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Q: Halite in the rock cycle ( No Answer,   2 Comments )
Subject: Halite in the rock cycle
Category: Science > Earth Sciences
Asked by: lynnjg-ga
List Price: $4.00
Posted: 06 Dec 2005 20:51 PST
Expires: 08 Dec 2005 20:05 PST
Question ID: 602460
What is the rock name for the mineral Halite in the form of a
Sedimentary Rock, Igneous Rock, and Metamorphic rock? If there is not
a specific name for it in a certain form, explain why. What is it's
role as Magma?

Clarification of Question by lynnjg-ga on 06 Dec 2005 21:20 PST
A description of Halite from a sediment, to changes to all three rock
types, (Sedimentary, Igneous, Metamorphic) including Magma, and Rock
Names of Halite in each stage (or why there is not a name in a certain
stage) is requested.
There is no answer at this time.

Subject: Re: Halite in the rock cycle
From: fstokens-ga on 07 Dec 2005 09:13 PST
I have only heard rock salt (sodium chloride) described as "halite". 
It is water soluble, so generally only found in significant quantities
in sedimentary evaporite deposits.
Subject: Re: Halite in the rock cycle
From: hfshaw-ga on 07 Dec 2005 17:25 PST
As fstokens commented, halite (NaCl) is really only common in
chemically precipitated sedimentary rocks called "evaporites", which
form by the evaporation of salt water (e.g., seawater).  It occurs
rarely in metamorphic rocks as inclusions in other minerals.  In such
cases, it is thought to have formed from sodium- and chlorine-rich
fluids that were present during the metamorphism.

The reason the halite in evaporites does not (usually) get buried deep
enough to reach metamorphic temperataures and pressures is that it has
a much lower density than the silicate minerals that make up most
rocks (2.1 gm/cm^3 for halite versus > 2.6 gm/cm^3 for most typical
silicate-dominated rocks.  In addition, when subjected to stress,
halite "flows" like  a very viscous liquid (more accurately, it
deforms plastically).  The net effect of both of these properties is
that the halite (and other salts) in buried evaporite deposits is
bouyant and deforms to make structures called "diapirs" (salt domes),
which are rising columns of salt (see
<>).  These
structures are of considerable economic interest because they often
form "traps" for hydrocarbons.

This is also the reason halite does not form magmas -- large
quantities of salt simply never get hot enough to form melts.  (Halite
melts at 801C at 1 atmosphere pressure, and higher temperatures at
higher pressures).  Small quantites of NaCl that do get buried deep
enough to reach the melting point either get dissolved in the water
produced by the metamorphic dehydration of hydrous silicate minerals
associated with the halite, or dissolve in any silicate melt that is
present.  Once the NaCl dissolved in a silicate melt, it dissociates
into Na+ and Cl- ions, which are then incorporated into more abundant
silicate minerals.

I know of a few instances of metamorphosed evaporites (try Googling
this phrase), but in all the cases I know of, only small quantities of
NaCl were present, and the NaCl reacted with surrounding silicates to
form Cl-containing silicates such as scapolite or Cl-rich micas.

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