There have been some tentative identifications of carbonate on Mars, though not confirmed definitively. See for example this report:

"The highlands crust of Mars and
the Pathfinder mission: a prospective view from Mariner 6 infrared
spectroscopy", Workshop on Early Mars,
http://www.lpi.usra.edu/meetings/earlymars/pdf/3030.pdf

This report made a prediction of hydrous magnesium carbonate at the Pathfinder site and other locations on Mars.
Note this was written before the Pathfinder landing. The observation of a hard, white material widespread at the Pathfinder site might be a confirmation of the predictions in this report:

----------------------------------------
Pathfinder found red, white and blue rocks on Mars.
"A University of Arizona scientist told the American Astronomical Society that Pathfinder pictures taken with different filters show a faint bluish tint on one side of exposed rocks, while the other side
of the rocks is a typical Martian red. Some partially buried rocks are a whitish color and may actually be layers of a hard material called caliche, a calcium carbonate rock material common in Arizona."
http://www.spacetoday.org/SolSys/Mars/MarsPathfinder.html

MARS PATHFINDER LANDING SITE:
GEOLOGY OF MARS
"During its traverse to Yogi the rover stirred the soil and exposed
material from several cm in depth. During one of the turns to
deploy the APXS (inset and white arrow), the wheels dug
particularly deeply and exposed white material. Spectra of this
white material show it is virtually identical to Scooby Doo, and
such white material may underly much of the site."
http://www.nd.edu/~cneal/eg.101/geology.1.html

Pathfinder's Finds
"Britt classifies Scooby Doo and its kin as a third kind of rock that he calls "caliche-like," after a cemented mineral common in the Tucson area.
When it rains in the desert, calcium-rich salts leach out of the upper soil and begin to percolate downward. But before they've penetrated more than a foot or so, the water evaporates in the
scorching heat, concentrating them into a layer—caliche—that has many of the properties of concrete.
Says Britt, "There's a big debate about
whether the caliche-like stuff should be classified as soil or rock. The people from moist climates who have never tried digging in it call it soil.
Those of us from arid regions call it rock. And since I'm the chair of the mineralogy and geochemistry science group, it's rock."
p. 9,
http://pr.caltech.edu/periodicals/EandS/articles/Pathfinder%20Feature.pdf
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Note as well that the extent of such deposits being revealed by the removal
of a surficial covering only centimeters deep might explain where are the
widespread carbonates expected to be on Mars: perhaps they are there, only
centimeters subsurface.
Indeed they may be widespread enough to be detected subsurface by the GRS
instrument on Mars Odyssey, though the resolution of GRS is 300km:

Mars Exploration Program
2001 Mars Odyssey Orbiter
"2.1.2 Gamma Ray Spectrometer (GRS)
"The experimental objective of the GRS is to determine the elemental
composition of the surface of Mars by full-planet mapping of
elemental abundance with an accuracy of 10% or better and a spatial
resolution of about 300 km by remote gamma ray spectrometry, and full
planet mapping of the hydrogen (with depth of water inferred) and CO2
abundances by remote neutron spectrometry."
p. 2-2
http://mars.jpl.nasa.gov/odyssey/pip/2001MarsOdyssey-PIP2.pdf

Also I found this report on White Rock:

Martian crater lake may have harboured life.
"His team's research, published in the Journal of the Geological Society
[Note 1] suggests that the so-called "white rock", a bright lens-shaped body
18 km by 15 km and up to 540 m thick, situated near the rim of the crater)
may represent a huge deposit of magnesium carbonate. It is possible that
this was precipitated in an evaporating lake from a groundwater seep with
the help of microbes."
http://www.gla.ac.uk/press/releases/originoflife.html

In the "highlands crust" report, it said the footprint for the Mariner
infrared spectrometer was 10 by 200 km, which might be too large for White
Rock. It would be interesting to find out if the Mariner data for Pollack crater, which contains White Rock, matches the carbonate signature seen at Ares Vallis, and also to find out if THEMIS can confirm the signature of magnesium carbonate seen in the Mariner data.

Bob Clark