Crystals in Minerals

[kingwinner]

1) How many sides do crystals of each of the 6 major crystal systems have?

 

I am really puzzled when by this question, since I can see two completely different shapes on the above 2 diagrams that I obtained from 2 different sources. For example, orthorthombic system on the top only has 6 sides while the bottom one has 14 sides (I think). Which one is right?

 

2) How would you use crystal structure to tell a crystal of pyrite from a crystal of gypsum?

The problem is I do not know whether "crystal structure" and "crystal system" are the same thing...I searched google definitions but I don't get what they mean

If so, the answer is simple -- cubic for pyrite and monoclinic for gypsum

 

3) I have learned that most crystals are restricted by its surroundings so large crystals usually can't grow. But do minerals still tend to appear in one of the 6 well-defined shapes? I mean, will they still develop into these nice, well-defined shapes?

 

 

Does anyone know??

[LOC]

1) How many sides do crystals of each of the 6 major crystal systems have?

 

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I am really puzzled when by this question, since I can see two completely different shapes on the above 2 diagrams that I obtained from 2 different sources. For example, orthorthombic system on the top only has 6 sides while the bottom one has 14 sides (I think). Which one is right?

They are both right. Crystallography is not my bag, but crystal systems are not defined by the number of sides. It is based on crystal symmetry and relationships between the axes of reference. There are six crystal systems and within those six are 32 crystal classes.

 

2) How would you use crystal structure to tell a crystal of pyrite from a crystal of gypsum?

Pyrite is isometric (3 axes are all equal length), while gypsum is monoclinic (3 axes are all different lengths where one is perpendicular to a plane formed by the other two).

 

 

 

The problem is I do not know whether "crystal structure" and "crystal system" are the same thing...I searched google definitions but I don't get what they mean

If so, the answer is simple -- cubic for pyrite and monoclinic for gypsum

Crystal structure refers to the internal arrangement of atoms. Crystal system is the classification system based on crystallographic axes.

 

Also, pyrite does not just occur in the cubic form. As I stated previously, pyrite is isometric (i.e, equant), but in addition to cubic also occurs as octohedrons, pyritohedrons, as well as in massive forms (no outward crystal structure) as nodular, stalactitic, fibrous, etc.

 

 

 

3) I have learned that most crystals are restricted by its surroundings so large crystals usually can't grow. But do minerals still tend to appear in one of the 6 well-defined shapes? I mean, will they still develop into these nice, well-defined shapes?

 

Does anyone know??

This is a somewhat complicated question and I'm not sure I understand it fully.

 

For the most part, crystals do form identifiable crystal faces and forms, but the whole crystal usually will not unless, as you stated, they are allowed to grow in unrestricted space, such as in open-space vugs or magma.

 

Additionally, ALL minerals form within the six systems, but they don't always exhibit such features externally.

[LOC]

King, are you studying geology?

[kingwinner]

Yes, I am taking geology!

 

1) I was given this question "How many sides do crystals of each of the 6 major crystal systems have?" in a workbook, but according to what you have said, this question is impossible to answer.........

 

2) "How would you use crystal structure to tell a crystal of pyrite from a crystal of gypsum?"

Then, by saying "crystal structure", the qeustion is bascially looking for one of the 6 crystal shapes or crystal systems! Therefore, it's the same answer if it is put in this way "How would you use crystal system to tell a crystal of pyrite from a crystal of gypsum?"

 

3) Then I guess in most cases, well-defined crystals won't form because there are usually in a restricted space.

[LOC]

Yes, I am taking geology!

Cool!

 

1) I was given this question "How many sides do crystals of each of the 6 major crystal systems have?" in a workbook, but according to what you have said, this question is impossible to answer.........

Well, if they are asking specifically for sides of the 6 crystal systems (which is a basic classification), then all of them have 6 sides, except the hexagonal system which has 8. It's when discussing minerals in general that the question becomes somewhat difficult to answer as the number of sides will depend on the mineral in question.

 

At least that's what I would guess. Sometimes questions are overly vague.

 

 

2) "How would you use crystal structure to tell a crystal of pyrite from a crystal of gypsum?"

 

Then, by saying "crystal structure", the qeustion is bascially looking for one of the 6 crystal shapes or crystal systems! Therefore, it's the same answer if it is put in this way "How would you use crystal system to tell a crystal of pyrite from a crystal of gypsum?"

Yes, I would say that.

 

When discussing crystal structure and crystal systems within the same context, they have different meanings. However, crystal structure is often used interchangeably with crystal system, crystal habit, crystal form, etc. At least that's been my experience.

 

 

3) Then I guess in most cases, well-defined crystals won't form because there are usually in a restricted space.

That really depends on the rocks and what sorts of crystals you have in mind. If you are referring to crystals that you often see at gem and mineral shows, then yes, these generally require unrestricted growth.

 

However, some rocks such as igneous rocks and metamorphic rocks, often contain euhedral minerals except they are contained within a groundmass.

[kingwinner]

Well, if they are asking specifically for sides of the 6 crystal systems (which is a basic classification), then all of them have 6 sides, except the hexagonal system which has 8. It's when discussing minerals in general that the question becomes somewhat difficult to answer as the number of sides will depend on the mineral in question.

1) How do you know that all of them have 6 sides, except hexagonal system with 8 sides? Clearly I can see that for the bottom figure, each of the 6 crystal systems has more than 6 sides, right? My textbook says that these (bottom figure) are the 6 basic, typical crystal systems / crystal shapes! I am confused because the top figure and the bottom figure shows completely different typical shapes for the same crystal system, why is it like that? :hihi:

 

 

Please allow me to ask just another quick question -- is glass a mineral?? Does glass have a definite chemical composition?

[LOC]

1) How do you know that all of them have 6 sides, except hexagonal system with 8 sides? Clearly I can see that for the bottom figure, each of the 6 crystal systems has more than 6 sides, right?

Instead of looking at the crystals themselves (which are examples of each system), look at the crystallographic axes.

 

 

My textbook says that these (bottom figure) are the 6 basic, typical crystal systems / crystal shapes! I am confused because the top figure and the bottom figure shows completely different typical shapes for the same crystal system, why is it like that? :hihi:

The textbook says, "Table 9-3 The Six Basic Crystal Systems." It is discussing the Isometric, tetragonal, hexagonal, etc. systems, not the individual examples of each (such as the pyrite example given for the Isometric System).

 

 

Please allow me to ask just another quick question -- is glass a mineral?? Does glass have a definite chemical composition?

Glass may have a definite chemical composition, but it is amorphous. Nature-formed glass is often called a mineraloid because, while if satisfies most requirements for a mineral, it lacks an internally-ordered crystal structure.