Assignment of part 4: Provenance and composition of mineral water

Weathering rates of minerals and rocks

Minerals are often thermodynamically not stable under ambient conditions. Therefore they alter to different minerals or dissolve entirely in the water that drains the soil, the top layer of the rock formation. The kinetics of the dissolution process determine whether a dilute or a more concentrated solution is the result. In this matter the soil plays an important role: the presence of degrading organic matter creates carbonic acid that promotes the weathering of minerals. Further, biological activity accelerates the turn-over of organic matter. The largest portion consists of bacterial consortia. Worms, ants, other animals, and plant roots aerate the soil creating micro-environments where oxidation and reduction can take place that add to the suite of possible weathering reactions. Intermittent water-logging as a consequence of rain fall assists in the formation of such micro-environments. So, despite that soils are often relatively thin (but keep in mind the very thick tropical laterite soils) they are a key player in weathering processes.

However, the residence time of water in the soil layer is generally short (days, up to a season). When water enters the bedrock below through fractures it can stay underground much longer (decades up to centuries). Hydrologists take advantage of the atmospheric hydrogen bomb testing in the 1950s and early 1960s to determine the age of groundwater by measuring the tritium content with a half-life of ~11 years. The long residence time enables the water to equilibrate with surrounding rock. Therefore, the geochemical composition of the water reflects the rocks it has percolated through. By simulating weathering experimentally the reaction of rocks with several kinds of water can be understood. Since the time of the experimentor is limited such studies are sometimes carried out at moderately elevated temperature to speed up reaction kinetics. Results are extrapolated to lower temperature by making use of laws that describe the temperature depedence of (geo)chemical reactions. Also it should be realized that waters warm up at depth because of the prevailing geothermal gradient.

Weathering experiments in "Spa Red" and "Spa Blue" waters

To experimentally simulate rock weathering, five rock types were powdered. The rock types are: granite, limestone, basalt, gypsiferous evaporite, and schist. Importantly the schist contains pyrite.

10 grams of each rock powder was put into bottles filled with either Spa Red (sparkling) or Spa Blue (still), kept at 40°C for one month and thoroughly shaken two times per day. For completeness also bottles filled only with the two Spa varieties were subjected to the same treatment (analytically referred to as ‘blanks’). After the experiment the waters of the 12 bottles were analyzed chemically. This was done with an Inductively Coupled Plasma Atomic Emission Spectroscopy instrument, which enables measurement of 36 elements simultaneously. In the instrument the reaction solution is sucked into a gas flame: the water is evaporized immediately and the dissolved contents are atomized (they become a plasma) in the very hot flame (>2500°C). Each element emits spectroscopic radiation at a specific wavelength. The intensity of the lines is calibrated with standards which allows calculation of concentration from the intensities measured.

Weathering reactions

To understand the water composition the dominant minerals of each potential parent rock type must be known. Further it must be known how amenable those minerals are to weathering, how reactive are those minerals? A third aspect is the reactivity of "Spa Red" and "Spa Blue". Answer the following questions.

a) Give the general weathering reaction of anorthite.
b) Give the general weathering reaction of albite.
c) How does calcite weather?
d) Provide the weathering reaction of pyrite. What makes it rather special?
e) How does olivine weather?
f) What is the weathering reaction of gypsum?

We now dwell a little longer on the pH of water. Pure water dissociates into [H⁺] and [OH^-] according to the reaction equation [H₂O] = [H⁺] + [OH^-]. The pH is the negative logarithm (base10) of [H⁺]. Pure water has a pH of 7. So, equilibrium concentrations of [H⁺] and [OH^-] are 10^-7 M/L and the equilibrium constant K_w = 10^-14.

Rain water is in equilibrium with the CO₂ in the atmosphere. We only consider CO₂ and ignore potential contributions of sulphuric and nitric acids. CO₂ is a weak acid. Therefore the pH of clean, natural rain water is lower than 7. You are going to calculate that pH for a CO₂ pressure (pCO₂) of 10^-3.5 bar (or atmosphere).

The following equibilibria apply:

pCO₂(gas) = [CO₂]aq (aq = aqueous, or 'in solution')

[CO₂]aq + [H₂O]aq = [H₂CO₃]aq

equilibrium constant: K₁ = [H₂CO₃]/pCO₂ = 10^-1.47 (with concentrations in Mole/L and pressure in bar)

[H₂CO₃]aq = [H⁺]aq + [HCO₃^-]aq

equilibrium constant: K₂ = [H⁺]*[HCO₃^-]/[H₂CO₃] = 10^-6.35 (with concentrations in Mole/L)

Keep in mind that electric charge neutrality applies.

g) What is the pH of clean rain?
h) What would be the pH if pCO₂ is raised 100 times?
i) What is the most reactive water: "Spa Red" or "Spa Blue"? Briefly explain why.

Your task

The 12 analyses are given in random order. They will be distributed by the instructor. Your task is to identify which analysis corresponds to which experiment. We have schist in Spa Red and Spa Blue, granite in Spa Red and Spa Blue, etc. Do not forget the two blanks.

Explain briefly how you came to your solution.

Strategy

Find out what are the dominant minerals in each rock type. Use your geology textbook or internet sources to this end. Assess the ‘weatherability’ of the important minerals, how amenable they are to weathering. Also take into account the reactivity of "Spa Red" and "Spa Blue".