public class PItem
{
    [BsonIgnoreIfDefault]
    public ObjectId ID { get; set; }

    [BsonElement("loc")]
    public PLocation PolyLocations { get; set; }
}

public class PLocation
{
    public String Type { get; set; }
    public GeoJsonPolygon<GeoJson2DGeographicCoordinates> Coordinates { get; set; }
}

"loc" : {
    "Type" : "Polygon",
    "coordinates" : {
        "type" : "Polygon",
        "coordinates" : [