Question

1. specific gravity. the specific gravity (sg) of a mineral is the ratio of its density to that of an equal volume of water. effectively, it is the density of the material. this property is very important in determining the behavior of rocks in the tectonic processes we will talk about later in class. this tray has specimens of three minerals: a) galena (pbs), b) pyroxene (mgfesi₂o₆), and c) orthoclase feldspar (kalsi₃o₈).

based on your observations only, which one do you think has the highest specific gravity? why do you say that?

use google to find the specific gravity of each sample.

a) galena
b) pyroxene (augite)
c) orthoclase feldspar

what are the elements that make up the mineral with the highest specific gravity? does it make sense that this is the one is the highest? why?

feldspar and pyroxene are two important rock - forming silicate minerals. feldspar is common in granite, whereas pyroxene is common in basalt.

Explanation:

Part 1: Observational Prediction of Highest SG

To predict the mineral with the highest specific gravity (SG) from observation, we analyze the atomic mass of constituent elements. Galena (\( \text{PbS} \)) contains lead (\( \text{Pb} \), atomic mass ~207.2 u) and sulfur (\( \text{S} \), ~32.07 u). Pyroxene (\( \text{MgFeSi}_2\text{O}_6 \)) has magnesium (\( \text{Mg} \), ~24.31 u), iron (\( \text{Fe} \), ~55.85 u), silicon (\( \text{Si} \), ~28.09 u), and oxygen (\( \text{O} \), ~16.00 u). Orthoclase feldspar (\( \text{KAlSi}_3\text{O}_8 \)) has potassium (\( \text{K} \), ~39.10 u), aluminum (\( \text{Al} \), ~26.98 u), silicon, and oxygen. Lead is much heavier than \( \text{Mg, Fe, K, Al} \), so Galena likely has the highest SG due to its dense \( \text{Pb} \) content.

Using online resources (e.g., mineral databases, Google):

• Galena (\( \text{PbS} \)): SG ≈ 7.2–7.6 (average ~7.4).
• Pyroxene (Augite, \( \text{MgFeSi}_2\text{O}_6 \)): SG ≈ 3.2–3.6.
• Orthoclase feldspar (\( \text{KAlSi}_3\text{O}_8 \)): SG ≈ 2.55–2.63.

Galena (\( \text{PbS} \)) is composed of lead (Pb) and sulfur (S). Lead has a very high atomic mass (~207.2 u) compared to lighter elements in the other minerals (e.g., Mg, Fe, K, Al). Higher atomic mass per unit volume increases density (and thus SG). Sulfur is also moderately dense, but Pb’s high mass dominates. This makes sense because SG depends on the density of the material, and dense elements like Pb (a heavy metal) increase the mineral’s overall density.

Answer:

Based on observation, A) Galena likely has the highest specific gravity. Lead (Pb) is a very dense element, so Galena’s composition (PbS) suggests it will be heavier than the silicate minerals (pyroxene, orthoclase) with lighter metal ions (Mg, Fe, K, Al).

Part 2: Google - Found Specific Gravity Values