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One strategy for sequestration of $\ce{CO2}$ is mineral trapping arising from geologic sequestration. Another strategy is mineral trapping in concrete via the Mineralization via Aqueous Precipitation or MAP process. In particular, Alkalinity Based on Low Energy or ABLE process offers an energy efficient MAP process. However the ABLE process produces $\ce{Cl2}$ (or $\ce{HCl}$), which, at scale, produces its own environmental impact.

One way of dealing with $\ce{Cl2}$ (or $\ce{HCl}$) might be mineral trapping via geologic sequestration.

Is this plausible?

James Bowery
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One of the problems of geological sequestration of chlorine is that most chloride minerals are very soluble.

The number of non-soluble (or very little) minerals is limited, but there are several options. The obvious ones are sodalite ($\ce{Na8(Al6Si6O24)Cl2}$) and chlorapatite ($\ce{Ca5(PO4)3Cl}$). There are other options though.

Webmineral lists a lot of chlorine-bearing minerals, and most of those are soluble. However, there could be several minerals with high percentages of chlorine that are insoluble enough under the conditions of your reservoir. But in general, your best bet are anhydrous silicates and phosphates, because those are the most likely to be stable over a long time.

That said, this stuff could be dumped in the ocean. The $\ce{HCl}$ could be neutralised with something like feldspars, turning it into clay and a solution of harmless sodium chloride.

Alternatively, you can oxidise the HCl and make chlorine gas, a useful product.

Gimelist
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  • The $\ce{Cl2}$ volumes projected for $\ce{CO2}$ sequestration via ABLE would totally saturate any market. Dumping in the ocean is unacceptable at these levels as that merely exchanges $\ce{CO3^{2-}}$ acidification for $\ce{Cl-}$ acidification.

    A possible oceanic geologic reservoir could be the pillow basalts that underlie pelagic sediments. As I understand it pillow basalts have low compressive strength due to their rapid cooling.

     – James Bowery Apr 24 '19 at 16:37
  • @JamesBowery that’s why I said neutralise it first. Chloride anions are harmless. – Gimelist Apr 24 '19 at 19:15