CEvolution of a magma can result in a variety of igneous rocks er thee a magma\'
ID: 154159 • Letter: C
Question
CEvolution of a magma can result in a variety of igneous rocks er thee a magma' extemece the discussion of crystal form in Exercise 3. Miner 76 Igneous Rocks chemical composition- thereby poducing a varietyals, it is shown that a mineral that grows free of a solid t obstraction-as within a water-filled cavity minerals and a variety of igneous rocks. How work or within magma or lava-sends to by crystal faces. But a mineral that grows against an earlier mineral Simple. The evolution of a magma is made develop a complete form bounded does this possihle by a single principlic will have less than a complete form extent by the earlier mineral. Q4.14 Applying the rule-the most complete foram temperatures. tha is, its form will be tramcated to some astualy, that shoukdet be too surprising. Fresh water and water freese (crystalline) at diftferent semperatures. right? So it is with different kinds of minerals. Peratur dentally, which does freeze at the lower tem- crystallices first, and the least complete form crystallizes afer or salt water? Hint: Recall that salt last-list the four minerals in Figure 4.11 (w.x. Y, and Qu2 on icy walkways melts the ice Z) in the order in which they crystallized. Caution: The card upon another: whereas minerals in the thin-section in Figure 4.11 have essentially no depth. View this thin- stack of cards in Figure 4.10 is 3-dimensional, with one as we lower the temperature of molten rock, crystallize earlier (ie, at higher temperatures ovens, we can show that, minerals ock, some minerals others section as 2-dimensional, with minerals side-by-side. te n sophisticated oenperatures). But long before ovens, geologists were able to de- the sequence of crystallization of minerals by study ing the first, a little warm-up exercise. e petrography (ie, the fabrics) of igneous rocks. ut 4.13 In what order were the sis cards in Figure 4.10 tossed onto the table? (Careful) Figure 4.11 This is a sketch of a thin-section of granite as viewed with a microscope. Its thickness is approximately that of tissue paper, so it is essentially 2-dimensional, ie, with no depth. 4.15 If the granite in Figure 4.11 were melted in the laboratory, what would be the order in which W, X, Y, and Z would melt? Hint: Imagine a pat of butter and a lump of sugar in a sauce pan. Add heat. Which melts first? Maintain the heat until both have melted, then turn off the heat. Which solidifies (crystallizes) first? It bears pointing out that the temperature of crystallization of a mineral is essentially the same as its temperature of melting. Analogy: As temperature falls, pure water freezes at 0 °C (32 °F). As temperature rises, ice melts at 0 C (32 °F). By studying a variety of rocks, as you did with granite, ge- ologists have been able to work out the sequence of crystal 0 Six playing cards were tossed face-up on a table. lization of common silicate minerals (Fig.4.12).Explanation / Answer
4.12. Saltwater freezes at lower temperature than the freshwater.
Freshwater crystallises to ice at 0oC (32oF) but seawater freezes to ice at -2oC (28.4oF). This happens due to the presence of the salt. Salt reduces the rate of reaction by which ice form from water. So more lowering of temperature is needed to freeze a salt water.
4.13. 6 cards are tossed face-up on the table. The cards when compared to the textures shown by minerals in a plutonic environment (i.e., magma chamber), it indicates that the grains represented by the cards are Euhedral (all faces developed) in shape and also a cumulate texture can also be thought of where the flattened grains of minerals (as represented by the cards) formed due to the crystal settling of the grains to the bottom of magma chamber abd forming cumulate layers of large flattened grains.
4.14. A first the most euhedral grains of 'X' develop. 'X' can represents minerals like olivine, pyroxene, Ca-rich plagioclase in the Bowens Reaction series. They attain their shape due to higher temperature of magma, very slow cooling of the melt/magma below the true crystallisation temperature of the mineral so that good diffusion occur.
'Z' grains may represent elongated prismatic pyroxene whose prismatic shape is not evident in 2-D. 'Z' grains are elongated in one direction but not on the another direction. This is due to the fact that some high energy faces which continue to grow (in a direction perpendicular to its surface) while some low energy faces do not grow much. As a result, the low energy faces become elongated and the elongated nature of the grain becomes obvious to us. The Z is older than Y because X grains are partially included in the Y grains.
Then slightly subhedral grain 'Y' can form. Subhedral grains (in which all faces are not developed) form because they do not get the enough time to crystallise like Euhedral grains. 'Y' grains can be of Amphibole, Na-rich plagioclase etc.
Last formed minerals like quartz, K-feldspar are repesented by 'W'. They form when temperature of magma is low and faster cooling of magma below the true crystallisation temperature of mineral occur.
Therefore the sequence is X, Z, Y, W.
4.15. The mineral which crystallised last will melt first and vice versa. That means the sequence will be W, Y, Z, X.