with an embedded document, regardless of the name of the field in the embedded document, the first field should contain the longitude value and the second field should contain the latitude value. For example:
db.zips2.insert( { _id: 1, city: "b", loc: { x: -73.974, y: 40.764 } } )
db.zips2.insert( { _id: 2, city: "b", loc: { x: -73.981, y: 40.768 } } )
Here the x field would be the longitude; and the y field would be the latitude.
Regards