For years, a rare minerals collector's trust has sought tens of thousands of dollars in annual property tax exemptions on a secluded Cove Neck estate, arguing its purpose is strictly educational, and that one day it would become a full museum.

And for years, Nassau denied the exemptions -- once calling the site no more than a "luxury mansion" in the woods. The county always lost in court.

Now, just as the county assessor has entered into an agreement that accepts the exemption, barring no changes to the land or its ownership, county lawmakers for the first time are questioning the break, worth about $80,000 this year, and nearly $1 million since 2004.

"Why are we entering into a stipulation when there's no museum there?" Legislative Minority Leader Kevan Abrahams (D-Freeport) asked at a county legislature meeting Sept. 21. "This is troubling to me."

"That exemption was based on certain conditions," Presiding Officer Norma Gonsalves (R-East Meadow) said before agreeing to table a measure that corrected the assessment roll and honored the exemption for this tax year. "I want to verify those conditions, and I did not get that today."

The waterfront land, valued at $5.5 million, according to land records, is just south of Sagamore Hill in one of Long Island's most affluent villages. Set several hundred feet off the road are two main structures: a 6,382-square-foot building the county dubbed "a luxury mansion" in court filings, and a smaller Colonial house with a backyard deck and a basketball hoop.

A sign at the foot of the driveway reads: "Private Property: Unauthorized persons on property will be prosecuted."

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Mineral, gem collection

Since December 2003, the property has been owned by The Mineral Trust, a Florida-based nonprofit chaired by Barry Yampol, a retired telecommunications company executive. Yampol had donated the land to the trust, which stated in its original application for a property tax exemption that its "primary purpose was to establish and maintain a mineral and related sciences museum."

A 1983 Washington Post story described Yampol, who has also lived in Cove Neck, as having minerals and gems "considered to be one of the finest private collections in the world." The trust in recent years has loaned some of its 200,000 items, worth nearly $100 million, to the American Museum of Natural History in Manhattan and the Smithsonian Institution in Washington, and has donated $2 million to the Smithsonian for its new mineralogy computer system.

"We're going to build a museum on that property," Jon Santemma, the trust's attorney, said by phone Monday. "We're firmly committed to it."

Santemma, whose offices are in Uniondale, noted that no one lives at the site's larger building, which he said does not even have a kitchen and is used solely for processing specimens. Security and custodial staff live at the smaller home, he said.

"We're following the law," Santemma said of the trust.

Of county lawmakers who believe a museum must be open for a nonprofit to receive a tax exemption, he added: "Their facts are totally wrong."

The trust applied for its first tax exemption in December 2003, shortly after it took title.

When Nassau denied the application, the trust sued, saying it had complied with state law that a nonprofit's property must be used exclusively for charitable or educational proposes in order to get exemptions. The trust claimed both.

The county responded that a "luxury mansion, which is utilized for storage . . . is not the type of building that is needed for the trust to further the trust's sole non-for-profit purpose." It said the trust had let a building permit expire for an accessory structure that was to be used as part of the museum.

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"The trust has failed to make a good-faith effort to build a suitable structure or structures in order to carry out its purpose," the county argued.

State Supreme Court Justice Edward McCarty III, in a brief ruling in March 2005, backed the trust's tax exemption.

County's legal losses

In later years, Nassau continued issuing denials, which courts overturned. In December, it entered into a stipulation that the trust would file annual exemption requests -- but that the county would "only make inquiries to Mineral Trust if there is either substantive physical change to the real property or its ownership of real property, or if there is an indicated change in Mineral Trust's annual tax exempt status."

Otherwise, the trust "shall be afforded full protection under" the state law governing nonprofit real property tax exemptions, the stipulation says.

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"The county continued to lose on the denial of the exemption in court," said acting assessor James Davis, explaining why Nassau had agreed to the 2014 stipulation. "The court deemed every year that the filing was true and accurate and overturned the exemption denial."

Although Santemma said he had no timeline for the museum's construction, his 2014 letter to the county estimated that the project could cost $40 million. He said the trust has spent $4.5 million on the site already.

Although the recession slowed things down, "the planned expansion remains very real," Santemma wrote. "The trust's work at the existing site continues and expands its important influence in the scientific community."

County lawmakers say they still want to know more. Toward the end of last week's debate, Legis. Dennis Dunne (R-Levittown) asked for the property's address and said, "I'd like to visit this museum."

Legis. Francis X. Becker (R-Lynbrook) suggested: "Let's get a bus and all visit this museum. You tell . . . [the owner] that if he's getting an exemption, the legislature would like to come, all 19 of us, and we want to visit."

"Is there a bus there?" Gonsalves asked.

"There's not even a museum there," Dunne replied.