using (var writer = new CsvWriter(new StreamWriter(exportCsv, append)))
{
    writer.Configuration.HasHeaderRecord = !append;
    writer.Configuration.RegisterClassMap<GeoVulcanCollarCsvMap>();
    writer.Configuration.TypeConverterOptionsCache.GetOptions<double?>().Formats = new[] { "0.######################" };
    writer.WriteRecords(list);
}