(CNBC) This is turning into one costly Instagram photo.

A watchdog group suspects Treasury Secretary Steven Mnuchin's trip to see the government gold in Fort Knox on the taxpayers' dime with his wife, Louise Linton, was actually spurred by a desire to be in the prime viewing area for Monday's total solar eclipse.

It was Linton's rant on Instagram against a woman critical of her bragging about pricey designer clothes she wore on the trip that drew the suspicions of that watchdog, Citizens for Responsibility and Ethics in Washington.

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