Lab 1 Rock Cycle

Geology 115/History 150 Name(s):

Lab 6: Minerals and metamorphic rocks

Minerals

A mineral is a naturally-occurring, solid, usually inorganic element or compound with a definite crystal structure and chemical composition which varies only within specific limits. Rocks are merely aggregates of minerals.

The mineralogical composition of a rock depends on the conditions under which that rock formed. Igneous rocks tend to have minerals that form at high temperatures; sedimentary rocks contain minerals that are stable at Earth-surface conditions. Metamorphic rocks consist of minerals that form under a range of pressure and temperature conditions within the Earth.

Common rock-forming minerals:

Mineral identification

The first part of this lab is to identify mineral specimens, using the charts provided in the Geology Lab Manual. Note that most rock samples will not have minerals as large as the ones you will see in this part of the lab, so take notice of diagnostic characteristics that do not depend on mineral size.

1. For instance, consider color, a seeming obvious choice: find the quartz display in the cabinet in the back of the lab room. Quartz is a mineral that also happens to be a gem in some of its forms (e.g., opal, tiger’s eye, amethyst). Consider all the different varieties of quartz; is there are unique color for quartz?

2. Now consider shape, another fairly obvious characteristic. Again examine the quartz display in the cabinet. Is the shape uniform for both specimens of the same mineral? In fact, if a mineral is left undisturbed as it precipitates, it can develop its crystal growth habit. The shape of such minerals is generally attractive, or, as mineralogists call it, “euhedral”.

Needed: Mineral testing kit (located in the Tub 1 space) and mineral samples M-1 through M-14 (Tub 2). Please label the minerals with their M-numbers (use the lab tape and a pen) so that they can be returned to their rightful box.

Using the charts:

So what characteristics are actually useful in identification? It turns out that the chemical composition of a mineral (which distinguishes one mineral from another, usually) manifests itself in certain ways.

The most apparent of these is the mineral's luster, which can be metallic or non-metallic. Luster refers to how the mineral reflects light; a metallic luster is how a piece of steel or bronze or copper would reflect light. Compare a piece of metal's luster to the luster of a piece of glass; the glass' luster (vitreous) is not a metallic luster. Of course, if the mineral has a dull or pearly luster, it is a non-metallic luster.

3. a. Look at minerals M-1, M-3 and M-7. Only one of these samples has a metallic luster. Which one?

b. Now examine minerals M-2 and M-10; again, only one of these has a metallic luster. Which one? Hint: you may need to look at different specimens of the same mineral. Why was this question harder to answer than part a?

Next, recall how color was problematic. However, we can still use it appropriately in mineral identification by simply determining if the mineral is dark-colored (black or one of the “cool” colors) or light-colored (white or one of the “warm” colors). The difference is due to the particular chemical elements that make up the mineral.

4. Look at samples M-11 and M-13. Which one is dark-colored? Which is light?

Now use the mineral identification charts found in the colored pages in the lab manuals located in the tub labeled “Lab Manuals”. There are three sheets: yellow, red and blue and they are divided according to the two useful characteristics already mentioned.

Since you’ll be eventually filling out the table on the next to last page of this lab, you can now enter the two useful characteristics for all of the minerals in this lab (M-1 through M-14) into the table: luster and color.

To identify a mineral’s name, once you have determined a mineral’s luster and color, note that you will be using one of the three colored sheets. Follow the headings of the sheet left to right on the sheet to determine what test to do next.

For all minerals, the relative hardness of the mineral may be determined by scratching a corner of the mineral on a piece of glass (or scratching a corner of the glass plate on the mineral). Hardness is the mineral's ability to resist scratching or abrasion. A mineral will scratch all softer minerals and will be scratched by all harder minerals. Certain index minerals define the Mohs Hardness Scale, so you can get a numerical value for hardness. Rather than finding the exact numerical value, you will simply need to determine whether the mineral is harder than glass.

5. Use the corner of a glass plate and scratch minerals M-1, M-6 and M-10 and record the results below. Then scratch a corner of each mineral on the flat surface and record the results. Be sure to brush off any flakes of mineral to make sure that you’ve actually left a scratch! Then combine the information to draw a conclusion. Hint: there’s one of each “type”.

Again for all minerals. cleavage is another property that helps narrow down the identity of the mineral. Cleavage is the ability of the mineral to split along closely spaced parallel planes. The planes along which a mineral cleaves (when hit with a hammer, for instance) are the planes where all the weak atomic bonds in the crystal structure exist. Notice that if all bonds are uniformly strong (like in a piece of quartz), the mineral will not cleave along a plane; instead, it will break unevenly and roughly...it will fracture. Cleavage is sometimes confusing because some minerals have good cleavage, some have poor cleavage and still others have no cleavage (they fracture). The table below should help identify different types of cleavages, but ask if this concept is confusing!

Also note that we discounted shape a while ago. Cleavage is not the same thing as shape – shape is how the mineral crystal grew, whereas cleavage is how the crystal broke.

6. a. How many cleavages does M-7 have? Hint: It’s called a “sheet silicate” for a good reason!

b. How many cleavages does M-10 have? Remember not to count parallel faces twice. What angle separates each distinct cleavage?

c. Look at the display of the quartz spar crystal. Note that the top of the crystal has a nearly perfect six-sided symmetry; then examine the bottom of the crystal where it was broken off. How many cleavages does this chunk of quartz have? So what cleavage-related property does quartz have?

Then there are more specialized tests that are not applicable to all minerals, but for the minerals that the test works on, they are diagnostic!

For instance, if the mineral has a metallic luster, determine the mineral's streak color. Streak refers to the color of the powderized mineral, most easily accomplished by rubbing a corner of the mineral sample against the porcelain streak plate provided.

7. a. Use the porcelain streak plate on sample M-4; what color does the streak turn out to be? Is it the same as the color of the mineral?

b. Try streaking a few of the nonmetallic luster minerals. What seems to be the problem with the streak test and nonmetallic minerals?

For minerals with metallic luster, magnetism may be used to identify the mineral.

8. Use the magnet to determine if M-2, M-3 or M-4 is magnetic. “Weakly magnetic”, “strongly magnetic” and “not magnetic” are acceptable answers. In addition to writing your answer here, enter the information under “other properties” on the appropriate row of the table.

9. Another test is a mineral’s reaction to weak acid. Obtain an acid dropper bottle and place one drop of (hydrochloric) acid on a sample of M-1 and M-8. Which one reacts? How can you tell?

10. All right, now put it all together and identify the rest of the mineral samples.

To make this simpler, here are the mineral names in alphabetic order: amphibole, biotite, calcite, chalcopyrite, galena, hematite, magnetite, muscovite, orthoclase (potassium feldspar or K-spar), plagioclase feldspar, pyroxene, pyrite, quartz.

Mineral ID Chart

11. Mineral summary: Name three physical characteristics you can use to distinguish quartz from calcite.

Mining economics

Consider the following selected assay results for layers of the Homestake breccia deposits, as reported by Noranda Exploration and Crown Butte Mines (Van Gosen, 2005).

12. Given that the tenor (concentration) of the various metals in each layer is report in troy ounces per ton of rock mined, for layer 1, how many tons of rock would have to be mined to yield 1 troy ounce of gold? The fixed costs of excavation, hauling, crushing and refining run about $480/ton of rock mined; is this layer economically feasible to mine for gold? Why or why not?

13. As of May 9, 2016, gold’s spot price in New York is $1266/troy ounce, silver is $17/troy ounce and copper is $0.13/troy ounce. Per ton of rock mined, which layer number is the most economic to mine? Prove it with a calculation. Is the silver and copper ever economic to mine here?

Metamorphic rocks

Metamorphic rocks have been subjected to sufficient heat and/or pressure to melt some of their constituent minerals, but not all of them. As a result of this selective mobilization of chemicals, only certain chemical reactions can occur, and so a whole new set of metamorphic minerals are crystallized.

Throw in the presence of fluids such as water and carbon dioxide (yes, at these pressures, even carbon dioxide can be a liquid), and nature has the means to create even more metamorphic minerals and therefore metamorphic rocks. Note that metamorphic rocks must be formed at depth; metamorphism is not a surface process, and so is distinguishable from mere sedimentation.

Rocks that have foliation (a sort of wavy layering, though it can resemble horizontal layering) are metamorphic rocks; the foliation indicates that directional pressure was applied to the rock while the mineralogical changes were occurring. On the other hand, some metamorphic rocks are not foliated; they appear crystalline, like coarse-grained igneous rocks. These metamorphic rocks were subjected to isotropic, or nondirected, pressure.

Because there are so many metamorphic minerals (of which you have seen but a few), there are all sorts of ways to name metamorphic rocks. We will concentrate on naming rocks by their metamorphic grade (that is, by the maximum degree of heat and pressure they were subjected to, and not their mineral composition), or, in some unusual cases, by their apparent composition (for instance, rocks like marble, quartzite or metaconglomerate, from which you cannot determine the metamorphic grade).

The parent rock of a metamorphic rock is the original rock that was metamorphosed into what you see today. As you can see from Table 6.1, the parent rock’s minerals really do determine the resulting metamorphic rock’s composition. Note the differences in mineralogy even at the same grade.

Table 6.1— Mineralogy of metamorphic rocks related to parent rock and grade

A metamorphic facies is a name of a set of metamorphic minerals which is uniquely created at a particular pressure and temperature. So, in addition to a metamorphic grade, a rock can belong to a particular metamorphic facies as well! Confused? You bet! However, realize that these terms all have their uses.

Note that not all minerals in a given cell in the table above will show up in every specimen of that grade/facies/parent rock, but all minerals in the specimen will be named in the cell!