerals on earth that break in very unique and predictable ways like this piece of calcite it almost looks like it was cut this way with a saw but in fact this is the natural way that calcite breaks this breakage is referred to as cleavage you'll notice it has predictable flat sides and if I were to smash this with a hammer the little pieces that it created would be shaped similarly this piece of Galena has a really interesting type of calcite has a really interesting type of cleavage if you look at it carefully you'll notice that it too breaks at these predictable 90-degree angles flat sites this was not cut this way this is how it naturally breaks as does this halite this displays cleavage there are some other types of cleavage as well this is called biotite mica and it looks like a flat sheet what's neat about biotite mica is you can actually peel off thin pieces of it and that is a different type of cleavage I have a piece of peeled off biotite mica here you notice it's paper-thin this was peeled off of a larger sample so just to sum up cleavage is predictable breaking like this calcite fracture is random breaking like this olivine streak refers to the color of a mineral as a powder in order to make a powdered sample of a mineral we use streak plates these are ceramic tiles one is black and one is white and what we do is we place them on the desk and we rub our mineral samples against them to create some powder and see what color that powder is let me demonstrate this is a piece of red hematite I'm gonna take this hematite and I'm gonna gently one time try and draw a line and what you notice is that it has a reddish brown streak I would refer to that as a colored streak if I try it on the dark plate I will see it and I can more clearly see the color so when we do streak test we often try it on both the black plate and the white plate let me give you another example this is pyrite notice how it looks gold but when I powder it on a streak plate it creates a grayish black streak still it is a colored streak that one might be difficult to see on a dark plate because it blends in with the background this is why we use both plates other minerals like sulfur might give you light colored streaks like a yellow streak which is more visible on a dark place other minerals might give you no streak at all like calcite does not leave any mark behind whatsoever even on the white plate it doesn't really leave anything clear maybe a little bit of white but nothing terribly clear so when you're doing these tests you're going to determine does your mineral leave a colored streak or does it leave nothing or white so again you have two categories colored or none or white beyond streak cleavage fracture hardness and luster there are a category of other properties that we might use to identify minerals and there are a variety of things for example this piece of halite could be identified based on taste halite is salt and if you licked it it would have a salty taste please don't lick any of your mineral samples over the next few days some minerals have a really unique property like this piece of magnetite now as the name implies it is in fact magnetic as you can see the paperclip is stuck to it other minerals have more unique characteristics like this calcite calcite has two really interesting properties one of which is referred to as double refraction if you put it over some type on a piece of paper and look at it carefully it doubles the type you notice how the word property is on my reference table here has been doubled as you look through it through the calcite in addition if I were to drop some hydrochloric acid on this calcite it would fizz up like soda so those are some of the other characteristics we're going to talk about

Channel: Mike Sammartano

Views: 806,169

Rating: 4.7426567 out of 5

Keywords: Mineral, Minerals, Mineral Identification, Luster, Streak, Hardness, Cleavage, Fracture, Quartz, Mineral Samples, Rocks and Minerals, ESRT, Earth Science Reference Tables

Id: 32NG9aeZ7_c

Length: 8min 33sec (513 seconds)

Published: Tue Jan 27 2015